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+# 0018. Deploying application logic as smart contracts on this framework
+
+- Status: Proposed
+- Date: 2026-06-20
+
+## Context
+
+This project is, structurally, a small **smart-contract execution substrate**:
+application logic is expressed as `Command` types (`consensus/types.ts`) that flow
+through an ordered, replicated log and are applied by a deterministic state
+machine (`stateMachine.ts`), with a tamper-evident hash-chained audit trail
+(`replicatedStateMachine.ts`) and exactly-once semantics. That is the same shape
+as "deterministic state transitions over an append-only, agreed-upon log" that a
+contract platform provides.
+
+A recurring question is therefore whether a conventional ("formal") API server's
+application logic should be deployed *as smart contracts on this framework* —
+i.e. modelling each mutating endpoint as a `Command` variant + an `apply()` case
++ a leader-side builder (`models/book.ts`), instead of an imperative handler over
+a database. This ADR records the analysis and the conditions under which that is
+(and is not) the right model.
+
+The decisive fact shaping the whole analysis: **Raft is Crash-Fault-Tolerant
+(CFT), not Byzantine-Fault-Tolerant (BFT).** It assumes peers are honest but may
+crash; it does not tolerate malicious participants. The framework gives the
+*form* of smart contracts (deterministic, replicated, audited execution) without
+the *substance* of trustlessness (agreement among mutually-distrusting parties).
+The "trustless agreement" framing in `docs/PHILOSOPHY.md` is aspirational in this
+sense.
+
+## Decision
+
+Characterize the framework as a **CFT replicated-state-machine contract
+substrate, scoped to a single trust domain**, and adopt the following guidance
+for modelling application logic as contracts on it:
+
+- **Recommended** where the goal is replication + tamper-evidence + exactly-once
+  within *one organization that controls all nodes*: compliance ledgers,
+  financial-ops journals, inventory/settlement, audit-critical CRUD. This is
+  effectively "event-sourcing + Raft + a hash-chained audit," which the codebase
+  already demonstrates.
+- **Not recommended** where the motivation is trust-minimization across
+  distrusting parties, public/permissionless participation, adversarial node
+  operators, large state, or logic requiring external I/O. Those are a category
+  mismatch for CFT consensus; use an actual BFT/blockchain platform.
+
+When logic *is* modelled this way, it must respect the existing invariants:
+determinism in `apply()` (ADR-0003), snapshot-relative indexing, leader-resolved
+non-determinism, and side effects kept off the apply path (leader/edge "oracle"
+pattern).
+
+## Consequences
+
+### Positive
+
+- **The execution model is already enforced.** Ordered, replicated, reproducible
+  state transitions come for free; the `apply()` switch is exhaustiveness-checked
+  and non-determinism is pushed to leader-side builders.
+- **Tamper-evident history = on-chain ledger.** `GET /audit` / `/audit/verify`
+  give a verifiable, replicated execution log per call, with no extra
+  event-sourcing infrastructure.
+- **Exactly-once semantics = transaction nonce.** A replayed `requestId` returns
+  the cached result, preventing double-application of stateful operations.
+- **Fault tolerance with no central DB.** Logic and state survive a minority of
+  node failures; there is no single database to lose.
+- **Determinism forces clean architecture.** Pure state transitions separated
+  from effects mirror smart-contract best practice and are trivially unit-testable.
+- **Cheaper and faster than a real blockchain.** No PoW/PoS, no gas market, no
+  global network; sub-second commits and optional linearizable reads.
+
+### Negative
+
+- **Not trustless / not BFT — the dealbreaker if trust-minimization is the goal.**
+  The leader builds every command and could forge `actor`/`timestamp`/`id`;
+  followers largely trust it. Any operator running modified `apply()` code
+  diverges silently. The `matchIndex` clamp in `replicateTo()` is the only
+  adversarial hardening.
+- **Determinism is a convention, not a sandbox.** Nothing in the runtime prevents
+  `Date.now()`/`Math.random()` inside `apply()`; it would silently diverge
+  replicas (the worst class of bug). No metering, no static guarantee.
+- **No gas / no resource metering.** A looping or over-allocating command blocks
+  the single-threaded apply path on every node, with no bound — it can halt the
+  cluster.
+- **Mutable logic undermines immutability.** Deployed TypeScript can be changed
+  and redeployed across a restart. The audit chain records *that* a command ran
+  and its result, but not *which code version* produced it — "tamper-evident"
+  covers the data, not the logic.
+- **Determinism excludes most real API work.** No external calls, randomness, or
+  wall-clock in `apply()`. Payment gateways, email, third-party reads need an
+  off-chain/leader-side oracle pattern — the genuinely awkward part of contract
+  development.
+- **Single-leader writes + full replication.** Mutating calls serialize through
+  one leader (no parallel execution); every node holds the full in-memory state
+  and an unbounded audit log. Same scaling pain as a full blockchain node.
+- **Schema/state migration friction.** State serializes as `RsmSnapshot`;
+  evolving its shape requires careful snapshot-format migration or `restore()`
+  breaks.
+
+## Alternatives considered
+
+- **Keep the conventional API-server model (imperative handlers over a shared
+  database).** Simpler for arbitrary side-effecting logic and large state, but
+  re-centralizes on one store and forfeits the replication, tamper-evident audit,
+  and exactly-once guarantees the substrate provides — the very properties that
+  motivate the contract framing.
+- **Deploy on an actual BFT / smart-contract platform (EVM, Cosmos/Tendermint,
+  Hyperledger Fabric).** The right choice when trust-minimization across
+  distrusting parties is required, with structural determinism (sandbox), gas
+  metering, and code-committing immutability. Rejected as default here because it
+  is far heavier and unnecessary within a single trust domain, where CFT + audit
+  already meets the need at a fraction of the cost.
+- **Hybrid: model only audit-critical, deterministic core logic as contracts and
+  keep side-effecting logic at the edge.** The pragmatic middle ground implied by
+  the guidance above; preferred over an all-or-nothing migration.
diff --git a/docs/adr/0019-native-backend-blockchain-benefits.md b/docs/adr/0019-native-backend-blockchain-benefits.md
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+# 0019. Bringing blockchain benefits to backend development natively
+
+- Status: Proposed
+- Date: 2026-06-20
+
+## Context
+
+ADR-0018 concluded that this framework provides the *form* of smart contracts
+(deterministic, replicated, audited execution) but not the *substance* of
+trustlessness, and is best scoped to a single trust domain (CFT, not BFT). It
+also enumerated the gaps that make "application logic as contracts" awkward for
+real backends: determinism is a convention not a guarantee, there is no story for
+external I/O, the single leader and in-memory state limit scale, the audit proves
+data but not logic, and there is no resource bound.
+
+This ADR answers the follow-up: **how can the framework deliver blockchain
+benefits (CFT availability, exactly-once, tamper-evident/provable audit,
+deterministic replay) natively in everyday backend development, without
+compromising the fundamental principles of backend systems** (developer
+productivity, side effects / external I/O, rich queries and large datasets,
+horizontal scale, schema evolution, a standard API contract, security and
+observability)?
+
+The decisive insight: backends are inherently *effectful* while a replicated
+state machine must be *pure and deterministic*. Rather than forcing backends to
+become pure, **split the system in two and connect the halves with the log**:
+
+```
+  Effectful EDGE          Deterministic CORE         Effectful EDGE (out)
+  (oracle resolution,  →  (consensus + audit +   →   (committed-intent
+   auth, validation)       exactly-once, state)       executor / outbox)
+                                   │
+                                   ▼
+                          Read projections (CQRS)
+                          rich queries, big data
+```
+
+The deterministic core gets the blockchain properties; the edges do the real
+backend work; the committed log is the exactly-once, auditable boundary between
+them. The existing seams (`Command`/`apply()`, the `Transport` and `RaftStorage`
+interfaces, leader-side builders, `CommandMeta`, the audit chain, snapshots) are
+the foundation this builds on.
+
+## Decision
+
+Adopt a layered "deterministic core + effectful edge" architecture and evolve the
+framework along the following pillars. Each closes a specific ADR-0018 gap while
+preserving a backend principle.
+
+1. **Module SDK (developer productivity).** Replace the four-touchpoint workflow
+   (extend the `Command` union, add an `apply()` case, add a leader-side builder,
+   wire a route) with one declarative unit: `defineModule({ state, commands,
+   queries, effects })`. Developers write **pure reducers** plus a schema; the
+   framework generates the namespaced command variants, the `apply()` dispatch,
+   REST routes + OpenAPI, and validation. The REST surface stays standard so
+   clients remain unaware they talk to a consensus cluster.
+
+2. **Determinism as a guarantee, not a convention.** Inject a deterministic `ctx`
+   (`ctx.now`, `ctx.random()`, `ctx.id()`) into every reducer, resolved on the
+   leader and baked into `CommandMeta` before the entry enters the log
+   (generalizing what `models/book.ts` already does for ids/timestamps). Run
+   reducers in a sandbox with frozen globals shadowing `Date`/`Math.random`/
+   `crypto`, add a lint rule, and add a CI check that replays the log on two
+   nodes and diffs state hashes.
+
+3. **Effects and external I/O via committed intents (the unblocker).** Reducers
+   never perform I/O; they **return declarative effect intents** that commit to
+   the log as part of the deterministic result. After commit, a single designated
+   executor (leader or leased worker) performs the effect exactly-once keyed by
+   `requestId`, then feeds the outcome back as a new command. Consequences: the
+   replicated log *is* a transactional outbox (exactly-once, for free), and
+   inbound external data is resolved on the leader before entering the log
+   (leader-as-oracle), keeping replicas identical.
+
+4. **Scale without compromising consensus.**
+   - *Reads / rich queries / big data:* project the committed log into a queryable
+     read model (CQRS) — a derived, rebuildable cache, not a central database, so
+     ADR-0002 still holds and the log remains source of truth.
+   - *Writes:* partition into multiple Raft groups (multi-Raft), one per keyspace/
+     aggregate, each with its own leader; cross-shard operations use a saga or 2PC
+     over groups. Small services stay single-group.
+   - *State > RAM:* swap the in-memory `Map` for a pluggable embedded LSM/KV store
+     behind the same interface (the `RaftStorage` seam); snapshots become store
+     checkpoints.
+
+5. **Auditability upgraded to real tamper-evidence.** Replace the linear hash
+   chain with a Merkle tree/accumulator (O(log n) inclusion proofs and a compact
+   root that can be externally anchored), and record a hash of the deployed module
+   code in a `DEPLOY` log entry referenced by each command's meta — so the audit
+   proves *which logic version* produced each result, closing ADR-0018's
+   data-but-not-logic gap.
+
+6. **Resource safety (gas analog).** Bound reducer execution with a deterministic
+   step/instruction budget (count operations, not wall-clock), validated on the
+   leader so over-budget commands are rejected before entering the log, plus input
+   size limits at the edge. No gas market — just a guard against a runaway command
+   halting the single-threaded apply path on every node.
+
+7. **Stay CFT, add accountability cheaply.** Sign commands with the originating
+   actor's key so the leader cannot forge `actor` in `CommandMeta`. Keep full BFT
+   (PBFT/Tendermint) as an optional swap behind the consensus seam, only if
+   multi-party trustlessness is ever required — not paid for by default.
+
+### Phased roadmap
+
+1. Determinism runtime + module SDK (pillars 1–2) — biggest DX/safety win, no
+   architecture change.
+2. Effect intents + post-commit executor/outbox (pillar 3) — unblocks real
+   backend use cases.
+3. CQRS read projections + pluggable state store (pillar 4) — queries and big data.
+4. Merkle audit + committed code version + signed commands (pillars 5, 7) — real
+   tamper-evidence.
+5. Multi-Raft sharding + step budget (pillars 4, 6) — scale and safety hardening.
+
+## Consequences
+
+### Positive
+
+- Blockchain benefits — CFT availability, exactly-once, provable/tamper-evident
+  audit with committed code versions, deterministic replay, a built-in outbox —
+  become available through ordinary-looking backend code.
+- Each fundamental backend principle is preserved: productivity (module SDK),
+  side effects (committed intents), rich queries/big data (CQRS + pluggable
+  store), horizontal scale (read replicas + multi-Raft), schema evolution
+  (logged, auditable `DEPLOY` migrations), standard API contract (unchanged REST),
+  security/observability (kept at the edge, off the deterministic path).
+- Builds on existing abstractions rather than replacing them; several pillars are
+  additive against current seams.
+
+### Negative
+
+- Materially larger surface area: a module runtime/sandbox, an effect executor,
+  projection infrastructure, multi-Raft routing, and a Merkle audit are each
+  non-trivial subsystems — well beyond a POC's current scope.
+- The committed-intent/outbox model adds eventual-consistency semantics for
+  side effects (an effect runs after commit, asynchronously), which application
+  authors must reason about.
+- Multi-Raft introduces cross-shard transactions (saga/2PC), the hardest part of
+  distributed data systems.
+- Sandboxing and step-budgeting add per-command overhead and constrain what
+  reducer code may do.
+- Signed commands and Merkle proofs add cryptographic machinery and key
+  management that the current trust-domain-internal model does without.
+
+## Alternatives considered
+
+- **Keep the framework as a focused POC and document the gaps only (ADR-0018).**
+  Lowest effort, but leaves the framework unusable for mainstream backend work;
+  this ADR is the constructive counterpart.
+- **Adopt an existing platform instead of evolving this one** — an event-sourcing
+  framework (Axon, EventStoreDB) for the core, or a permissioned ledger
+  (Hyperledger Fabric) for the contract/audit properties. Pragmatic, but forfeits
+  the single-substrate integration (consensus + audit + exactly-once + REST in one
+  place) that is this project's thesis, and Fabric pays BFT-class cost unneeded in
+  a single trust domain.
+- **All-at-once rewrite rather than the phased roadmap.** Higher risk; the phases
+  are independently valuable and each ships a usable increment, so incremental is
+  preferred.
+- **Make backends pure instead of splitting core/edge.** Rejected outright:
+  forbidding external I/O contradicts the fundamental nature of backend systems;
+  the core/edge split is precisely what avoids that compromise.
+
+## Prototype status (2026-06-20)
+
+A working prototype lives under `src/runtime/` (see
+[`src/runtime/README.md`](../../src/runtime/README.md)), built and reviewed in
+seven incremental milestones. The first three are pure runtime layered on top of
+the consensus core; M4–M7 wire it into real Raft and harden it.
+
+- **M1 — Module SDK + deterministic runtime (pillars 1–2).** `defineModule`, a
+  `ModuleHost` that dispatches commands to pure reducers, and a deterministic
+  `ReducerContext` (`now`/`random`/`id`) derived entirely from a leader-resolved
+  `Seed`. A convergence test proves two independent hosts fed the same seeded
+  command stream reach byte-identical state.
+- **M2 — Committed-intent effects + outbox executor (pillar 3).** Reducers emit
+  declarative `EffectIntent`s into a deterministic outbox; a post-commit
+  `EffectExecutor` runs each effect and feeds the result back as a committed
+  `EffectResultEntry`. Exactly-once at the state level is enforced by outbox
+  dedup + idempotent result application + an in-flight guard; handler execution
+  is honestly at-least-once.
+- **M3 — Merkle audit + committed code version (pillar 5).** A Merkle
+  accumulator with O(log n) inclusion proofs and a domain-separated root, plus a
+  per-command module code-version hash so the audit proves which logic version
+  produced each result.
+- **M4 — Consensus wiring (pillars 1–3 end-to-end).** The runtime plugs into the
+  pluggable `StateMachine` seam (ADR-0017): a `ModuleStateMachine` adapts the
+  `ModuleHost` so a `ModuleAppCommand` (carrying a leader-resolved seed) is an
+  ordinary application command — no bespoke consensus-core support. A 3-node
+  cluster converges on identical module state *and* Merkle audit root, with
+  substrate idempotency and the seed flowing through the log.
+- **M5 — Determinism enforcement + resource bounds (pillars 2, 6).** A static
+  determinism lint rejects reducers that touch non-deterministic globals
+  (`defineModule` strict-by-default), and a deterministic `413` resource bound
+  caps effect count and result size — rejected uniformly on every replica. A
+  shared canonical serializer removes the audit/size-bound drift hazard.
+- **M6 — CQRS read projections (pillar 4, reads).** A derived, rebuildable read
+  model folded from the committed command stream, answering indexed queries the
+  raw module state doesn't — strictly off the apply path; `rebuild()` proves it
+  is a cache, not a source of truth (ADR-0002 upheld).
+- **M7 — Signed commands (pillar 7).** Module commands are ed25519-signed by the
+  originating actor over the *logical* payload (not the leader's seed); every node
+  verifies on the apply path against an actor→key registry, so a forged `actor` is
+  rejected `401` identically on all nodes. Opt-in and fully backward-compatible.
+- **M8 — Pluggable key-oriented state store (pillar 4, state > RAM).** A
+  `StateStore` abstraction (in-memory default; the seam a persistent embedded
+  KV/LSM drops into) plus a transactional keyed module model (`defineKeyedModule`)
+  where reducers touch individual records through a copy-on-write `StoreView` —
+  only the touched keys in memory per command. Sorted iteration + clone-on-read
+  keep it deterministic (a cross-incidental-order test proves the store converges
+  regardless of apply order of independent commands); keyed and whole-state
+  modules coexist in snapshot/restore. Additive — the consensus core is untouched.
+- **M9 — `vm` determinism sandbox + step meter (pillars 2, 6).** Opt-in
+  (`defineModule(def, { sandbox: true })`) execution of whole-state reducers inside
+  a Node `vm` context that enforces determinism by *removal* — `Date`, timers,
+  `crypto`, `Intl` and the locale-sensitive prototype methods are deleted/thrown,
+  so a careless reducer reaching for ambient non-determinism throws instead of
+  silently diverging replicas. The apply path runs the sandbox timeout-free
+  (deterministic); a leader-side `admit()` dry-runs with a `vm` wall-clock timeout
+  to reject runaways before they enter the log (the gas/CFT model). Honestly
+  scoped: `vm` is a determinism aid, not a security boundary.
+- **M10 — Multi-Raft sharding (pillar 4, write scaling; see ADR-0020).** Multiple
+  independent Raft groups (shards) with a deterministic `sha256(key) mod N` shard
+  router, so writes to different shards commit in parallel and state is
+  partitioned. Cross-shard transactions use a saga (try/compensate) — atomicity by
+  compensation, not isolation; a throwing compensation is surfaced
+  (`compensationFailures`), not hidden. Additive: each shard is an ordinary
+  single-group cluster, so the consensus core is untouched.
+
+**Remaining frontier:** an optional **BFT consensus swap** (pillar 7) for
+multi-party trustlessness beyond the single trust domain. This is a protocol-level
+replacement (different messages, `3f+1` quorums, signed votes) well beyond a
+prototype increment; **ADR-0021** records the honest design, the real seam (the
+commit-ordered-log contract above the log, not the Raft `Transport`), and why it
+is deliberately not built. Everything that ports unchanged under BFT — the
+deterministic state machine, Merkle audit, signing, and the core/edge split — is
+already in place, so a future BFT effort starts from a clean boundary.
diff --git a/docs/adr/0020-multi-raft-sharding.md b/docs/adr/0020-multi-raft-sharding.md
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+# 0020. Multi-Raft sharding for write scaling
+
+- Status: Proposed
+- Date: 2026-06-21
+
+## Context
+
+ADR-0019 pillar 4 names a hard limit of the prototype: **all writes serialize
+through a single Raft leader.** One leader orders every committed entry, so write
+throughput is bounded by one node, and the entire state must fit on every member.
+M6 (CQRS read projections) and M8 (pluggable key-oriented state store) scale
+*reads* and *state size*, but neither scales *writes* — a single Raft group is
+still one ordering bottleneck.
+
+The standard answer, used by every production system built on Raft (CockroachDB,
+TiKV, YugabyteDB, etcd-at-scale), is **multi-Raft**: partition the keyspace into
+shards, give each shard its own independent Raft group with its own leader, and
+let writes to different shards proceed in parallel. The cost is that an operation
+spanning two shards is no longer a single atomic log append — it needs a
+cross-shard transaction protocol.
+
+This ADR records the design for a multi-Raft *prototype* on top of the existing
+runtime, and the decision to demonstrate cross-shard atomicity with a **saga**
+(try/compensate) rather than blocking two-phase commit.
+
+## Decision
+
+Introduce a **shard router + per-shard Raft groups + a saga coordinator**, built
+additively on the existing consensus core and runtime (no changes to `RaftNode`):
+
+- **Sharding key.** A deterministic function maps each command to exactly one
+  shard. For keyed modules (M8) the shard is derived from the record key
+  (`hash(key) mod N`, or an explicit range map); for whole-state modules the
+  whole module is pinned to one shard. The mapping is configuration, identical on
+  every participant, so routing is deterministic.
+- **Per-shard groups.** Each shard is an independent Raft cluster — its own set of
+  `RaftNode`s, its own leader election, its own replicated log and `ModuleHost`.
+  A write to shard *s* is proposed only to *s*'s leader; shards commit in parallel.
+  Within a shard, every guarantee from M1–M9 still holds unchanged.
+- **Shard router.** A thin front door maps an incoming command to its shard and
+  submits to that shard's current leader (forwarding as today). It holds no state
+  of its own — it is derivable from the shard map and each group's leadership.
+- **Cross-shard transactions via saga.** An operation touching shards *A* and *B*
+  (e.g. transfer from an account on *A* to one on *B*) runs as an ordered set of
+  per-shard local steps, each with a **compensating** step. The coordinator
+  executes forward; on a step failure it runs the compensations for the steps that
+  already committed, in reverse. Each step and each compensation is an ordinary,
+  idempotent, single-shard command (so it inherits exactly-once via the requestId
+  dedup). This yields **atomicity by eventual compensation**, not isolation.
+
+### Why saga, not 2PC
+
+- Two-phase commit holds locks across shards for the duration of the transaction
+  and blocks if the coordinator fails mid-commit — it trades availability for
+  isolation, which is at odds with the project's availability-first thesis.
+- A saga keeps each shard independently available and uses the runtime's existing
+  idempotency + audit to make steps safe to retry and compensations safe to
+  replay. It surfaces intermediate states (no isolation), which is the accepted
+  trade-off and is exactly what the hash-chained audit is there to record.
+
+## Consequences
+
+### Positive
+
+- Writes to different shards commit in parallel — throughput scales with shard
+  count instead of being pinned to one leader.
+- State is partitioned, so no single node must hold the whole dataset.
+- Built additively: each shard is just an existing single-group cluster, so all
+  per-shard correctness (determinism, snapshots, signed commands, sandbox, audit)
+  carries over untouched.
+- The saga steps are ordinary signed/audited module commands, so a cross-shard
+  transaction is itself fully audit-traceable across shards.
+
+### Negative
+
+- **No cross-shard isolation.** A saga exposes intermediate states (the debit is
+  visible before the credit). Reads spanning shards can observe a partially
+  applied transaction; only per-shard linearizability is preserved.
+- **Compensation, not rollback.** A failed cross-shard transaction is undone by
+  forward compensating actions, which must be designed per operation and be
+  idempotent — more application-author burden than a single-shard command.
+- **Cross-shard ordering is not globally defined.** There is no single log across
+  shards, so there is no global total order; the per-shard audit roots no longer
+  compose into one root (a cross-shard transaction appears as correlated entries
+  in two audits). A global audit needs a separate cross-shard correlation id.
+- **Resharding is out of scope.** Changing the shard count/map (rebalancing) is a
+  hard problem (key migration between groups) and is not addressed here.
+- **Routing/membership complexity.** The router must track each shard's current
+  leader, and a client must retry across leader changes per shard.
+
+## Alternatives considered
+
+- **Stay single-group (status quo).** Simplest and globally ordered, but the write
+  bottleneck and whole-state-on-every-node limit are exactly what this addresses.
+- **Two-phase commit / Percolator-style transactions.** Gives cross-shard
+  isolation (snapshot/serializable), but blocks on coordinator failure and holds
+  locks — rejected for the prototype in favor of availability; a real system might
+  layer a transaction protocol on top of the same per-shard groups later.
+- **Single big Raft group with parallel apply.** Doesn't help: the bottleneck is
+  *ordering/commit* at one leader, not apply.
+- **Hash- vs range-sharding.** Hash spreads load evenly but kills range scans;
+  range keeps locality but risks hotspots. The prototype uses a simple,
+  configurable map and treats the choice as orthogonal.
+
+## Prototype scope (tracked under ADR-0019 milestone M10)
+
+A runtime-level prototype: a deterministic `ShardRouter` over N independent
+in-process groups, a `Saga` coordinator with compensations, and a demonstrated
+cross-shard account transfer (debit on shard A, credit on shard B, with
+compensation when the credit leg fails). Resharding, range scans, and a global
+cross-shard audit root are explicitly out of scope.
diff --git a/docs/adr/0021-pluggable-bft-consensus-seam.md b/docs/adr/0021-pluggable-bft-consensus-seam.md
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+# 0021. Pluggable BFT consensus — the seam, and why it is not built
+
+- Status: Proposed
+- Date: 2026-06-21
+
+## Context
+
+ADR-0018 established that this framework is **Crash-Fault-Tolerant (CFT)**, not
+**Byzantine-Fault-Tolerant (BFT)**: Raft assumes peers are honest-but-may-crash.
+ADR-0019 (pillar 7) listed an "optional BFT consensus swap behind the existing
+consensus seam" as a way to extend the system to **multi-party trustlessness**
+(mutually-distrusting operators) when a single trust domain is not enough. M7
+(signed commands) already added per-actor accountability, but it does not make
+the *ordering* Byzantine-tolerant — a malicious leader can still equivocate
+(propose different logs to different followers) or censor, and CFT consensus does
+not defend against that.
+
+This ADR honestly assesses what a BFT swap would actually require on this
+codebase, and records the decision **not** to build it in the prototype.
+
+## The honest state of the "seam"
+
+ADR-0019 implies BFT could drop in "behind the existing consensus seam." On
+inspection that is optimistic. The current seam is the `Transport` interface
+(`src/consensus/transport.ts`), whose methods are **Raft-specific**:
+`sendRequestVote`, `sendAppendEntries`, `sendInstallSnapshot`. A BFT protocol
+(PBFT, Tendermint) does not use those messages at all — it uses a different,
+multi-phase exchange (e.g. pre-prepare → prepare → commit, plus view-change),
+needs `3f+1` members to tolerate `f` Byzantine faults (vs Raft's `2f+1` for `f`
+crashes), and requires every consensus message to be **signed** and cross-checked
+so no participant can be taken at its word.
+
+So the real seam is not `Transport` but the boundary *above the log*: the
+`RaftNode` API the rest of the system depends on — `submit(command) → committed,
+ordered, applied result`, plus leadership/term/membership status. Everything in
+`src/runtime/` (M1–M10) and the HTTP layer depends only on that
+commit-ordered-log contract, **not** on Raft internals. That contract is the true
+pluggability point.
+
+## Decision
+
+**Do not implement BFT in the prototype.** Instead:
+
+1. **Name the real seam.** Define (in documentation, and as a refactoring target)
+   a `Consensus` interface capturing exactly what the runtime needs: `submit`,
+   `readBarrier`, membership, and a leadership/status view — the subset of
+   `RaftNode` that `ReplicatedStateMachine` and the controllers actually use. A
+   BFT engine would implement *that*, not the Raft `Transport`.
+2. **Reuse M7 as a building block.** BFT requires signed, non-repudiable messages;
+   the ed25519 signing + `KeyRegistry` from M7 is the cryptographic primitive a
+   BFT layer would build on (signing *consensus votes*, not just client commands).
+3. **Keep determinism as the invariant that ports unchanged.** The deterministic
+   state machine, the Merkle audit (M3), the canonical serialization (M5), and the
+   leader-resolved-seed discipline are all consensus-agnostic — they work
+   identically under BFT, because they only assume a committed, totally-ordered
+   command log. This is the payoff of the core/edge split (ADR-0019): the *edge*
+   and *core* do not change when the *ordering* protocol does.
+4. **Treat BFT as a distinct project** gated on a real multi-party requirement,
+   not a milestone of this prototype.
+
+## Consequences
+
+### Positive
+
+- Honest scoping: we do not ship a fake or half-correct BFT (a subtly-wrong BFT is
+  worse than none — it gives false trust-minimization guarantees).
+- The work that *does* port — determinism, audit, signing, the core/edge split —
+  is already done and validated, so a future BFT effort starts from a clean
+  log-contract boundary rather than a rewrite of the runtime.
+- Documents a real correction to ADR-0019's "swap behind the existing seam"
+  framing, so future readers aren't misled into thinking it's a `Transport` swap.
+
+### Negative
+
+- The system remains single-trust-domain (CFT) — unsuitable for adversarial,
+  multi-organization deployments, exactly as ADR-0018 concluded.
+- The application boundary *below* the log — the `StateMachine` interface — is now
+  extracted (ADR-0017), so applications already plug in cleanly. But the ordering
+  boundary *above* the log (a `Consensus` interface a BFT engine would implement)
+  is still not extracted: `RaftNode` is referenced directly (e.g. `ShardRouter`,
+  controllers), so a real BFT swap would first require that second refactor.
+
+## Alternatives considered
+
+- **Build a minimal PBFT/Tendermint engine now.** Rejected: thousands of lines,
+  a different protocol with view-changes and `3f+1` quorums, and high risk of
+  subtle safety bugs — not a prototype increment, and dangerous to ship half-done.
+- **Claim the `Transport` interface is the BFT seam (as ADR-0019 implied).**
+  Rejected as inaccurate: BFT does not use Raft's RPCs; pretending otherwise would
+  mislead.
+- **Adopt an external BFT platform (Tendermint/CometBFT, HotStuff) and run the
+  deterministic state machine on top via ABCI.** The most realistic path to actual
+  BFT — the state machine, audit, and signing port directly onto an ABCI-style
+  app interface. Recorded as the recommended direction *if* multi-party
+  trustlessness is ever required; deliberately out of scope here.
diff --git a/docs/adr/README.md b/docs/adr/README.md
index 7daf7e5..54b2e98 100644
--- a/docs/adr/README.md
+++ b/docs/adr/README.md
@@ -30,6 +30,10 @@ supersedes the old one rather than editing history.
 | [0015](./0015-dynamic-cluster-membership.md) | Dynamic cluster membership via single-server changes | Accepted |
 | [0016](./0016-crash-consistent-snapshot-persistence.md) | Crash-consistent snapshot/log persistence and snapshot transfer | Accepted |
 | [0017](./0017-generic-pluggable-state-machine.md) | Generic pluggable state machine (framework core) | Accepted |
+| [0018](./0018-application-logic-as-smart-contracts.md) | Deploying application logic as smart contracts on this framework | Proposed |
+| [0019](./0019-native-backend-blockchain-benefits.md) | Bringing blockchain benefits to backend development natively | Proposed |
+| [0020](./0020-multi-raft-sharding.md) | Multi-Raft sharding for write scaling | Proposed |
+| [0021](./0021-pluggable-bft-consensus-seam.md) | Pluggable BFT consensus — the seam, and why it is not built | Proposed |
 
 ## Template
 
diff --git a/src/index.ts b/src/index.ts
index 75dbf8e..87461a0 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -40,3 +40,39 @@ export { isControlCommand, isConfigCommand } from './consensus/types';
 export { MetricsRegistry, metrics } from './platform/metrics';
 export { createLogger } from './platform/logger';
 export type { Logger } from './platform/logger';
+
+// ---- Module runtime (ADR-0019): an opinionated application you can plug in via
+// the StateMachine seam above, instead of writing one by hand. Write business
+// logic as pure module reducers and inherit determinism enforcement, an
+// exactly-once effect outbox, a Merkle audit, signed commands, a keyed store,
+// CQRS read projections, and cross-shard sagas. See src/runtime/README.md.
+export { ModuleStateMachine } from './runtime/moduleStateMachine';
+export { ModuleHost } from './runtime/moduleHost';
+export { defineModule } from './runtime/defineModule';
+export { defineKeyedModule } from './runtime/keyedModule';
+export type { KeyedModuleDefinition, KeyedReducer } from './runtime/keyedModule';
+export { buildModuleCommand, buildSignedModuleCommand } from './runtime/command';
+export { resolveSeed, createContext } from './runtime/context';
+export { EffectExecutor } from './runtime/effectExecutor';
+export { defineProjection } from './runtime/projection';
+export { ProjectionHost } from './runtime/projectionHost';
+export { MemoryStateStore, StoreView } from './runtime/stateStore';
+export { MerkleAudit, EMPTY_ROOT } from './runtime/merkleAudit';
+export { generateActorKeypair, signCommand, verifyCommand, KeyRegistry } from './runtime/signing';
+export { ShardRouter, NoShardLeaderError } from './runtime/shardRouter';
+export { runSaga } from './runtime/saga';
+export type {
+    ModuleDefinition,
+    ModuleAppCommand,
+    ModuleCommand,
+    ModuleApplyResult,
+    Reducer,
+    ReducerContext,
+    EffectIntent,
+    EffectHandler,
+    Seed,
+} from './runtime/types';
+export type { StateStore } from './runtime/stateStore';
+export type { ProjectionDefinition, ProjectionEvent } from './runtime/projection';
+export type { SagaStep, SagaResult } from './runtime/saga';
+export type { ShardHandle, ShardNode } from './runtime/shardRouter';
diff --git a/src/runtime/README.md b/src/runtime/README.md
new file mode 100644
index 0000000..6e5805e
--- /dev/null
+++ b/src/runtime/README.md
@@ -0,0 +1,101 @@
+# Runtime — ADR-0019 prototype
+
+A prototype of the **deterministic core + effectful edge** design from
+[ADR-0019](../../docs/adr/0019-native-backend-blockchain-benefits.md): a way to
+write ordinary-looking backend modules that nonetheless inherit the blockchain
+benefits of the consensus substrate — deterministic replicated execution,
+exactly-once effects, and a provable tamper-evident audit.
+
+This layer plugs into the consensus core through the pluggable `StateMachine`
+seam (ADR-0017): a `ModuleStateMachine` adapts the `ModuleHost` to the framework
+contract, so the whole runtime rides real Raft as an ordinary application — no
+bespoke consensus-core support. The book service is just *another* `StateMachine`;
+this is the recommended one.
+
+## The model
+
+```
+  EDGE (effectful)          CORE (deterministic)          EDGE (effectful)
+  resolveSeed(),       →    ModuleHost.apply():       →   EffectExecutor.drain():
+  oracle reads,             pure reducers over             runs handlers once,
+  validation                replicated state +             feeds results back as
+                            outbox + Merkle audit          committed entries
+```
+
+Non-determinism (time, randomness, ids, external data) is resolved **once on the
+edge** and baked into the committed command/seed, so every replica applies the
+exact same values — the same discipline `src/models/book.ts` already uses for the
+book demo.
+
+## Pieces
+
+| File | Role | Milestone |
+|------|------|-----------|
+| `types.ts` | `ModuleDefinition`, `Reducer`, `ReducerContext`, `EffectIntent`, `Seed`, audit/outbox/host-option types | M1–M5 |
+| `defineModule.ts` | Declarative module definition + validation (rejects `__`-reserved names; strict determinism lint) | M1, M5 |
+| `context.ts` | `resolveSeed()` (the sole edge entropy site) and the deterministic `ReducerContext` | M1 |
+| `moduleHost.ts` | `ModuleHost`: dispatch to pure reducers, queries, outbox, Merkle audit, resource bounds, signature check, snapshot/restore | M1–M7 |
+| `effectExecutor.ts` | Post-commit executor: runs effect handlers exactly-once and submits results back | M2 |
+| `merkleAudit.ts` | Append-only Merkle accumulator: inclusion proofs + domain-separated root | M3 |
+| `codeHash.ts` | Deterministic module code-version hash (POC stand-in for hashing the built artifact) | M3 |
+| `canonical.ts` | Single shared canonical (sorted-key) JSON serializer used by every hash/size path | M5 |
+| `determinism.ts` | Static determinism lint + `canonicalBytes` for the deterministic resource bound | M5 |
+| `moduleStateMachine.ts` | Adapts `ModuleHost` to the framework `StateMachine` seam (ADR-0017) — plug it into a `RaftNode` | M4 |
+| `command.ts` | Leader-side `buildModuleCommand` / `buildSignedModuleCommand` (resolve seed, optionally sign) | M4, M7 |
+| `signing.ts` | ed25519 sign/verify over the logical payload + `KeyRegistry` (actor→authorized key) | M7 |
+| `projection.ts` / `projectionHost.ts` | CQRS read side: derived, rebuildable read model off the apply path | M6 |
+| `stateStore.ts` | `StateStore` interface + in-memory `MemoryStateStore` + copy-on-write `StoreView` (the seam for a persistent KV) | M8 |
+| `keyedModule.ts` | `defineKeyedModule` / `KeyedReducer` — record-addressed modules that touch only the keys they need | M8 |
+| `sandbox.ts` | `vm` determinism sandbox (enforce-by-removal) + leader-side step/CPU meter (`admit`) | M9 |
+| `shardRouter.ts` / `saga.ts` | Multi-Raft sharding: deterministic shard router over independent groups + cross-shard saga (try/compensate) | M10 |
+| `modules/` | Demo modules: `counter`, `notes` (leader-resolved id/time via `ctx`), `payments` (effect → settle), `accounts` (keyed), `compute` (sandboxed) | M1–M2, M8–M9 |
+| `projections/` | Demo projection: `noteIndex` (notes indexed by actor) | M6 |
+
+The runtime rides **real consensus** (M4): a `ModuleAppCommand` is an ordinary
+application command, and `ModuleStateMachine` is the application a `RaftNode`
+replicates — so a multi-node cluster converges on identical module state and
+audit root with no special core support. See
+`tests/runtime/consensusIntegration.test.ts` and `tests/runtime/signing.test.ts`
+for the end-to-end proofs.
+
+## Writing a module
+
+```ts
+import { defineModule } from '../defineModule';
+
+export const counter = defineModule<{ value: number }>({
+    name: 'counter',
+    version: '1',
+    initialState: () => ({ value: 0 }),
+    commands: {
+        // Pure: state in, { state, result?, effects? } out. ctx is the only
+        // source of "now"/randomness/ids, all derived from the leader's seed.
+        increment: (state, input: any, _ctx) => ({
+            state: { value: state.value + (input?.by ?? 1) },
+        }),
+    },
+    queries: {
+        value: (state) => state.value,
+    },
+});
+```
+
+## Invariants (do not break)
+
+- **Reducers are pure.** Never read `Date`/`Math.random`/`crypto`/network inside a
+  reducer — use `ctx.now` / `ctx.random()` / `ctx.id()`. Divergence is silent and
+  severe (same rule as ADR-0003 for the core state machine).
+- **Effects are described, not performed, in reducers.** Return `EffectIntent`s;
+  the `EffectExecutor` performs them after commit and feeds results back as
+  committed entries.
+- **Everything hashed uses canonical (sorted-key) serialization** so replicas
+  compute identical audit roots.
+
+## Tests
+
+```bash
+LOG_SILENT=true npx jest tests/runtime
+```
+
+Covers determinism/convergence, snapshot-restore fidelity, exactly-once effects,
+and Merkle proof/verify + audit convergence.
diff --git a/src/runtime/canonical.ts b/src/runtime/canonical.ts
new file mode 100644
index 0000000..6ec4217
--- /dev/null
+++ b/src/runtime/canonical.ts
@@ -0,0 +1,91 @@
+/**
+ * Single shared canonical-JSON serializer (ADR-0019 runtime).
+ *
+ * Canonical JSON means: stringify with object keys recursively SORTED, so two
+ * logically-equal values serialize to the SAME bytes regardless of property
+ * insertion order. This is the determinism linchpin for both consumers below —
+ * without sorted keys, two replicas building the same object with different key
+ * order would produce different bytes and diverge.
+ *
+ * WHY ONE MODULE: there were previously two near-identical copies of this logic
+ * (one for audit-leaf hash preimages in `merkleAudit.ts`, one for the size bound
+ * in `determinism.ts`). Two copies of a determinism-critical serializer are a
+ * drift hazard — a fix or feature applied to one but not the other could make a
+ * leaf hash and a size measurement disagree about the same value. This module is
+ * the single source of truth; the only sanctioned variation is how `undefined`
+ * is handled, expressed as an explicit option (see below).
+ *
+ * THE TWO `undefined` MODES (and why they differ):
+ *  - `'throw'` (default) — for HASH PREIMAGES (audit leaves). A hash preimage
+ *    must be LOSSLESS and unambiguous: bare `JSON.stringify(undefined)` yields
+ *    the value `undefined` (not a string), and a property whose value is
+ *    `undefined` is silently dropped. Either would corrupt a leaf hash or make
+ *    it ambiguous, so we refuse `undefined` loudly rather than hash garbage.
+ *  - `'drop'` — for the SIZE bound. A byte-length measurement is not a preimage,
+ *    so it can be lenient: a property whose value is `undefined` is dropped and
+ *    an `undefined` array element is normalized to `null`, exactly mirroring
+ *    `JSON.stringify`. This never throws on the state shapes reducers return.
+ */
+
+/** Controls how a `undefined` value is treated during serialization. */
+export interface CanonicalOptions {
+    /**
+     * `'throw'` (default): reject `undefined` anywhere — required for lossless
+     * hash preimages. `'drop'`: omit `undefined` object properties and normalize
+     * `undefined` array elements to `null`, matching `JSON.stringify` — used for
+     * the lenient size bound.
+     */
+    onUndefined?: 'throw' | 'drop';
+}
+
+/**
+ * Serialize `value` to canonical (recursively sorted-key) JSON.
+ *
+ * The byte output is identical to the previous per-module serializers for each
+ * caller's chosen `onUndefined` mode — `'throw'` reproduces `merkleAudit.ts`'s
+ * old `canonical()` exactly (so audit-leaf hashes and the Merkle root are
+ * unchanged), and `'drop'` reproduces `determinism.ts`'s old `canonicalJson()`.
+ */
+export function canonicalJson(value: unknown, opts: CanonicalOptions = {}): string {
+    const onUndefined = opts.onUndefined ?? 'throw';
+
+    if (value === undefined) {
+        if (onUndefined === 'throw') {
+            throw new Error('canonical: undefined is not serializable (would corrupt the hash preimage)');
+        }
+        // 'drop' mode only reaches here for a top-level/array `undefined`; array
+        // callers normalize to null below, so this serves the top-level case.
+        return JSON.stringify(null);
+    }
+
+    if (value === null || typeof value !== 'object') {
+        return JSON.stringify(value);
+    }
+
+    if (Array.isArray(value)) {
+        return `[${value
+            .map((v) => {
+                // In 'drop' mode, `undefined` array holes/elements become `null`
+                // exactly as `JSON.stringify` would; in 'throw' mode the recursive
+                // call rejects them.
+                if (v === undefined && onUndefined === 'drop') {
+                    return 'null';
+                }
+                return canonicalJson(v, opts);
+            })
+            .join(',')}]`;
+    }
+
+    const obj = value as Record<string, unknown>;
+    const keys = Object.keys(obj).sort();
+    const parts: string[] = [];
+    for (const k of keys) {
+        // In 'drop' mode, skip `undefined`-valued properties (matching
+        // `JSON.stringify`); in 'throw' mode the recursive call rejects them.
+        if (obj[k] === undefined && onUndefined === 'drop') {
+            continue;
+        }
+        parts.push(`${JSON.stringify(k)}:${canonicalJson(obj[k], opts)}`);
+    }
+    return `{${parts.join(',')}}`;
+}
diff --git a/src/runtime/codeHash.ts b/src/runtime/codeHash.ts
new file mode 100644
index 0000000..ab87b8a
--- /dev/null
+++ b/src/runtime/codeHash.ts
@@ -0,0 +1,61 @@
+import { createHash } from 'crypto';
+
+/**
+ * Module code-version hashing (ADR-0019 pillar 5). The audit records a hash of
+ * the LOGIC that produced each result, so the history proves not just *what* the
+ * data became but *which version of the code* computed it — closing ADR-0018's
+ * "tamper-evident data but not logic" gap.
+ *
+ * PROTOTYPE CAVEAT (stated honestly): `fn.toString()` is a STAND-IN for hashing
+ * the built/deployed artifact. It captures a reducer's source text, which is
+ * enough to detect that the logic changed (a different body or a bumped
+ * `version` yields a different hash) and to bind that change into the audit. It
+ * is NOT a production code-provenance mechanism: source text omits captured
+ * closure variables and imported helpers, can be defeated by semantically-equal
+ * rewrites, and is sensitive to incidental formatting. A real deployment would
+ * hash the immutable build output recorded in a `DEPLOY` log entry. For this
+ * POC, source-text identity is sufficient to demonstrate the audit captures
+ * which logic version ran.
+ *
+ * DETERMINISM: command names are sorted and the material is assembled with no
+ * clock/randomness, so every replica computes the identical code hash for a
+ * given module definition.
+ */
+
+/** sha256 hex of a string. */
+function sha256(s: string): string {
+    return createHash('sha256').update(s).digest('hex');
+}
+
+/**
+ * The minimal structural shape `moduleCodeHash` needs. Both whole-state
+ * `ModuleDefinition` and `KeyedModuleDefinition` satisfy it (each has a `name`,
+ * an optional `version`, and a `commands` record of functions), so the hash
+ * accepts the heterogeneous registry's union without an `as any` cast at the
+ * call site — it only reads `name`/`version`/the reducers' source text.
+ */
+interface HashableModule {
+    name: string;
+    version?: string;
+    commands: Record<string, (...args: never[]) => unknown>;
+}
+
+/**
+ * Deterministic identity hash of a module's logic. Built from the module name,
+ * its `version` (defaulting to `'0'` when unset), and each command name paired
+ * with its reducer's source text, with command names SORTED for a stable order.
+ */
+export function moduleCodeHash(def: HashableModule): string {
+    const commandNames = Object.keys(def.commands).sort();
+    const commandMaterial = commandNames
+        // `JSON.stringify` on the name + body keeps the delimiter unambiguous so
+        // distinct (name, body) pairs cannot alias into the same concatenation.
+        .map((name) => `${JSON.stringify(name)}:${JSON.stringify(def.commands[name].toString())}`)
+        .join(',');
+    const material = [
+        `name:${JSON.stringify(def.name)}`,
+        `version:${JSON.stringify(def.version ?? '0')}`,
+        `commands:[${commandMaterial}]`,
+    ].join('|');
+    return sha256(material);
+}
diff --git a/src/runtime/command.ts b/src/runtime/command.ts
new file mode 100644
index 0000000..29fa343
--- /dev/null
+++ b/src/runtime/command.ts
@@ -0,0 +1,75 @@
+import { resolveSeed } from './context';
+import { signCommand } from './signing';
+import { ModuleAppCommand } from './types';
+
+/**
+ * LEADER-SIDE builder for a module application command (ADR-0019). This is where
+ * the runtime's non-determinism is resolved up front: {@link resolveSeed}
+ * captures the ambient clock + a fresh random nonce ONCE on the leader, and that
+ * seed is baked into the command before it enters the replicated log. Every
+ * replica then derives the identical `ReducerContext` from the committed seed, so
+ * module reducers stay pure and the cluster converges — exactly the discipline
+ * `src/models/book.ts` uses to resolve ids/timestamps for the book demo,
+ * generalized to the module runtime.
+ *
+ * The returned command is an ordinary `AppCommand` (type `'MODULE'`): it plugs
+ * into a `RaftNode` whose state machine is a {@link ModuleStateMachine}, with no
+ * bespoke consensus-core support. `actor`/`requestId` are carried on the command
+ * (the adapter has no separate meta) and MUST match the `CommandMeta` it is
+ * submitted with. Followers/replicas never call this; they apply the committed
+ * command as-is.
+ */
+export function buildModuleCommand(
+    module: string,
+    command: string,
+    input: unknown,
+    meta: { actor: string; requestId: string },
+): ModuleAppCommand {
+    return {
+        type: 'MODULE',
+        module,
+        command,
+        input,
+        seed: resolveSeed(),
+        actor: meta.actor,
+        requestId: meta.requestId,
+    };
+}
+
+/**
+ * Build a SIGNED module command (ADR-0019 pillar 7). The originating actor signs
+ * the LOGICAL command — `{ module, command, input, actor, requestId }` — with its
+ * private key BEFORE this reaches the leader; the leader then resolves the
+ * deterministic `seed` and forwards the signature unchanged.
+ *
+ * The `seed` is INTENTIONALLY excluded from the signed payload: the leader picks
+ * it after the actor signs, so the actor cannot have signed over it, and (being
+ * a non-security convergence value) it does not need to be authenticated. On
+ * apply, every node recomputes this same logical payload from the committed
+ * command and verifies the signature against the actor's registered key, so a
+ * leader that forged `actor` could not have produced a matching signature.
+ */
+export function buildSignedModuleCommand(
+    module: string,
+    command: string,
+    input: unknown,
+    opts: { actor: string; requestId: string; privateKeyPem: string },
+): ModuleAppCommand {
+    const sig = signCommand(opts.privateKeyPem, {
+        module,
+        command,
+        input,
+        actor: opts.actor,
+        requestId: opts.requestId,
+    });
+    return {
+        type: 'MODULE',
+        module,
+        command,
+        input,
+        seed: resolveSeed(),
+        actor: opts.actor,
+        requestId: opts.requestId,
+        sig,
+    };
+}
diff --git a/src/runtime/context.ts b/src/runtime/context.ts
new file mode 100644
index 0000000..8340032
--- /dev/null
+++ b/src/runtime/context.ts
@@ -0,0 +1,69 @@
+import { createHash, randomBytes } from 'crypto';
+import { ReducerContext, Seed } from './types';
+
+/**
+ * Seed resolution and deterministic context construction (ADR-0019 pillar 2).
+ *
+ * `resolveSeed` is the ONLY place real non-determinism enters; it runs on the
+ * leader. `createContext` then rebuilds a fully deterministic `ReducerContext`
+ * from that seed, so any replica (or a fresh host during replay) reconstructs
+ * the identical clock/PRNG/id stream and converges byte-for-byte.
+ */
+
+/**
+ * LEADER-SIDE ONLY. Capture the ambient clock and a fresh random nonce, exactly
+ * as `models/book.ts` resolves ids/timestamps up front before a command enters
+ * the log. The resulting seed is replicated verbatim; replicas never call this.
+ */
+export function resolveSeed(): Seed {
+    return {
+        timestamp: new Date().toISOString(),
+        nonce: randomBytes(16).toString('hex'),
+    };
+}
+
+/** Derive a 32-bit unsigned seed integer from the nonce via sha256. */
+function seedStateFromNonce(nonce: string): number {
+    const digest = createHash('sha256').update(nonce).digest();
+    // Take the first 4 bytes as a big-endian uint32. `>>> 0` forces unsigned.
+    return digest.readUInt32BE(0) >>> 0;
+}
+
+/**
+ * Build a deterministic context from a seed. Every capability below is a pure
+ * function of `seed` (plus per-context call counters), so the same seed always
+ * yields the same `now`/`random()` stream/`id()` stream.
+ */
+export function createContext(seed: Seed, meta: { actor: string; requestId: string }): ReducerContext {
+    // mulberry32: a tiny, fast, well-distributed PRNG. Deterministic given its
+    // 32-bit state, which we derive from the nonce. We keep `randomState` in a
+    // closure so successive `random()` calls advance the same stream.
+    let randomState = seedStateFromNonce(seed.nonce);
+    const random = (): number => {
+        // mulberry32 step.
+        randomState = (randomState + 0x6d2b79f5) >>> 0;
+        let t = randomState;
+        t = Math.imul(t ^ (t >>> 15), t | 1);
+        t ^= t + Math.imul(t ^ (t >>> 7), t | 61);
+        return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
+    };
+
+    // Deterministic id stream: sha256(nonce:counter) hex, truncated to 32 chars.
+    // The counter is independent of the PRNG so the two streams don't interfere.
+    let idCounter = 0;
+    const id = (): string => {
+        const hex = createHash('sha256')
+            .update(`${seed.nonce}:${idCounter}`)
+            .digest('hex');
+        idCounter += 1;
+        return hex.slice(0, 32);
+    };
+
+    return {
+        now: seed.timestamp,
+        random,
+        id,
+        actor: meta.actor,
+        requestId: meta.requestId,
+    };
+}
diff --git a/src/runtime/defineModule.ts b/src/runtime/defineModule.ts
new file mode 100644
index 0000000..13a0c92
--- /dev/null
+++ b/src/runtime/defineModule.ts
@@ -0,0 +1,128 @@
+import { lintReducer } from './determinism';
+import { ModuleDefinition } from './types';
+
+/**
+ * Options for {@link defineModule}.
+ */
+export interface DefineModuleOptions {
+    /**
+     * Enforce the determinism lint at definition time (ADR-0019 pillar 2).
+     * Defaults to TRUE: a reducer that references a non-deterministic global
+     * (`Date.now`, `Math.random`, `crypto`, network, timers, …) is REJECTED with
+     * a thrown error before the module can ever be registered.
+     *
+     * OPT-OUT: pass `{ strict: false }` for a vetted reducer whose flagged token
+     * is a false positive (e.g. a banned word appearing only in a string/comment)
+     * or that has been audited by other means. Opting out does not silence the
+     * lint — the violations are attached to the returned definition under the
+     * reserved `__lint` field for inspection/logging — it only skips the throw.
+     */
+    strict?: boolean;
+    /**
+     * Opt into the `vm` determinism sandbox (ADR-0019 pillars 2, 6 — M9). When
+     * set here it is stamped onto the returned definition so the host compiles the
+     * reducers into a frozen `vm` context at registration. Additive and
+     * default-off; the static lint stays on as defense-in-depth (a sandboxed
+     * reducer is self-contained and still passes the lint). See `sandbox.ts` for
+     * the honest "not a security boundary" scoping. WHOLE-STATE modules only.
+     */
+    sandbox?: boolean;
+    /**
+     * Wall-clock budget (ms) for the leader-side admission step meter. Only
+     * meaningful with `sandbox: true`. Stamped onto the returned definition.
+     * Default applied by the host: 50.
+     */
+    stepBudgetMs?: number;
+}
+
+/**
+ * A module definition that failed the determinism lint with `strict: false`.
+ * The violations are recorded under the reserved `__lint` field so an operator
+ * can still surface them, even though registration was allowed to proceed.
+ */
+export type LintedModuleDefinition<S> = ModuleDefinition<S> & { __lint?: string[] };
+
+/**
+ * Validate and return a module definition (ADR-0019 pillar 1: the single
+ * declarative unit that replaces the four-touchpoint command workflow).
+ *
+ * Validation happens here, at definition time, so a malformed module fails loud
+ * on startup rather than surfacing as a confusing dispatch error later. The
+ * function is otherwise an identity — it returns the same object, typed — which
+ * keeps authoring a module a single `defineModule({ ... })` call.
+ *
+ * Determinism lint (ADR-0019 pillar 2): every command reducer is statically
+ * linted for non-deterministic globals. With `strict` (the default), any
+ * violation THROWS, turning the "reducers must be pure" convention into an
+ * enforced guarantee. With `{ strict: false }` the violations are recorded on
+ * the returned definition's `__lint` field instead of throwing — the documented
+ * escape hatch for a vetted reducer.
+ */
+export function defineModule<S>(
+    rawDef: ModuleDefinition<S>,
+    opts: DefineModuleOptions = {},
+): LintedModuleDefinition<S> {
+    const strict = opts.strict ?? true;
+    // Stamp the sandbox opt-in (ADR-0019 M9) from the options onto the definition
+    // so the host sees it at registration. A field set directly on `def` wins over
+    // the option (explicit on the definition is the more specific intent); options
+    // are the ergonomic place to pass it alongside `strict`.
+    const def: ModuleDefinition<S> = { ...rawDef };
+    const sandbox = rawDef.sandbox ?? opts.sandbox;
+    if (sandbox !== undefined) def.sandbox = sandbox;
+    const stepBudgetMs = rawDef.stepBudgetMs ?? opts.stepBudgetMs;
+    if (stepBudgetMs !== undefined) def.stepBudgetMs = stepBudgetMs;
+    if (!def.name || def.name.trim() === '') {
+        throw new Error('Module definition requires a non-empty name');
+    }
+
+    // Names starting with `__` are reserved for runtime internals — the snapshot
+    // stores the outbox under the reserved `__outbox` key in the same flat
+    // module-states object, so a `__`-prefixed module would silently collide.
+    // Fail closed at definition time rather than corrupt a snapshot later.
+    if (def.name.startsWith('__')) {
+        throw new Error(
+            `Module name "${def.name}" is reserved: names starting with "__" are reserved for runtime internals`,
+        );
+    }
+
+    const commandNames = Object.keys(def.commands ?? {});
+    if (commandNames.length === 0) {
+        throw new Error(`Module "${def.name}" must define at least one command`);
+    }
+
+    // Validate names exactly as they will be dispatched (untrimmed keys are what
+    // `moduleHost.apply` looks up), so validation can't diverge from dispatch.
+    // `Object.keys` already collapses duplicate literal keys, so only the
+    // empty-name case needs rejecting here; an empty name makes dispatch ambiguous.
+    for (const name of commandNames) {
+        if (name === '') {
+            throw new Error(`Module "${def.name}" has a command with an empty name`);
+        }
+    }
+
+    // Determinism lint (ADR-0019 pillar 2): scan every reducer's source for
+    // non-deterministic globals. This is the always-on static stand-in for the
+    // deferred vm/worker sandbox — it catches the common footgun at definition
+    // time, before a single command can be applied.
+    const violations: string[] = [];
+    for (const name of commandNames) {
+        violations.push(...lintReducer(name, def.commands[name]));
+    }
+
+    if (violations.length > 0) {
+        if (strict) {
+            throw new Error(
+                `Module "${def.name}" failed the determinism lint (ADR-0019 pillar 2):\n` +
+                    violations.map((v) => `  - ${v}`).join('\n') +
+                    `\nReducers must be pure: use ctx.now / ctx.random() / ctx.id() instead of ambient ` +
+                    `globals, or pass { strict: false } to defineModule for a vetted reducer.`,
+            );
+        }
+        // strict: false — record the violations rather than throw, so a vetted
+        // module can opt out while the findings remain inspectable.
+        return Object.assign({}, def, { __lint: violations });
+    }
+
+    return def;
+}
diff --git a/src/runtime/determinism.ts b/src/runtime/determinism.ts
new file mode 100644
index 0000000..c95734a
--- /dev/null
+++ b/src/runtime/determinism.ts
@@ -0,0 +1,127 @@
+/**
+ * Determinism lint + size accounting (ADR-0019 pillar 2 "determinism as a
+ * guarantee, not a convention", and pillar 6 "resource safety / gas").
+ *
+ * Pillar 2 calls for turning the *convention* "reducers must be pure" into an
+ * enforced *guarantee*. The ideal enforcement is a `vm`/worker sandbox with
+ * frozen globals; that needs a separate execution context and is explicitly
+ * DEFERRED (see the ADR's prototype-status note). This module is the cheap,
+ * always-on first line of defence instead: a STATIC lint that inspects a
+ * reducer's source for the common non-determinism footguns and rejects them at
+ * registration, before any command is ever applied.
+ *
+ * Pillar 6 wants a deterministic resource bound. `canonicalBytes` below is the
+ * size-accounting primitive the host uses to cap result/state amplification —
+ * every replica computes the identical byte length and rejects identically, so
+ * the bound never causes divergence (unlike a wall-clock CPU meter would).
+ */
+
+import { canonicalJson } from './canonical';
+
+/**
+ * Non-deterministic access patterns to detect in reducer source.
+ *
+ * Each entry is a targeted regex chosen to match the ACCESS to a banned global,
+ * not an innocent identifier that merely shares a name. For example we match
+ * `Math.random` (the property access), not a local variable or object field
+ * literally named `random` — so a module that has a `random` field in its state
+ * is not flagged. `ctx.now` / `ctx.id()` / `ctx.random()` are the SANCTIONED
+ * deterministic substitutes and go through `ctx`, never these globals, so the
+ * demo modules (counter/notes/payments) pass cleanly.
+ *
+ * This list is the documented footgun set, not an exhaustive proof of purity.
+ */
+export const BANNED: { pattern: RegExp; reason: string }[] = [
+    // Clock + randomness: the two classic determinism breakers.
+    { pattern: /\bMath\s*\.\s*random\b/, reason: 'Math.random (use ctx.random())' },
+    { pattern: /\bDate\s*\.\s*now\b/, reason: 'Date.now (use ctx.now)' },
+    // Any `new Date(` or bare `Date(` is suspicious: even `new Date(arg)` is a
+    // footgun magnet, and we cannot statically prove the arg is deterministic.
+    // Authors should derive time from ctx.now instead of constructing Dates.
+    { pattern: /\bnew\s+Date\s*\(/, reason: 'new Date( (use ctx.now)' },
+    { pattern: /(?<!\.)\bDate\s*\(/, reason: 'Date( call (use ctx.now)' },
+    // Ambient crypto / entropy. Match ACCESS patterns, not the bare identifier:
+    // a property access (`crypto.`), the global (`globalThis.crypto`), and a
+    // direct require — never a legitimate local named `crypto` (or `cryptoKey` /
+    // `cryptocurrency`). Dynamic `require('crypto')` is also caught by the
+    // general `require(` rule below; these are the targeted crypto-specific hits.
+    { pattern: /\bcrypto\s*\./, reason: 'crypto. (non-deterministic; use ctx.id()/ctx.random())' },
+    {
+        pattern: /\brequire\s*\(\s*['"]crypto['"]\s*\)/,
+        reason: "require('crypto') (non-deterministic; use ctx.id()/ctx.random())",
+    },
+    { pattern: /\bglobalThis\s*\.\s*crypto\b/, reason: 'globalThis.crypto (non-deterministic; use ctx.id()/ctx.random())' },
+    // Dynamic code / module loading: opens an unbounded escape hatch.
+    { pattern: /\brequire\s*\(/, reason: 'require( (dynamic module load)' },
+    { pattern: /\bimport\s*\(/, reason: 'import( (dynamic module load)' },
+    // Ambient process / global object: env, argv, hrtime, etc.
+    { pattern: /\bprocess\s*\./, reason: 'process. (ambient environment)' },
+    { pattern: /\bglobalThis\b/, reason: 'globalThis (ambient global access)' },
+    // Network / IO: must go through an effect intent, never a reducer.
+    { pattern: /\bfetch\s*\(/, reason: 'fetch( (network I/O; emit an EffectIntent)' },
+    // Timers: wall-clock scheduling has no place on the deterministic path.
+    { pattern: /\bsetTimeout\b/, reason: 'setTimeout (wall-clock timer)' },
+    { pattern: /\bsetInterval\b/, reason: 'setInterval (wall-clock timer)' },
+    // High-resolution clock.
+    { pattern: /\bperformance\s*\./, reason: 'performance. (wall-clock timing)' },
+    // Reflection: a common way to reach banned globals indirectly.
+    { pattern: /\bReflect\s*\./, reason: 'Reflect. (reflective global access)' },
+    // Locale / ICU formatting & collation: results depend on the host locale and
+    // ICU build, so two replicas can format/compare differently from the same
+    // input. Match the METHOD access (`.toLocaleString`, `.localeCompare`, any
+    // `.toLocale*(`) and the `Intl` namespace access — never a bare identifier
+    // that merely shares a name. The sandbox neutralizes these structurally; this
+    // is the defense-in-depth lint for non-sandboxed modules.
+    { pattern: /\.\s*toLocaleString\b/, reason: '.toLocaleString (locale-dependent formatting; format deterministically)' },
+    { pattern: /\.\s*localeCompare\b/, reason: '.localeCompare (locale-dependent collation; compare deterministically)' },
+    { pattern: /\.\s*toLocale[A-Za-z]*\s*\(/, reason: '.toLocale*( (locale-dependent; non-deterministic)' },
+    { pattern: /\bIntl\s*\./, reason: 'Intl. (locale/ICU formatting; non-deterministic)' },
+];
+
+/**
+ * Statically lint a single reducer's source for non-deterministic globals,
+ * returning a list of human-readable violation strings (empty = clean).
+ *
+ * HONEST LIMITATION: this is a STATIC stand-in for a real sandbox. It operates
+ * on `fn.toString()` — the reducer's source text — so it is trivially
+ * bypassable by obfuscation or indirection (e.g. `globalThis['Ma'+'th']`,
+ * aliasing a global, or pulling entropy through a closed-over helper defined
+ * elsewhere), and it can produce false positives on source that merely mentions
+ * a banned token in a string/comment. It catches the COMMON footgun — a reducer
+ * author reflexively reaching for `Date.now()` or `Math.random()` — at zero
+ * runtime cost. It is NOT a security boundary; the real boundary (a `vm`/worker
+ * sandbox with frozen globals) is deferred per ADR-0019.
+ *
+ * SCOPE: this lint deliberately covers COMMAND REDUCERS only — they are the code
+ * on the convergence path, run on every replica for every command. It is NOT
+ * applied to `queries` (read-only and off the convergence path, so impurity
+ * there cannot diverge replicated state) nor to `initialState` (runs once at
+ * registration, not per command).
+ */
+export function lintReducer(commandName: string, fn: Function): string[] {
+    const source = fn.toString();
+    const violations: string[] = [];
+    for (const { pattern, reason } of BANNED) {
+        if (pattern.test(source)) {
+            violations.push(`command "${commandName}" references ${reason}`);
+        }
+    }
+    return violations;
+}
+
+/**
+ * Byte length of the canonical (sorted-key) JSON serialization of `value`. The
+ * size primitive behind the host's deterministic result/state bound: every
+ * replica computes the identical byte count from the identical state, so an
+ * over-budget reducer is rejected uniformly on every node — no divergence. Uses
+ * UTF-8 byte length (not string length) so multi-byte content is measured
+ * honestly.
+ *
+ * Uses the shared `canonicalJson` in DROP-`undefined` mode: this is a
+ * byte-length bound, not a hash preimage, so a property whose value is
+ * `undefined` is dropped exactly as `JSON.stringify` would — lenient on purpose,
+ * and it never throws on the state shapes reducers return.
+ */
+export function canonicalBytes(value: unknown): number {
+    return Buffer.byteLength(canonicalJson(value, { onUndefined: 'drop' }), 'utf8');
+}
diff --git a/src/runtime/effectExecutor.ts b/src/runtime/effectExecutor.ts
new file mode 100644
index 0000000..75311e3
--- /dev/null
+++ b/src/runtime/effectExecutor.ts
@@ -0,0 +1,78 @@
+import { resolveSeed } from './context';
+import { EffectHandler, EffectIntent, EffectResultEntry } from './types';
+
+/**
+ * The effectful EDGE of the core/edge split (ADR-0019 pillar 3). The deterministic
+ * core records `pending` effects in its outbox; this executor runs them post-commit
+ * — the ONE non-deterministic actor in the system — and feeds each outcome back as
+ * a committed `EffectResultEntry` that replicas apply identically.
+ *
+ * Exactly-once is a collaboration, not a single mechanism:
+ *  - HANDLER execution is at-least-once across executor restarts: a crash after the
+ *    side effect but before `submit` lands forces a retry on the next drain.
+ *  - COMMITTED STATE is exactly-once regardless, because (a) the outbox dedups
+ *    enqueue on `idempotencyKey`, (b) `applyEffectResult` is idempotent on that key,
+ *    and (c) the in-flight guard below stops a single executor double-running an
+ *    effect concurrently. Handlers should therefore be idempotent where the external
+ *    system allows, but the core stays correct even if a handler runs twice.
+ */
+export class EffectExecutor {
+    /**
+     * Keys currently being executed by THIS executor. Guards against a concurrent
+     * double-drain firing the same handler twice before the first `submit` has
+     * marked the outbox `done`. Cleared in a `finally` so a failed handler can be
+     * retried by a later drain.
+     */
+    private readonly inFlight = new Set<string>();
+
+    constructor(
+        private readonly handlers: Record<string, EffectHandler>,
+        private readonly submit: (entry: EffectResultEntry) => void | Promise<void>,
+    ) {}
+
+    /**
+     * Run every pending intent whose key is not already in flight. On success,
+     * resolve a `Seed` here on the edge (committed verbatim so any `onResult`
+     * reducer stays deterministic on every replica), package the handler's result
+     * into an `EffectResultEntry`, and `submit` it back
+     * into the log. On handler failure, leave the effect pending (do NOT submit) so a
+     * later drain retries it.
+     */
+    async drain(pending: EffectIntent[]): Promise<void> {
+        await Promise.all(pending.map((intent) => this.run(intent)));
+    }
+
+    private async run(intent: EffectIntent): Promise<void> {
+        const key = intent.idempotencyKey;
+        // In-flight guard: a second concurrent drain seeing the same pending key
+        // skips it rather than re-running the handler.
+        if (this.inFlight.has(key)) {
+            return;
+        }
+
+        const handler = this.handlers[intent.kind];
+        if (!handler) {
+            // No handler for this kind: leave it pending. A handler may be
+            // registered later; we never drop the intent or fabricate a result.
+            return;
+        }
+
+        this.inFlight.add(key);
+        try {
+            const result = await handler(intent);
+            // The seed is resolved once on the edge by the executor, then committed
+            // verbatim so every replica applies the same value (convergence comes
+            // from committing the value, not from where it was generated). That
+            // deterministic seed keeps any consuming `onResult` reducer's `ctx`
+            // identical on every replica.
+            const entry: EffectResultEntry = { idempotencyKey: key, result, seed: resolveSeed() };
+            await this.submit(entry);
+        } catch {
+            // Handler failed (e.g. network error): swallow and leave the effect
+            // pending so the next drain retries. Submitting nothing keeps the
+            // outbox entry `pending`, preserving at-least-once handler semantics.
+        } finally {
+            this.inFlight.delete(key);
+        }
+    }
+}
diff --git a/src/runtime/keyedModule.ts b/src/runtime/keyedModule.ts
new file mode 100644
index 0000000..6170fc3
--- /dev/null
+++ b/src/runtime/keyedModule.ts
@@ -0,0 +1,123 @@
+/**
+ * Keyed (key-oriented, transactional) module model (ADR-0019 pillar 4, the
+ * "state larger than RAM" half).
+ *
+ * A WHOLE-STATE module (`defineModule`) hands each reducer the entire module
+ * state and takes back the next whole state. A KEYED module instead hands the
+ * reducer a transactional {@link StoreView} over a per-module {@link StateStore}:
+ * the reducer READS and WRITES individual records by key and returns no state
+ * blob at all. The host commits the view's buffered writes only on success, so a
+ * keyed reducer gets the SAME atomicity guarantee as a whole-state one — without
+ * ever loading the whole dataset.
+ *
+ * This is ADDITIVE. Keyed modules carry `kind: 'keyed'` so the host can route
+ * them separately; whole-state modules are unchanged and keep `kind` absent
+ * (treated as `'whole'`).
+ */
+
+import { lintReducer } from './determinism';
+import { StateStore, StoreView } from './stateStore';
+import { EffectIntent, ReducerContext } from './types';
+
+/**
+ * A keyed reducer. Mutations go through `store` (a transactional {@link StoreView}
+ * buffering its writes); the reducer returns NO next-state — only an optional
+ * explicit `result` to surface to the caller and any `effects` to enqueue. Same
+ * purity contract as a whole-state reducer: read `now`/randomness/ids from `ctx`
+ * only, never ambient globals.
+ */
+export type KeyedReducer = (
+    store: StoreView,
+    input: unknown,
+    ctx: ReducerContext,
+) => { result?: unknown; effects?: EffectIntent[] };
+
+/**
+ * A read query over a keyed module's store. Receives the module's
+ * {@link StateStore} (read surface; reads return clones) and the query args.
+ */
+export type KeyedQuery = (store: StateStore, args: unknown) => unknown;
+
+/**
+ * A keyed module definition. The `kind: 'keyed'` discriminant lets the
+ * {@link ModuleHost} tell it apart from a whole-state `ModuleDefinition` (which
+ * has no `kind`).
+ */
+export interface KeyedModuleDefinition {
+    name: string;
+    version?: string;
+    kind: 'keyed';
+    commands: Record<string, KeyedReducer>;
+    queries?: Record<string, KeyedQuery>;
+}
+
+/**
+ * A keyed module that failed the determinism lint with `strict: false`. As with
+ * `defineModule`, the violations are recorded under the reserved `__lint` field
+ * rather than thrown.
+ */
+export type LintedKeyedModuleDefinition = KeyedModuleDefinition & { __lint?: string[] };
+
+/** Options for {@link defineKeyedModule}, mirroring `defineModule`. */
+export interface DefineKeyedModuleOptions {
+    /**
+     * Enforce the determinism lint at definition time (ADR-0019 pillar 2).
+     * Defaults to TRUE — identical semantics to `defineModule`: a violation
+     * throws, or with `{ strict: false }` is recorded under `__lint` instead.
+     */
+    strict?: boolean;
+}
+
+/**
+ * Validate and return a keyed module definition. Mirrors `defineModule`'s
+ * validation exactly (non-empty name, reject `__`-reserved names, ≥1 command,
+ * run the determinism lint strict-by-default over each reducer) and stamps
+ * `kind: 'keyed'` so the host routes it through the keyed (StoreView) path.
+ */
+export function defineKeyedModule(
+    def: Omit<KeyedModuleDefinition, 'kind'> & { kind?: 'keyed' },
+    opts: DefineKeyedModuleOptions = {},
+): LintedKeyedModuleDefinition {
+    const strict = opts.strict ?? true;
+    if (!def.name || def.name.trim() === '') {
+        throw new Error('Module definition requires a non-empty name');
+    }
+    // Same reserved-name guard as defineModule/register: `__`-prefixed names
+    // collide with the snapshot's reserved keys (`__outbox`/`__audit`).
+    if (def.name.startsWith('__')) {
+        throw new Error(
+            `Module name "${def.name}" is reserved: names starting with "__" are reserved for runtime internals`,
+        );
+    }
+
+    const commandNames = Object.keys(def.commands ?? {});
+    if (commandNames.length === 0) {
+        throw new Error(`Module "${def.name}" must define at least one command`);
+    }
+    for (const name of commandNames) {
+        if (name === '') {
+            throw new Error(`Module "${def.name}" has a command with an empty name`);
+        }
+    }
+
+    // Determinism lint (ADR-0019 pillar 2) over every keyed reducer — the same
+    // static stand-in for the deferred sandbox that whole-state modules get.
+    const violations: string[] = [];
+    for (const name of commandNames) {
+        violations.push(...lintReducer(name, def.commands[name]));
+    }
+
+    const result: KeyedModuleDefinition = { ...def, kind: 'keyed' };
+    if (violations.length > 0) {
+        if (strict) {
+            throw new Error(
+                `Module "${def.name}" failed the determinism lint (ADR-0019 pillar 2):\n` +
+                    violations.map((v) => `  - ${v}`).join('\n') +
+                    `\nReducers must be pure: use ctx.now / ctx.random() / ctx.id() instead of ambient ` +
+                    `globals, or pass { strict: false } to defineKeyedModule for a vetted reducer.`,
+            );
+        }
+        return Object.assign(result, { __lint: violations });
+    }
+    return result;
+}
diff --git a/src/runtime/merkleAudit.ts b/src/runtime/merkleAudit.ts
new file mode 100644
index 0000000..ab7295f
--- /dev/null
+++ b/src/runtime/merkleAudit.ts
@@ -0,0 +1,256 @@
+import { createHash } from 'crypto';
+import { canonicalJson } from './canonical';
+
+/**
+ * Merkle accumulator audit (ADR-0019 pillar 5). Upgrades the consensus core's
+ * LINEAR hash chain (`replicatedStateMachine.ts`) to a Merkle tree: instead of
+ * an O(n) walk to prove an entry, a verifier needs only a compact root plus an
+ * O(log n) sibling path. The root is a single hash that can be externally
+ * anchored, and any inclusion proof verifies against it without the full log —
+ * the property the linear chain could not offer.
+ *
+ * DETERMINISM: the audit is replicated state, so every leaf hash and the root
+ * MUST be a pure function of the applied command stream. We therefore hash a
+ * CANONICAL serialization (object keys sorted) of each leaf — no `Date`, no
+ * `Math.random`, no key-order dependence — so all replicas compute byte-identical
+ * hashes and converge on the same root.
+ */
+
+/**
+ * One audited fact: which logic version (`codeHash`) produced which result
+ * (`status`) for which command, and on whose behalf. `seq` is the leaf's index
+ * in the accumulator (and its position in the tree). `codeHash` is what closes
+ * ADR-0018's "data but not logic" gap — see {@link moduleCodeHash}.
+ */
+export interface AuditLeaf {
+    seq: number;
+    module: string;
+    command: string;
+    actor: string;
+    requestId: string;
+    status: number;
+    codeHash: string;
+}
+
+/**
+ * An inclusion proof: the leaf's own hash plus the ordered sibling hashes from
+ * leaf level up to the root. `side` says whether the sibling sits to the LEFT or
+ * RIGHT of the running hash at that level, so a verifier knows the concatenation
+ * order (`sha256(0x01 + left + right)`, the internal-node domain). A level where
+ * this node was promoted unchanged
+ * (odd-count tail — see below) contributes NO sibling, exactly as the tree built.
+ */
+export interface MerkleProof {
+    leafHash: string;
+    siblings: { hash: string; side: 'left' | 'right' }[];
+}
+
+/** sha256 hex of the given string. The single hashing primitive used throughout. */
+function sha256(s: string): string {
+    return createHash('sha256').update(s).digest('hex');
+}
+
+/**
+ * RFC-6962-style domain-separation tags. Leaves and internal nodes are hashed in
+ * DISTINCT domains (`0x00` for leaves, `0x01` for internal nodes) so a hash can
+ * never be valid as both. Without this, an internal-node hash (a bare 64-hex
+ * string) could be presented as a `MerkleProof.leafHash` and verify against the
+ * root — the classic second-preimage ambiguity. The prefix byte closes that gap.
+ */
+const LEAF_TAG = '\x00';
+const NODE_TAG = '\x01';
+
+/** Hash an internal node from its two child hashes, tagged in the node domain. */
+function hashNode(left: string, right: string): string {
+    return sha256(NODE_TAG + left + right);
+}
+
+/**
+ * Root of the EMPTY tree: a fixed, documented constant `sha256('')`. Returning a
+ * stable sentinel (rather than throwing) lets `auditRoot()` be called before any
+ * command is applied and keeps the root well-defined for the zero-leaf case.
+ * Deliberately UNTAGGED (neither leaf nor node domain): an empty tree has no
+ * leaf, so it cannot collide with any real leaf or internal hash.
+ */
+export const EMPTY_ROOT = sha256('');
+
+/**
+ * Hash a leaf via its canonical serialization, in the LEAF domain (`0x00`
+ * prefix). The tag keeps leaf hashes disjoint from internal-node hashes so a
+ * proof's `leafHash` can never be a smuggled internal node. Pure and
+ * replica-stable.
+ *
+ * The serializer (shared `canonicalJson` from `./canonical`) sorts object keys
+ * recursively so logically-equal leaves serialize to the SAME bytes regardless
+ * of property insertion order — the determinism linchpin for the leaf hash. We
+ * use the THROW-on-`undefined` mode because this is a hash PREIMAGE: it must be
+ * lossless and unambiguous, so any `undefined` (which `JSON.stringify` would
+ * silently drop or emit as the value `undefined`) is rejected loudly rather than
+ * corrupting the leaf hash. No current `AuditLeaf` field is ever `undefined`;
+ * the throw keeps that enforced for any future reuse.
+ */
+function hashLeaf(leaf: AuditLeaf): string {
+    return sha256(LEAF_TAG + canonicalJson(leaf, { onUndefined: 'throw' }));
+}
+
+/**
+ * Append-only Merkle accumulator over audit leaves.
+ *
+ * DOMAIN SEPARATION (RFC-6962-style): leaves and internal nodes are hashed in
+ * disjoint domains via a one-byte prefix — leaves as `sha256(0x00 + canonical)`,
+ * internal nodes as `sha256(0x01 + left + right)`. Because the two domains can
+ * never collide, an internal-node hash can never be passed off as a leaf hash
+ * (or vice versa), which closes the second-preimage ambiguity for proofs.
+ *
+ * ODD-NODE SCHEME (documented, and applied identically in build and proof): when
+ * a tree level has an odd number of nodes, the unpaired LAST node is **promoted
+ * unchanged** to the next level (it is NOT duplicated and re-hashed with itself).
+ * Combined with the leaf/internal domain tags above, this avoids the classic
+ * "duplicate-last" second-preimage ambiguity (where a tree and a tampered variant
+ * could share a root). A promoted level contributes no proof element, because no
+ * concatenation happened there.
+ *
+ * ATTESTATION SCOPE (read this before trusting the root for more than it offers):
+ * the audit attests *which command + logic version (`codeHash`) + status* was
+ * applied, and in what ORDER. It does NOT by itself attest STATE agreement: two
+ * replicas that processed an identical command stream share the same audit root
+ * even if a non-determinism bug left their materialized state divergent. State
+ * convergence is a separate property, verified elsewhere — the audit root is a
+ * cross-check on the command/logic stream, not a proof of equal state.
+ */
+export class MerkleAudit {
+    /** Leaves in append order; `leaf.seq` equals its index here. */
+    private readonly storedLeaves: AuditLeaf[] = [];
+    /** Cached leaf hashes, parallel to `storedLeaves`. */
+    private readonly leafHashes: string[] = [];
+
+    /**
+     * Append a leaf, returning its index (which equals `leaf.seq`). The leaf is
+     * canonical-hashed once and cached; the tree itself is recomputed on demand
+     * by `root()`/`proof()`, which keeps append O(1) and the structure trivially
+     * correct for any leaf count.
+     */
+    append(leaf: AuditLeaf): number {
+        const index = this.storedLeaves.length;
+        this.storedLeaves.push({ ...leaf });
+        this.leafHashes.push(hashLeaf(leaf));
+        return index;
+    }
+
+    /**
+     * Build all tree levels bottom-up from the cached leaf hashes. `levels[0]` is
+     * the leaf level; the last entry is the single-node root level. Returns an
+     * empty array for an empty tree (callers handle that as {@link EMPTY_ROOT}).
+     */
+    private buildLevels(): string[][] {
+        if (this.leafHashes.length === 0) {
+            return [];
+        }
+        const levels: string[][] = [this.leafHashes.slice()];
+        let current = levels[0];
+        while (current.length > 1) {
+            const next: string[] = [];
+            for (let i = 0; i < current.length; i += 2) {
+                if (i + 1 < current.length) {
+                    next.push(hashNode(current[i], current[i + 1]));
+                } else {
+                    // Odd tail: promote the last node unchanged (no self-pairing).
+                    next.push(current[i]);
+                }
+            }
+            levels.push(next);
+            current = next;
+        }
+        return levels;
+    }
+
+    /**
+     * Merkle root over all leaf hashes. Empty tree → {@link EMPTY_ROOT}
+     * (`sha256('')`). Stable for a given leaf set regardless of how the tree is
+     * cached or recomputed.
+     */
+    root(): string {
+        const levels = this.buildLevels();
+        if (levels.length === 0) {
+            return EMPTY_ROOT;
+        }
+        return levels[levels.length - 1][0];
+    }
+
+    /**
+     * Inclusion proof for the leaf at `index`. Walks level by level, recording
+     * the sibling hash and which side it sits on. Where this node is the promoted
+     * odd tail (no sibling at that level), it records nothing and simply rises —
+     * the SAME rule `buildLevels` used, so `verify` reconstructs the identical
+     * root. Throws on an out-of-range index.
+     */
+    proof(index: number): MerkleProof {
+        if (index < 0 || index >= this.storedLeaves.length) {
+            throw new Error(`Audit proof index out of range: ${index}`);
+        }
+        const levels = this.buildLevels();
+        const siblings: { hash: string; side: 'left' | 'right' }[] = [];
+        let pos = index;
+        // Walk from the leaf level up to (but not including) the root level.
+        for (let level = 0; level < levels.length - 1; level += 1) {
+            const nodes = levels[level];
+            const isRightChild = pos % 2 === 1;
+            const siblingPos = isRightChild ? pos - 1 : pos + 1;
+            if (siblingPos < nodes.length) {
+                siblings.push({
+                    hash: nodes[siblingPos],
+                    // If we are the right child, the sibling is to our left.
+                    side: isRightChild ? 'left' : 'right',
+                });
+            }
+            // else: we are the promoted odd tail — no sibling, no concatenation.
+            pos = Math.floor(pos / 2);
+        }
+        return { leafHash: this.leafHashes[index], siblings };
+    }
+
+    /**
+     * Recompute the root from a proof and compare to `root`. Folds each sibling
+     * into the running hash on the recorded side, then checks equality. Returns
+     * false on any mismatch, so a tampered leaf or wrong-index proof fails.
+     */
+    static verify(root: string, proof: MerkleProof): boolean {
+        let running = proof.leafHash;
+        for (const sib of proof.siblings) {
+            running = sib.side === 'left' ? hashNode(sib.hash, running) : hashNode(running, sib.hash);
+        }
+        return running === root;
+    }
+
+    /** A defensive copy of every leaf in append order. */
+    leaves(): AuditLeaf[] {
+        return this.storedLeaves.map((l) => ({ ...l }));
+    }
+
+    /** Number of appended leaves. */
+    size(): number {
+        return this.storedLeaves.length;
+    }
+
+    /**
+     * Snapshot the accumulator as just its leaves — the tree (and every hash) is
+     * a pure function of them, so storing the leaves alone is sufficient and
+     * keeps the snapshot minimal. `restore` rebuilds the hashes deterministically.
+     */
+    snapshot(): { leaves: AuditLeaf[] } {
+        return { leaves: this.leaves() };
+    }
+
+    /**
+     * Rebuild from a snapshot: drop all state, then re-append each leaf so the
+     * cached hashes are recomputed canonically. Two hosts restoring the same
+     * snapshot therefore reproduce the identical root.
+     */
+    restore(snap: { leaves: AuditLeaf[] }): void {
+        this.storedLeaves.length = 0;
+        this.leafHashes.length = 0;
+        for (const leaf of snap.leaves) {
+            this.append(leaf);
+        }
+    }
+}
diff --git a/src/runtime/moduleHost.ts b/src/runtime/moduleHost.ts
new file mode 100644
index 0000000..c75809d
--- /dev/null
+++ b/src/runtime/moduleHost.ts
@@ -0,0 +1,767 @@
+import { createContext } from './context';
+import { moduleCodeHash } from './codeHash';
+import { canonicalBytes } from './determinism';
+import { KeyedModuleDefinition } from './keyedModule';
+import { AuditLeaf, MerkleAudit, MerkleProof } from './merkleAudit';
+import {
+    BudgetExceededError,
+    CompiledReducer,
+    compileReducer,
+    runReducer,
+    runReducerWithBudget,
+} from './sandbox';
+import { KeyRegistry, SignablePayload, verifyCommand } from './signing';
+import { MemoryStateStore, StateStore, StoreView } from './stateStore';
+import {
+    EffectIntent,
+    EffectResultEntry,
+    ModuleApplyResult,
+    ModuleCommand,
+    ModuleDefinition,
+    ModuleHostOptions,
+    OutboxEntry,
+} from './types';
+
+/** Default ceiling on effects emitted by a single command. See `ModuleHostOptions`. */
+const DEFAULT_MAX_EFFECTS = 16;
+/** Default ceiling on the canonical byte size of a command's next-state (64 KiB). */
+const DEFAULT_MAX_RESULT_BYTES = 64 * 1024;
+/** Default ceiling on the buffered writes a single KEYED command may make. */
+const DEFAULT_MAX_WRITES = 256;
+/** Default wall-clock step budget (ms) for the leader-side admission dry-run. */
+const DEFAULT_STEP_BUDGET_MS = 50;
+
+/**
+ * A registrable module: either a whole-state `ModuleDefinition` (no `kind`, the
+ * original model) or a key-oriented `KeyedModuleDefinition` (`kind: 'keyed'`,
+ * ADR-0019 pillar 4). The host discriminates on `kind`, defaulting absent to
+ * whole-state for back-compat.
+ */
+export type AnyModuleDefinition = ModuleDefinition<unknown> | KeyedModuleDefinition;
+
+/** Narrow an `AnyModuleDefinition` to the keyed variant. */
+function isKeyed(def: AnyModuleDefinition): def is KeyedModuleDefinition {
+    return (def as KeyedModuleDefinition).kind === 'keyed';
+}
+
+/**
+ * Deep-clone via JSON round-trip. Used so snapshots are decoupled from live
+ * state (mutating a host after snapshotting must not retroactively change the
+ * snapshot, and vice versa). Module state is plain serializable data — the same
+ * assumption the consensus-core snapshots already make — so JSON is sufficient
+ * and avoids depending on `structuredClone` typings under the ES2017 lib target.
+ */
+function deepClone<T>(value: T): T {
+    return JSON.parse(JSON.stringify(value)) as T;
+}
+
+/**
+ * Registry of modules plus their live state (ADR-0019 pillars 1–2). This is the
+ * deterministic runtime that a later milestone will drive from the replicated
+ * `apply()` path. Today it stands alone: register modules, then `apply` commands
+ * whose seeds were resolved on the leader. Because the host derives every
+ * context from the command's seed, two hosts fed the same command stream reach
+ * identical state — the property the convergence test asserts.
+ */
+export class ModuleHost {
+    private readonly modules = new Map<string, AnyModuleDefinition>();
+    /** Live per-module WHOLE-STATE blob, keyed by module name. */
+    private readonly states = new Map<string, unknown>();
+    /**
+     * Per-module record store for KEYED modules (ADR-0019 pillar 4), keyed by
+     * module name. A keyed module has an entry here and NONE in `states`; a
+     * whole-state module is the reverse. The default `MemoryStateStore` is the
+     * in-memory backend; the `StateStore` seam is where a persistent embedded
+     * KV/LSM store would drop in (snapshots becoming store checkpoints).
+     */
+    private readonly stores = new Map<string, StateStore>();
+    /**
+     * The deterministic outbox (ADR-0019 pillar 3), keyed by `idempotencyKey`.
+     * Every host derives it from the same committed command stream, so it is
+     * itself replicated state — part of `snapshot()`/`restore()`. Keying on the
+     * idempotency key is what makes enqueue idempotent: a replayed command can
+     * re-run its reducer but never re-enqueue the same effect.
+     */
+    private readonly outbox = new Map<string, OutboxEntry>();
+    /**
+     * Per-module code-version hash (ADR-0019 pillar 5), computed at `register()`
+     * from the module's logic. Every audited leaf stamps the hash of the module
+     * that produced it, so the history proves WHICH logic version ran.
+     */
+    private readonly codeHashes = new Map<string, string>();
+    /**
+     * Compiled sandbox reducers for `sandbox: true` whole-state modules (ADR-0019
+     * M9), keyed `module/command`. Built ONCE at `register()` by re-evaluating
+     * each reducer's source inside a fresh `vm` context with a frozen safe-global
+     * set. Present only for sandboxed modules; absent modules dispatch directly as
+     * before. Both the apply path (`runReducer`, no timeout) and leader-side
+     * admission (`runReducerWithBudget`, timeout) drive the SAME compiled handle.
+     */
+    private readonly compiled = new Map<string, CompiledReducer>();
+    /**
+     * The Merkle audit accumulator (ADR-0019 pillar 5). Replicated state: it is
+     * derived purely from the applied command stream, so two hosts fed the same
+     * stream produce the same `auditRoot()`. Folded into `snapshot()`/`restore()`.
+     */
+    private readonly audit = new MerkleAudit();
+    /**
+     * Monotonic audit sequence, advanced once per audited leaf. Deterministic
+     * (driven only by the command stream), it is each leaf's `seq` / index.
+     */
+    private auditSeq = 0;
+
+    /**
+     * Deterministic resource bound (ADR-0019 pillar 6): the max effects one
+     * command may emit and the max canonical byte size of the next-state it may
+     * produce. Computed identically on every replica, so over-budget commands
+     * are rejected uniformly — NOT a CPU meter (that is deferred; needs a vm).
+     */
+    private readonly maxEffects: number;
+    private readonly maxResultBytes: number;
+    private readonly maxWrites: number;
+    /**
+     * Default wall-clock step budget (ms) for the leader-side admission meter
+     * (ADR-0019 pillar 6 — M9), used when a sandboxed module does not set its own
+     * `stepBudgetMs`. Only the admission dry-run consults it; the apply path is
+     * always timeout-free.
+     */
+    private readonly stepBudgetMs: number;
+
+    /**
+     * Optional actor->public-key registry (ADR-0019 pillar 7). When SET, every
+     * caller `apply()` must carry a valid actor signature over its logical
+     * command or it is rejected (401) before its reducer runs. When UNSET
+     * (default), no verification happens and behavior is exactly as before —
+     * the back-compat guarantee that keeps all existing tests green. Configured
+     * per node before start (like modules), identical on every node, so
+     * verification converges.
+     */
+    private keyRegistry?: KeyRegistry;
+
+    constructor(opts: ModuleHostOptions = {}) {
+        this.maxEffects = opts.maxEffects ?? DEFAULT_MAX_EFFECTS;
+        this.maxResultBytes = opts.maxResultBytes ?? DEFAULT_MAX_RESULT_BYTES;
+        this.maxWrites = opts.maxWrites ?? DEFAULT_MAX_WRITES;
+        this.stepBudgetMs = opts.stepBudgetMs ?? DEFAULT_STEP_BUDGET_MS;
+    }
+
+    /**
+     * Configure the actor signature registry (ADR-0019 pillar 7). Call before
+     * start, identically on every node. Once set, caller commands are verified
+     * on the apply path; until set, verification is skipped (back-compat).
+     */
+    setKeyRegistry(reg: KeyRegistry): void {
+        this.keyRegistry = reg;
+    }
+
+    /** Authorize `publicKeyPem` as the signer for `actor`, creating the registry if needed. */
+    registerActorKey(actor: string, publicKeyPem: string): void {
+        if (!this.keyRegistry) this.keyRegistry = new KeyRegistry();
+        this.keyRegistry.registerActor(actor, publicKeyPem);
+    }
+
+    /**
+     * Register a module and initialize its state. Accepts BOTH a whole-state
+     * `ModuleDefinition` and a key-oriented `KeyedModuleDefinition` (discriminated
+     * on `kind`, ADR-0019 pillar 4); a keyed module gets its own `StateStore`
+     * instead of a whole-state blob. Throws on a duplicate name.
+     */
+    register(def: AnyModuleDefinition): void {
+        if (!def.name || def.name.trim() === '') {
+            throw new Error('Module definition requires a non-empty name');
+        }
+        // Reject reserved `__`-prefixed names: the snapshot stores the outbox
+        // under the reserved `__outbox` key in the same flat module-states object,
+        // so a `__`-prefixed module would silently collide (its state overwritten
+        // on snapshot, misread as an outbox map on restore). Fail closed.
+        if (def.name.startsWith('__')) {
+            throw new Error(
+                `Module name "${def.name}" is reserved: names starting with "__" are reserved for runtime internals`,
+            );
+        }
+        if (this.modules.has(def.name)) {
+            throw new Error(`Module "${def.name}" is already registered`);
+        }
+        // Sandbox is a WHOLE-STATE feature for this milestone (ADR-0019 M9):
+        // sandboxing keyed StoreView-mutating reducers is out of scope, so reject
+        // the combination at registration rather than silently ignore the flag.
+        if (isKeyed(def) && (def as { sandbox?: boolean }).sandbox) {
+            throw new Error(
+                `Module "${def.name}": the determinism sandbox (sandbox: true) is supported ` +
+                    `for whole-state modules only in this milestone, not keyed modules`,
+            );
+        }
+
+        this.modules.set(def.name, def);
+        if (isKeyed(def)) {
+            // Keyed module: give it an empty record store. The default in-memory
+            // backend; the StateStore interface is the persistent-KV seam.
+            this.stores.set(def.name, new MemoryStateStore());
+        } else {
+            this.states.set(def.name, def.initialState());
+            // If the module opted into the sandbox, compile each reducer ONCE now
+            // into its own frozen vm context (ADR-0019 M9). A non-self-contained
+            // reducer source (e.g. method-shorthand) fails loudly here, at
+            // registration, rather than at first dispatch.
+            if (def.sandbox) {
+                for (const [name, reducer] of Object.entries(def.commands)) {
+                    this.compiled.set(`${def.name}/${name}`, compileReducer(reducer.toString()));
+                }
+            }
+        }
+        // Stamp the module's logic version now; every leaf it later produces
+        // records this hash (ADR-0019 pillar 5).
+        this.codeHashes.set(def.name, moduleCodeHash(def));
+    }
+
+    /** Register several modules (whole-state and/or keyed) in order. */
+    registerModules(defs: AnyModuleDefinition[]): void {
+        for (const def of defs) this.register(def);
+    }
+
+    /**
+     * Dispatch a command: look up the module + reducer, build a deterministic
+     * context from the command's seed, run the (pure) reducer against current
+     * state, and adopt the returned state. Failures are returned as status codes
+     * rather than thrown, mirroring the state machine's HTTP-style results, so a
+     * single bad command never crashes the apply loop.
+     */
+    apply(cmd: ModuleCommand, meta: { actor: string; requestId: string }): ModuleApplyResult {
+        // Actor signature verification (ADR-0019 pillar 7), if a registry is
+        // configured. This runs FIRST so a forged/tampered/unsigned command never
+        // reaches its reducer and never touches state or the outbox. Verification
+        // is a pure, deterministic function of the committed command + meta
+        // (canonical JSON + ed25519 verify, both deterministic), so every replica
+        // rejects identically — the cluster cannot diverge on a bad signature.
+        const denied = this.verifySignature(cmd, meta);
+        if (denied) {
+            // Audit the rejection at 401 for uniformity with other failure
+            // statuses (a forged command IS part of the history), but do NOT run
+            // the reducer and do NOT mutate state/outbox.
+            this.recordAudit(cmd, meta, denied.status);
+            return denied;
+        }
+
+        const result = this.dispatch(cmd, meta);
+        if (result.status === 200) {
+            // Record each emitted effect into the outbox as `pending`, but only if
+            // its key is unknown — a replayed command must re-run deterministically
+            // without re-enqueuing the same effect (the exactly-once dedup point).
+            for (const intent of result.effects) {
+                if (!this.outbox.has(intent.idempotencyKey)) {
+                    this.outbox.set(intent.idempotencyKey, { intent, status: 'pending' });
+                }
+            }
+        }
+        return result;
+    }
+
+    /**
+     * Leader-side admission / step meter (ADR-0019 pillar 6 "gas" — M9). A
+     * pre-log DRY RUN: the LEADER calls this BEFORE submitting a command to the
+     * log; if it returns `ok: false`, the command is rejected and never enters the
+     * log. Followers do NOT re-admit — under the CFT trust model they trust that a
+     * committed entry was already admitted by the leader, which is exactly why the
+     * apply path stays timeout-free and deterministic.
+     *
+     * For a SANDBOXED module it runs the reducer inside the vm with a wall-clock
+     * `stepBudgetMs` timeout against a CLONE of current state — NO mutation, NO
+     * commit, NO effects enqueued, NO audit. A runaway reducer (e.g. an infinite
+     * loop) trips the vm timeout and is reported as `503` (`BudgetExceededError`);
+     * any other reducer throw is reported as `500` — the SAME status the apply
+     * path (`dispatch`) gives that deterministic throw, so admission and apply
+     * stay in status parity. Because the dry run touches a clone and discards its
+     * result, the timeout's non-determinism never reaches replicated state.
+     *
+     * For a NON-SANDBOXED module admission is a NO-OP that returns ok: the step
+     * meter is a sandbox feature (a direct-execution reducer cannot be safely
+     * interrupted by the vm), and those modules already gate fan-out/size on the
+     * deterministic apply path. Unknown module/command also admit ok and surface
+     * their 404 on apply, keeping admission strictly about the CPU/step budget.
+     */
+    admit(
+        cmd: ModuleCommand,
+        meta: { actor: string; requestId: string },
+    ): { ok: true } | { ok: false; status: number; message: string } {
+        const compiled = this.compiled.get(`${cmd.module}/${cmd.command}`);
+        if (!compiled) {
+            // Not a sandboxed command (non-sandboxed module, keyed module, or
+            // unknown module/command): nothing to meter, admit and let apply run.
+            return { ok: true };
+        }
+
+        const def = this.modules.get(cmd.module) as ModuleDefinition<unknown>;
+        const budgetMs = def.stepBudgetMs ?? this.stepBudgetMs;
+        const ctx = createContext(cmd.seed, meta);
+        // Clone current state so the dry run cannot mutate committed state even if
+        // the reducer mutates its argument in place before looping/throwing.
+        const working = deepClone(this.states.get(cmd.module));
+
+        try {
+            runReducerWithBudget(compiled, working, cmd.input, ctx, budgetMs);
+            return { ok: true };
+        } catch (err) {
+            if (err instanceof BudgetExceededError) {
+                return {
+                    ok: false,
+                    status: 503,
+                    message:
+                        `Command "${cmd.command}" on module "${cmd.module}" exceeded the ` +
+                        `${budgetMs}ms step budget and was rejected before entering the log`,
+                };
+            }
+            // Any other reducer error during admission: reject too, so a command
+            // that would deterministically 500 on apply is caught at the edge.
+            // Report 500 (NOT 400) for admit/apply STATUS PARITY: the apply path
+            // (`dispatch`) maps the same deterministic reducer throw to 500, so the
+            // pre-log gate must surface the identical status it would get on apply.
+            const message = err instanceof Error ? err.message : String(err);
+            return {
+                ok: false,
+                status: 500,
+                message: `Command "${cmd.command}" on module "${cmd.module}" failed admission: ${message}`,
+            };
+        }
+    }
+
+    /**
+     * Verify a caller command's actor signature (ADR-0019 pillar 7). Returns a
+     * deterministic 401 `ModuleApplyResult` to REJECT, or `undefined` to ALLOW.
+     *
+     * Back-compat: with no registry configured, always allows (returns
+     * `undefined`) — verification is opt-in, so existing unsigned flows are
+     * untouched. With a registry configured, a command is allowed ONLY if it
+     * carries a `sig` AND the actor has a registered key AND the signature
+     * verifies over the canonical LOGICAL payload (excluding `seed`). A missing
+     * signature, an unknown actor, or an invalid signature all reject — this is
+     * what stops a leader forging `actor`.
+     */
+    private verifySignature(
+        cmd: ModuleCommand,
+        meta: { actor: string; requestId: string },
+    ): ModuleApplyResult | undefined {
+        if (!this.keyRegistry) return undefined; // back-compat: no verification
+
+        if (!cmd.sig) {
+            return { status: 401, effects: [], message: 'Missing actor signature' };
+        }
+        const publicKey = this.keyRegistry.get(meta.actor);
+        if (!publicKey) {
+            return { status: 401, effects: [], message: `No registered key for actor "${meta.actor}"` };
+        }
+        // Reconstruct the EXACT logical payload the actor signed — the seed is
+        // deliberately excluded (the leader adds it after signing).
+        const payload: SignablePayload = {
+            module: cmd.module,
+            command: cmd.command,
+            input: cmd.input,
+            actor: meta.actor,
+            requestId: meta.requestId,
+        };
+        if (!verifyCommand(publicKey, payload, cmd.sig)) {
+            return { status: 401, effects: [], message: 'Invalid actor signature' };
+        }
+        return undefined; // verified
+    }
+
+    /**
+     * The reducer dispatch path, shared by `apply` (caller commands) and
+     * `applyEffectResult` (the committed `onResult` follow-up). It runs a pure
+     * reducer against a clone of current state and adopts the result atomically;
+     * it does NOT touch the outbox so callers can layer their own bookkeeping.
+     */
+    private dispatch(cmd: ModuleCommand, meta: { actor: string; requestId: string }): ModuleApplyResult {
+        const def = this.modules.get(cmd.module);
+        if (!def) {
+            return { status: 404, effects: [], message: `Unknown module: ${cmd.module}` };
+        }
+
+        if (!def.commands[cmd.command]) {
+            return {
+                status: 404,
+                effects: [],
+                message: `Unknown command "${cmd.command}" on module "${cmd.module}"`,
+            };
+        }
+
+        // Keyed modules take a separate, transactional (StoreView) path.
+        if (isKeyed(def)) {
+            return this.dispatchKeyed(def, cmd, meta);
+        }
+
+        const reducer = def.commands[cmd.command];
+        const ctx = createContext(cmd.seed, meta);
+        // Hand the reducer a deep clone, never the live reference: a reducer that
+        // mutates `state` in place and then throws must not corrupt committed
+        // host state. Live state is replaced atomically only on a clean return.
+        const working = deepClone(this.states.get(cmd.module));
+
+        // The reducer is about to run, so this command WILL be audited regardless
+        // of its outcome (success, business-failure status, or thrown→500). The
+        // earlier unknown-module/unknown-command 404s returned before this point
+        // and are intentionally NOT audited — no logic ran for them.
+        // For a sandboxed module, run the reducer inside its frozen vm context on
+        // the APPLY path with NO timeout (ADR-0019 M9): the call must be a pure,
+        // deterministic function of (state, input, ctx) so every replica computes
+        // the same next-state. A banned global (Date, Math.random, …) is absent in
+        // the sandbox and throws here, caught below as a 500 — structurally, not
+        // by the lint. Non-sandboxed modules run the reducer directly, as before.
+        const sandboxed = this.compiled.get(`${cmd.module}/${cmd.command}`);
+
+        let outcome: ModuleApplyResult;
+        try {
+            const result = sandboxed
+                ? runReducer(sandboxed, working, cmd.input, ctx)
+                : reducer(working, cmd.input, ctx);
+            const effects = result.effects ?? [];
+
+            // Deterministic resource bound (ADR-0019 pillar 6). Computed AFTER the
+            // reducer returns, from its output alone — every replica derives the
+            // same effect count and the same canonical byte size, so an
+            // over-budget command is rejected identically everywhere (no
+            // divergence). On rejection we DO NOT adopt the next-state and DO NOT
+            // surface the effects: it is treated as a failed apply, audited with a
+            // 413 status for uniformity (consistent with a business-failure
+            // status), leaving committed state and the outbox untouched.
+            if (effects.length > this.maxEffects) {
+                outcome = {
+                    status: 413,
+                    effects: [],
+                    message:
+                        `Command "${cmd.command}" on module "${cmd.module}" emitted ${effects.length} ` +
+                        `effects, exceeding the limit of ${this.maxEffects}`,
+                };
+            } else {
+                const stateBytes = canonicalBytes(result.state);
+                if (stateBytes > this.maxResultBytes) {
+                    outcome = {
+                        status: 413,
+                        effects: [],
+                        message:
+                            `Command "${cmd.command}" on module "${cmd.module}" produced a ${stateBytes}-byte ` +
+                            `state, exceeding the limit of ${this.maxResultBytes} bytes`,
+                    };
+                } else {
+                    // Under budget: adopt the next-state and surface the reducer's
+                    // explicit `result` value (not the whole ReducerResult).
+                    this.states.set(cmd.module, result.state);
+                    outcome = { status: 200, result: result.result, effects };
+                }
+            }
+        } catch (err) {
+            // A throwing reducer must not corrupt state or halt the host; report it.
+            const message = err instanceof Error ? err.message : String(err);
+            outcome = { status: 500, effects: [], message };
+        }
+
+        this.recordAudit(cmd, meta, outcome.status);
+        return outcome;
+    }
+
+    /**
+     * The KEYED reducer dispatch path (ADR-0019 pillar 4). Mirrors `dispatch`'s
+     * atomicity and resource-bound envelope, but the reducer mutates a
+     * transactional {@link StoreView} instead of returning a next-state blob:
+     *
+     *  - Build a copy-on-write `StoreView` over the module's `StateStore`. The
+     *    reducer reads through it (clones, reads-its-own-writes) and buffers any
+     *    `put`/`delete` — nothing reaches the store yet.
+     *  - On a CLEAN, in-budget return, `commit()` the view atomically. On a thrown
+     *    reducer OR a budget rejection, DISCARD the view (drop it) so the store is
+     *    untouched — the same all-or-nothing guarantee whole-state modules get.
+     *  - Resource bounds (deterministic, computed from the buffer after return):
+     *    `maxEffects` (fan-out), `maxWrites` (records touched), and `maxResultBytes`
+     *    over the canonical bytes of the buffered puts (write amplification). Over
+     *    budget ⇒ 413, view discarded.
+     *  - Audit exactly as the whole-state path (one envelope leaf per run).
+     */
+    private dispatchKeyed(
+        def: KeyedModuleDefinition,
+        cmd: ModuleCommand,
+        meta: { actor: string; requestId: string },
+    ): ModuleApplyResult {
+        const reducer = def.commands[cmd.command];
+        const ctx = createContext(cmd.seed, meta);
+        const store = this.stores.get(cmd.module)!;
+        // The view buffers all writes; the live store is touched only on commit().
+        const view = new StoreView(store);
+
+        let outcome: ModuleApplyResult;
+        try {
+            const result = reducer(view, cmd.input, ctx);
+            const effects = result.effects ?? [];
+
+            if (effects.length > this.maxEffects) {
+                // Over fan-out budget: discard the view (no commit), 413.
+                outcome = {
+                    status: 413,
+                    effects: [],
+                    message:
+                        `Command "${cmd.command}" on module "${cmd.module}" emitted ${effects.length} ` +
+                        `effects, exceeding the limit of ${this.maxEffects}`,
+                };
+            } else if (view.pendingWriteCount() > this.maxWrites) {
+                // Over write-count budget: discard the view, 413.
+                outcome = {
+                    status: 413,
+                    effects: [],
+                    message:
+                        `Command "${cmd.command}" on module "${cmd.module}" buffered ` +
+                        `${view.pendingWriteCount()} writes, exceeding the limit of ${this.maxWrites}`,
+                };
+            } else {
+                const writeBytes = canonicalBytes(view.pendingPuts());
+                if (writeBytes > this.maxResultBytes) {
+                    // Over write-size budget: discard the view, 413.
+                    outcome = {
+                        status: 413,
+                        effects: [],
+                        message:
+                            `Command "${cmd.command}" on module "${cmd.module}" buffered ${writeBytes} bytes ` +
+                            `of writes, exceeding the limit of ${this.maxResultBytes} bytes`,
+                    };
+                } else {
+                    // Clean + in budget: commit the buffer atomically and surface
+                    // the reducer's explicit result value.
+                    view.commit();
+                    outcome = { status: 200, result: result.result, effects };
+                }
+            }
+        } catch (err) {
+            // A throwing keyed reducer commits NOTHING: the view is dropped here
+            // with its buffer unapplied, so the store is exactly as before.
+            const message = err instanceof Error ? err.message : String(err);
+            outcome = { status: 500, effects: [], message };
+        }
+
+        this.recordAudit(cmd, meta, outcome.status);
+        return outcome;
+    }
+
+    /**
+     * Append one audit leaf for a command whose reducer ran. The leaf is a pure
+     * function of the command + meta + outcome status + the producing module's
+     * code hash, so every replica appends an identical leaf and the Merkle root
+     * stays convergent. `seq` is the monotonic, deterministic audit index.
+     */
+    private recordAudit(cmd: ModuleCommand, meta: { actor: string; requestId: string }, status: number): void {
+        const leaf: AuditLeaf = {
+            seq: this.auditSeq,
+            module: cmd.module,
+            command: cmd.command,
+            actor: meta.actor,
+            requestId: meta.requestId,
+            status,
+            codeHash: this.codeHashes.get(cmd.module) ?? '',
+        };
+        this.audit.append(leaf);
+        this.auditSeq += 1;
+    }
+
+    /** All outbox entries still awaiting execution at the edge. */
+    pendingEffects(): EffectIntent[] {
+        const out: EffectIntent[] = [];
+        for (const name of [...this.outbox.keys()].sort()) {
+            const entry = this.outbox.get(name)!;
+            if (entry.status === 'pending') {
+                // Deep-clone (like getOutbox) so a handler that mutates the intent
+                // it receives cannot mutate the committed outbox state behind it.
+                out.push(deepClone(entry.intent));
+            }
+        }
+        return out;
+    }
+
+    /**
+     * Apply a committed `EffectResultEntry` — the follow-up entry the executor
+     * fed back after performing the effect at the edge. This runs on the
+     * deterministic apply path on EVERY replica, so it must be a pure function of
+     * the entry + current state, and it MUST be idempotent: a redelivered or
+     * replayed result must not re-dispatch the `onResult` reducer.
+     *
+     * Idempotency rule: if the key is unknown or already `done`, no-op (200, no
+     * dispatch). Otherwise mark the entry `done`, store the edge-resolved
+     * `result`, and — if the intent named an `onResult` command — dispatch it
+     * through the normal reducer path with the entry's leader-resolved `seed`, so
+     * the consuming reducer stays deterministic.
+     */
+    applyEffectResult(entry: EffectResultEntry, meta: { actor: string; requestId: string }): ModuleApplyResult {
+        // First-applied wins. A handler retry that fires before the first result
+        // is applied can commit multiple `EffectResultEntry` values for one key
+        // (each drain that completes submits one). That is fine: the FIRST entry
+        // applied flips the key to `done`; every later entry for that key hits the
+        // `done` guard below and no-ops. Convergence holds because the committed
+        // log order is identical on every replica, so all replicas apply the same
+        // "first" entry and discard the same rest.
+        const slot = this.outbox.get(entry.idempotencyKey);
+        if (!slot || slot.status === 'done') {
+            // Unknown or already-applied: exactly-once at the state level means
+            // this is a harmless no-op, never a second `onResult` dispatch.
+            return { status: 200, effects: [] };
+        }
+
+        slot.status = 'done';
+        slot.result = entry.result;
+
+        const onResult = slot.intent.onResult;
+        if (!onResult) {
+            return { status: 200, effects: [] };
+        }
+
+        // Feed the edge-resolved result to the consuming reducer. We go through
+        // `apply` (not bare `dispatch`) so any effects the onResult reducer emits
+        // are themselves enqueued into the outbox.
+        return this.apply(
+            {
+                module: onResult.module,
+                command: onResult.command,
+                input: { idempotencyKey: entry.idempotencyKey, result: entry.result },
+                seed: entry.seed,
+            },
+            meta,
+        );
+    }
+
+    /**
+     * The outbox as a list in deterministic (sorted-by-key) order. For tests and
+     * inspection; the canonical store is the keyed map.
+     */
+    getOutbox(): OutboxEntry[] {
+        return [...this.outbox.keys()].sort().map((k) => deepClone(this.outbox.get(k)!));
+    }
+
+    /** Run a read query against current state. Never mutates. */
+    query(module: string, name: string, args?: unknown): unknown {
+        const def = this.modules.get(module);
+        if (!def) {
+            throw new Error(`Unknown module: ${module}`);
+        }
+        const q = def.queries?.[name];
+        if (!q) {
+            throw new Error(`Unknown query "${name}" on module "${module}"`);
+        }
+        if (isKeyed(def)) {
+            // A keyed query reads the module's StateStore (reads return clones).
+            return q(this.stores.get(module)!, args);
+        }
+        return (q as (state: unknown, args: unknown) => unknown)(this.states.get(module), args);
+    }
+
+    /** Current live state of a module (for tests/snapshots). */
+    getState(module: string): unknown {
+        return this.states.get(module);
+    }
+
+    /**
+     * The `StateStore` backing a keyed module (for tests/inspection). Returns the
+     * LIVE store, not a copy — callers must NOT mutate it in production code
+     * (writes must go through `apply()` so they are audited and replicated);
+     * direct mutation here would diverge a replica.
+     */
+    getStore(module: string): StateStore | undefined {
+        return this.stores.get(module);
+    }
+
+    /**
+     * Serializable map of module name -> deep-cloned state. Keys are emitted in
+     * sorted order so a `JSON.stringify` over the snapshot is stable regardless
+     * of module registration order (helps the later audit/hash-chain milestone).
+     */
+    snapshot(): Record<string, unknown> {
+        const states: Record<string, unknown> = {};
+        for (const name of [...this.states.keys()].sort()) {
+            states[name] = deepClone(this.states.get(name));
+        }
+        // KEYED modules contribute their store dump (SORTED entries) under the
+        // module name, alongside whole-state blobs. `store.snapshot()` already
+        // returns sorted, cloned entries — deterministic across replicas.
+        for (const name of [...this.stores.keys()].sort()) {
+            states[name] = this.stores.get(name)!.snapshot();
+        }
+        // The outbox is replicated state too: emit it under a reserved key, also
+        // in sorted-key order so `JSON.stringify` over the snapshot is stable.
+        const outbox: Record<string, OutboxEntry> = {};
+        for (const key of [...this.outbox.keys()].sort()) {
+            outbox[key] = deepClone(this.outbox.get(key)!);
+        }
+        // The Merkle audit + its seq are replicated state too: emit them under the
+        // reserved `__audit` key (collision-safe — `__`-prefixed module names are
+        // rejected at registration). Storing the leaves alone is sufficient; the
+        // tree and root are a pure function of them, rebuilt on restore.
+        const auditSnap = { seq: this.auditSeq, ...this.audit.snapshot() };
+        return { ...states, __outbox: outbox, __audit: auditSnap };
+    }
+
+    /**
+     * Replace the state of registered modules from a snapshot. Only modules that
+     * are both registered AND present in the snapshot are restored; unknown keys
+     * in the snapshot are ignored (a module may have been removed), and modules
+     * absent from the snapshot keep their initialized state.
+     */
+    restore(snap: Record<string, unknown>): void {
+        for (const [name, def] of this.modules) {
+            if (!Object.prototype.hasOwnProperty.call(snap, name)) continue;
+            if (isKeyed(def)) {
+                // Route by registered kind: a keyed module restores its store from
+                // the dumped [key, value][] entries (cloned by the store on restore).
+                const entries = (snap[name] as [string, unknown][]) ?? [];
+                this.stores.get(name)!.restore(entries);
+            } else {
+                this.states.set(name, deepClone(snap[name]));
+            }
+        }
+        // Rebuild the outbox from its reserved key. Replacing wholesale (not
+        // merging) keeps restore a faithful point-in-time reconstruction.
+        this.outbox.clear();
+        const saved = snap.__outbox;
+        if (saved && typeof saved === 'object') {
+            for (const [key, entry] of Object.entries(saved as Record<string, OutboxEntry>)) {
+                this.outbox.set(key, deepClone(entry));
+            }
+        }
+        // Rebuild the Merkle audit from its reserved key. The accumulator
+        // recomputes all leaf hashes (and thus the root) deterministically, so a
+        // restored host reports the identical `auditRoot()`.
+        const auditSaved = snap.__audit as { seq?: number; leaves?: AuditLeaf[] } | undefined;
+        this.audit.restore({ leaves: auditSaved?.leaves ?? [] });
+        this.auditSeq = auditSaved?.seq ?? this.audit.size();
+    }
+
+    // ---- audit access (ADR-0019 pillar 5) ----
+
+    /**
+     * The compact Merkle root over all audited commands. A single hash that
+     * summarizes the entire history and can be externally anchored; identical on
+     * every replica that applied the same command stream.
+     */
+    auditRoot(): string {
+        return this.audit.root();
+    }
+
+    /**
+     * An O(log n) inclusion proof that the leaf at `seq` is part of the audited
+     * history. Verifies against `auditRoot()` via `MerkleAudit.verify`.
+     */
+    auditProof(seq: number): MerkleProof {
+        return this.audit.proof(seq);
+    }
+
+    /** Every audit leaf in append order (for inspection/tests). */
+    auditEntries(): AuditLeaf[] {
+        return this.audit.leaves();
+    }
+
+    /** Number of audited commands. */
+    auditSize(): number {
+        return this.audit.size();
+    }
+
+    /**
+     * The code-version hash recorded for a module (or `undefined` if the module
+     * is not registered). The same value every audited leaf from that module
+     * carries — letting a verifier confirm which logic version produced a result.
+     */
+    moduleCodeHash(module: string): string | undefined {
+        return this.codeHashes.get(module);
+    }
+}
diff --git a/src/runtime/moduleStateMachine.ts b/src/runtime/moduleStateMachine.ts
new file mode 100644
index 0000000..4c10335
--- /dev/null
+++ b/src/runtime/moduleStateMachine.ts
@@ -0,0 +1,59 @@
+import { StateMachine } from '../consensus/stateMachine';
+import { ApplyResult } from '../consensus/types';
+import { ModuleHost } from './moduleHost';
+import { ModuleAppCommand } from './types';
+
+/**
+ * Adapts a {@link ModuleHost} to the framework's {@link StateMachine} contract
+ * (ADR-0017) so the whole module runtime — pure reducers, effects/outbox, the
+ * Merkle audit, the keyed store, signing — plugs into a `RaftNode` as an ordinary
+ * pluggable application:
+ *
+ *   const sm = new ModuleStateMachine();
+ *   sm.host.registerModules([counter, notes]);
+ *   const node = new RaftNode({ id, peers, stateMachine: sm }, transport);
+ *
+ * This is the seam ADR-0021 calls the real pluggability point — the
+ * commit-ordered-log contract above the log. The module runtime no longer needs
+ * a bespoke `MODULE` command baked into the consensus core: a `ModuleAppCommand`
+ * is just an application command `C`, and this adapter is the application.
+ *
+ * The only impedance to bridge is that `StateMachine.apply(command)` receives no
+ * separate `CommandMeta` (audit/idempotency are the substrate's concern), whereas
+ * `ModuleHost.apply(cmd, meta)` needs `actor`/`requestId` (for the audit leaf and
+ * signature verification). We therefore carry those two fields ON the command
+ * (see {@link ModuleAppCommand}) and split them back out here. `ModuleHost`
+ * itself is untouched.
+ */
+export class ModuleStateMachine implements StateMachine<ModuleAppCommand, unknown> {
+    /** The wrapped runtime — use it to register modules/keys and to read state. */
+    readonly host: ModuleHost;
+
+    constructor(host: ModuleHost = new ModuleHost()) {
+        this.host = host;
+    }
+
+    apply(command: ModuleAppCommand): ApplyResult<unknown> {
+        const result = this.host.apply(
+            {
+                module: command.module,
+                command: command.command,
+                input: command.input,
+                seed: command.seed,
+                sig: command.sig,
+            },
+            { actor: command.actor, requestId: command.requestId },
+        );
+        // The runtime's `effects` stay in the host's outbox (drained post-commit
+        // by the EffectExecutor); the substrate only needs status/data/message.
+        return { status: result.status, data: result.result, message: result.message };
+    }
+
+    snapshot(): unknown {
+        return this.host.snapshot();
+    }
+
+    restore(data: unknown): void {
+        this.host.restore(data as Record<string, unknown>);
+    }
+}
diff --git a/src/runtime/modules/accounts.ts b/src/runtime/modules/accounts.ts
new file mode 100644
index 0000000..507717e
--- /dev/null
+++ b/src/runtime/modules/accounts.ts
@@ -0,0 +1,128 @@
+import { defineKeyedModule } from '../keyedModule';
+
+/**
+ * A demo KEYED module (ADR-0019 pillar 4, "state larger than RAM"): a ledger of
+ * accounts where state is ONE RECORD PER ACCOUNT, addressed by account id. Every
+ * command touches only the keys it needs — `deposit` reads/writes a single
+ * account, `transfer` exactly two — so the whole ledger is NEVER loaded as a
+ * blob. That is the per-key access the in-memory `Map` model could not express;
+ * it is the property a persistent embedded KV/LSM backend exploits to scale state
+ * past RAM behind the same `StateStore` seam.
+ *
+ * Determinism: `openedAt` comes from `ctx.now` (the leader-resolved seed), never
+ * an ambient `Date` — the same purity contract every reducer obeys.
+ */
+
+/** One account record, stored at key = its id. */
+interface Account {
+    id: string;
+    balance: number;
+    openedAt: string;
+}
+
+export const accounts = defineKeyedModule({
+    name: 'accounts',
+    version: '1',
+    commands: {
+        /**
+         * Open an account at key = id. Idempotent-ish: rejects 400 if the key is
+         * already present rather than silently overwriting an existing balance.
+         */
+        open: (store, input, ctx) => {
+            const { id } = (input ?? {}) as { id: string };
+            if (!id) {
+                throw new Error('open requires an id');
+            }
+            if (store.has(id)) {
+                throw new Error(`account "${id}" already exists`);
+            }
+            const account: Account = { id, balance: 0, openedAt: ctx.now };
+            store.put(id, account);
+            return { result: account };
+        },
+
+        /** Read the single account by key, add `amount`, write it back. */
+        deposit: (store, input) => {
+            const { id, amount } = (input ?? {}) as { id: string; amount: number };
+            if (!(amount > 0)) {
+                throw new Error('deposit amount must be positive');
+            }
+            const account = store.get(id) as Account | undefined;
+            if (!account) {
+                throw new Error(`account "${id}" not found`);
+            }
+            account.balance += amount;
+            store.put(id, account);
+            return { result: account };
+        },
+
+        /**
+         * Read the single account by key, subtract `amount`, write it back. The
+         * single-key debit half of a CROSS-SHARD transfer (ADR-0020): a cross-shard
+         * transfer cannot be one `transfer` command (the two accounts live in
+         * different Raft groups), so the saga composes `withdraw` on the source
+         * shard with `deposit` on the target shard. Rejects (non-200) on a missing
+         * account or insufficient balance so the saga can compensate; because the
+         * host commits the StoreView only on a clean return, a rejection writes
+         * nothing — the balance is unchanged, never partially debited.
+         */
+        withdraw: (store, input) => {
+            const { id, amount } = (input ?? {}) as { id: string; amount: number };
+            if (!(amount > 0)) {
+                throw new Error('withdraw amount must be positive');
+            }
+            const account = store.get(id) as Account | undefined;
+            if (!account) {
+                throw new Error(`account "${id}" not found`);
+            }
+            if (account.balance < amount) {
+                throw new Error(`insufficient balance in "${id}"`);
+            }
+            account.balance -= amount;
+            store.put(id, account);
+            return { result: account };
+        },
+
+        /**
+         * Move funds between two accounts, touching ONLY the two involved keys.
+         * Rejects on a missing account or insufficient balance; because the host
+         * commits the StoreView atomically, a rejection (thrown) writes neither
+         * side — no partial transfer.
+         */
+        transfer: (store, input) => {
+            const { from, to, amount } = (input ?? {}) as { from: string; to: string; amount: number };
+            if (!(amount > 0)) {
+                throw new Error('transfer amount must be positive');
+            }
+            if (from === to) {
+                throw new Error('cannot transfer to the same account');
+            }
+            const src = store.get(from) as Account | undefined;
+            if (!src) {
+                throw new Error(`account "${from}" not found`);
+            }
+            const dst = store.get(to) as Account | undefined;
+            if (!dst) {
+                throw new Error(`account "${to}" not found`);
+            }
+            if (src.balance < amount) {
+                throw new Error(`insufficient balance in "${from}"`);
+            }
+            src.balance -= amount;
+            dst.balance += amount;
+            store.put(from, src);
+            store.put(to, dst);
+            return { result: { from: src, to: dst } };
+        },
+    },
+    queries: {
+        /** Balance of one account, or `undefined` if it does not exist. */
+        balance: (store, args) => {
+            const { id } = (args ?? {}) as { id: string };
+            const account = store.get(id) as Account | undefined;
+            return account?.balance;
+        },
+        /** Number of accounts in the ledger. */
+        count: (store) => store.size(),
+    },
+});
diff --git a/src/runtime/modules/compute.ts b/src/runtime/modules/compute.ts
new file mode 100644
index 0000000..f7cf962
--- /dev/null
+++ b/src/runtime/modules/compute.ts
@@ -0,0 +1,68 @@
+import { defineModule } from '../defineModule';
+
+/**
+ * A demo SANDBOXED whole-state module (ADR-0019 pillars 2, 6 — M9). Defined with
+ * `{ sandbox: true }`, so the host re-compiles each reducer into a frozen `vm`
+ * context at registration: the reducers below run with NO ambient `Date`,
+ * `Math.random`, `crypto`, timers, etc. — determinism is enforced STRUCTURALLY,
+ * not merely by the static lint.
+ *
+ * Reducers MUST be self-contained arrow/`function` expressions that reference
+ * only their parameters, `ctx`, and the curated safe globals (`Object`, `Array`,
+ * `Math` without `random`, `JSON`, …). They derive any time/randomness/ids from
+ * `ctx`, exactly like the non-sandboxed demos — the sandbox merely makes a
+ * regression (reaching for `Date.now()`) impossible to apply silently.
+ */
+interface ComputeState {
+    /** Last computed value (deterministic). */
+    last: number;
+    /** A deterministic id stamped on the last computation, from `ctx.id()`. */
+    lastId: string;
+}
+
+/** Input to `sumTo`: compute the bounded sum 1..n. */
+interface SumToInput {
+    n: number;
+}
+
+export const compute = defineModule<ComputeState>(
+    {
+        name: 'compute',
+        initialState: () => ({ last: 0, lastId: '' }),
+        commands: {
+            // A terminating arithmetic reducer: the triangular number 1..n. Bounded
+            // by a hard cap so it always halts well within any step budget. Uses
+            // ctx.id() (deterministic) — never crypto/Date — to stamp the result.
+            sumTo: (state, input, ctx) => {
+                const n = Math.max(0, Math.min(((input ?? {}) as SumToInput).n | 0, 100000));
+                let total = 0;
+                for (let i = 1; i <= n; i++) {
+                    total += i;
+                }
+                return {
+                    state: { last: total, lastId: ctx.id() },
+                    result: { sum: total },
+                };
+            },
+            // TEST FIXTURE ONLY — do not call on a real apply path. `spin` loops
+            // forever; it exists solely to prove the leader-side admission step
+            // meter (`ModuleHost.admit`) interrupts a runaway reducer via the vm
+            // timeout and rejects it BEFORE it could enter the log. On the apply
+            // path (no timeout) this would hang, which is precisely why admission
+            // must reject it first.
+            spin: (state) => {
+                // eslint-disable-next-line no-constant-condition
+                while (true) {
+                    // busy-loop; the vm wall-clock budget interrupts this during
+                    // the leader's admission dry-run.
+                }
+                // Unreachable, but keeps the reducer a well-formed expression.
+                return { state };
+            },
+        },
+        queries: {
+            last: (state) => state.last,
+        },
+    },
+    { sandbox: true },
+);
diff --git a/src/runtime/modules/counter.ts b/src/runtime/modules/counter.ts
new file mode 100644
index 0000000..fa8b30f
--- /dev/null
+++ b/src/runtime/modules/counter.ts
@@ -0,0 +1,26 @@
+import { defineModule } from '../defineModule';
+
+/** A trivial demo module: a single integer with increment/reset commands. */
+interface CounterState {
+    value: number;
+}
+
+/** Input to `increment`: an optional step (defaults to 1). */
+interface IncrementInput {
+    by?: number;
+}
+
+export const counter = defineModule<CounterState>({
+    name: 'counter',
+    initialState: () => ({ value: 0 }),
+    commands: {
+        increment: (state, input) => {
+            const { by = 1 } = (input ?? {}) as IncrementInput;
+            return { state: { value: state.value + by } };
+        },
+        reset: () => ({ state: { value: 0 } }),
+    },
+    queries: {
+        value: (state) => state.value,
+    },
+});
diff --git a/src/runtime/modules/notes.ts b/src/runtime/modules/notes.ts
new file mode 100644
index 0000000..b5f921e
--- /dev/null
+++ b/src/runtime/modules/notes.ts
@@ -0,0 +1,45 @@
+import { defineModule } from '../defineModule';
+
+/** A single note. Its `id` and `createdAt` come from the deterministic ctx. */
+interface Note {
+    id: string;
+    text: string;
+    createdAt: string;
+}
+
+interface NotesState {
+    notes: Note[];
+}
+
+/** Input to `create`. */
+interface CreateInput {
+    text: string;
+}
+
+/**
+ * A demo module that mints ids and timestamps. It exists to prove the central
+ * runtime contract: the id and createdAt below are NOT pulled from `crypto` or
+ * `Date` inside the reducer — they flow from `ctx`, which is rebuilt identically
+ * on every replica from the leader-resolved seed. Replace ctx with ambient
+ * randomness here and the convergence test would fail.
+ */
+export const notes = defineModule<NotesState>({
+    name: 'notes',
+    initialState: () => ({ notes: [] }),
+    commands: {
+        create: (state, input, ctx) => {
+            const { text } = (input ?? {}) as CreateInput;
+            const note: Note = {
+                id: ctx.id(),
+                text,
+                createdAt: ctx.now,
+            };
+            // Return the created note as the explicit `result` so callers get it
+            // directly at `apply().result`, decoupled from the next-state object.
+            return { state: { notes: [...state.notes, note] }, result: note };
+        },
+    },
+    queries: {
+        list: (state) => state.notes,
+    },
+});
diff --git a/src/runtime/modules/payments.ts b/src/runtime/modules/payments.ts
new file mode 100644
index 0000000..2a1bb53
--- /dev/null
+++ b/src/runtime/modules/payments.ts
@@ -0,0 +1,78 @@
+import { defineModule } from '../defineModule';
+
+/** An order tracked through the charge lifecycle. */
+interface Order {
+    id: string;
+    amount: number;
+    status: 'pending' | 'paid' | 'failed';
+}
+
+interface PaymentsState {
+    orders: Record<string, Order>;
+}
+
+/** Input to `charge`. */
+interface ChargeInput {
+    orderId: string;
+    amount: number;
+}
+
+/**
+ * Input to `settle`, the `onResult` target. The host feeds it the effect's key
+ * plus the edge-resolved `result`; here `result` carries the gateway outcome.
+ */
+interface SettleInput {
+    idempotencyKey: string;
+    result: { orderId: string; ok: boolean };
+}
+
+/**
+ * A demo module for the committed-intent effect model (ADR-0019 pillar 3).
+ *
+ * `charge` is a pure reducer: it records the order as `pending` and EMITS an
+ * effect intent rather than calling a payment gateway itself. The intent commits
+ * to the log via the outbox; the edge executor performs the real charge and feeds
+ * the outcome back, which `settle` folds into state. Both reducers are pure — the
+ * only side effect lives in the executor's handler, off the deterministic path.
+ */
+export const payments = defineModule<PaymentsState>({
+    name: 'payments',
+    initialState: () => ({ orders: {} }),
+    commands: {
+        charge: (state, input, ctx) => {
+            const { orderId, amount } = (input ?? {}) as ChargeInput;
+            const order: Order = { id: orderId, amount, status: 'pending' };
+            return {
+                state: { orders: { ...state.orders, [orderId]: order } },
+                result: order,
+                // `ctx.id()` is deterministic, so the idempotency key is identical
+                // on every replica — the outbox dedups consistently across hosts.
+                effects: [
+                    {
+                        kind: 'http',
+                        idempotencyKey: ctx.id(),
+                        payload: { orderId, amount },
+                        onResult: { module: 'payments', command: 'settle' },
+                    },
+                ],
+            };
+        },
+        settle: (state, input) => {
+            const { result } = (input ?? {}) as SettleInput;
+            const existing = state.orders[result.orderId];
+            if (!existing) {
+                // Nothing to settle; return state unchanged. Pure no-op.
+                return { state };
+            }
+            const settled: Order = { ...existing, status: result.ok ? 'paid' : 'failed' };
+            return {
+                state: { orders: { ...state.orders, [result.orderId]: settled } },
+                result: settled,
+            };
+        },
+    },
+    queries: {
+        order: (state, args) => state.orders[(args as { orderId: string }).orderId],
+        list: (state) => Object.values(state.orders),
+    },
+});
diff --git a/src/runtime/projection.ts b/src/runtime/projection.ts
new file mode 100644
index 0000000..29b07ae
--- /dev/null
+++ b/src/runtime/projection.ts
@@ -0,0 +1,98 @@
+/**
+ * CQRS read-projection layer (ADR-0019 pillar 4: "Reads / rich queries / big
+ * data").
+ *
+ * The authoritative state is the replicated log applied by `ModuleHost` — that
+ * is the source of truth and the only thing consensus commits. This layer is the
+ * OTHER half of CQRS: a DERIVED, indexed read model built by folding the stream
+ * of committed module commands. It exists to answer rich queries the raw module
+ * state does not index (e.g. "all note ids by a given actor") without bloating
+ * the deterministic apply path or the snapshots the consensus core replicates.
+ *
+ * WHY SEPARATE: a projection is a CACHE, not a database. It is never consulted
+ * to decide whether a command is valid, never feeds back into the log, and can
+ * be thrown away and rebuilt from the committed command stream at any time
+ * (`ProjectionHost.rebuild`). Because the fold is PURE and DETERMINISTIC, every
+ * node that replays the same committed stream reconstructs an identical read
+ * model — read-scaling that preserves ADR-0002 (the log stays source of truth).
+ */
+
+/**
+ * A committed module command as seen by the READ side. This is the projection
+ * layer's input event: the same information the write path produced when it
+ * applied the command, captured AFTER commit so the read model only ever folds
+ * facts that consensus already agreed on.
+ *
+ * It is derived from a `ModuleHost.apply` outcome plus the command's meta — see
+ * the projection tests for how a host apply result maps onto one of these. The
+ * read side intentionally does not depend on `ModuleHost` internals; it consumes
+ * this flat, serializable event so a projection could equally be fed from a log
+ * reader, a replication stream, or a test harness.
+ */
+export interface ProjectionEvent {
+    /** Monotonic position of the command in the committed stream (its log order). */
+    seq: number;
+    /** Target module name. */
+    module: string;
+    /** Command name dispatched on the module. */
+    command: string;
+    /** The command input that was committed. */
+    input: unknown;
+    /** The explicit result the reducer surfaced (e.g. the created entity). */
+    result: unknown;
+    /** Who issued the command (from `CommandMeta`). */
+    actor: string;
+    /** Request correlation id (from `CommandMeta`). */
+    requestId: string;
+    /** Leader-resolved commit timestamp, if the caller chose to carry it. */
+    timestamp?: string;
+}
+
+/**
+ * A declarative read projection over the committed command stream.
+ *
+ * `on` is a PURE fold: given the current view and the next committed event, it
+ * returns the NEXT view. It is responsible for its own filtering — it inspects
+ * `event.module` / `event.command` and ignores events it does not care about by
+ * returning the view unchanged. Keeping the fold pure (no `Date`, no
+ * `Math.random`, no I/O) is what makes the read model rebuildable and convergent
+ * across nodes: the same committed stream always folds to the same view.
+ *
+ * `queries` are read-only views over the folded state — the rich, indexed answers
+ * the projection exists to provide. They never mutate the view.
+ */
+export interface ProjectionDefinition<V> {
+    name: string;
+    /** Builds the empty initial view. A factory so each host gets its own copy. */
+    init: () => V;
+    /** Pure fold: next view given the current view and the next committed event. */
+    on: (view: V, event: ProjectionEvent) => V;
+    /** Named read-only queries over the folded view. */
+    queries: Record<string, (view: V, args: unknown) => unknown>;
+}
+
+/**
+ * Validate and return a projection definition. Like `defineModule`, this is an
+ * identity with light validation so a malformed projection fails loud at
+ * definition time rather than as a confusing fold/query error later.
+ *
+ * Requirements: a non-empty name (projections are registered by name and
+ * duplicates are rejected) and at least one query (a projection with no queries
+ * is dead weight — it folds a view nothing can read).
+ */
+export function defineProjection<V>(def: ProjectionDefinition<V>): ProjectionDefinition<V> {
+    if (!def.name || def.name.trim() === '') {
+        throw new Error('Projection definition requires a non-empty name');
+    }
+    if (typeof def.init !== 'function') {
+        throw new Error(`Projection "${def.name}" requires an init() factory`);
+    }
+    if (typeof def.on !== 'function') {
+        throw new Error(`Projection "${def.name}" requires an on() fold`);
+    }
+    const queryNames = Object.keys(def.queries ?? {});
+    if (queryNames.length === 0) {
+        throw new Error(`Projection "${def.name}" must define at least one query`);
+    }
+    return def;
+}
diff --git a/src/runtime/projectionHost.ts b/src/runtime/projectionHost.ts
new file mode 100644
index 0000000..d6ce48c
--- /dev/null
+++ b/src/runtime/projectionHost.ts
@@ -0,0 +1,132 @@
+import { canonicalJson } from './canonical';
+import { ProjectionDefinition, ProjectionEvent } from './projection';
+
+/**
+ * The READ side of CQRS (ADR-0019 pillar 4). A registry of projections plus their
+ * live, folded views, fed by the stream of committed module commands.
+ *
+ * It is architecturally SEPARATE from the authoritative state:
+ *  - It never participates in the consensus apply path — `ModuleHost` and the
+ *    replicated log remain the single source of truth.
+ *  - It is a DERIVED cache: every view here can be discarded and reconstructed
+ *    from the committed command stream via `rebuild`. That is the proof the read
+ *    model is not authoritative — if the cache and a from-scratch replay disagree,
+ *    the replay (the log) wins, by construction.
+ *  - Folds are pure and deterministic (`ProjectionDefinition.on`), so two hosts
+ *    fed the same committed stream converge to deep-equal views — the same
+ *    discipline the deterministic core uses, applied to the read model so read
+ *    replicas stay consistent.
+ */
+export class ProjectionHost {
+    private readonly defs = new Map<string, ProjectionDefinition<unknown>>();
+    /** Live per-projection view, keyed by projection name. */
+    private readonly views = new Map<string, unknown>();
+
+    /** Register a projection and initialize its view. Throws on a duplicate name. */
+    register<V>(def: ProjectionDefinition<V>): void {
+        if (!def.name || def.name.trim() === '') {
+            throw new Error('Projection definition requires a non-empty name');
+        }
+        if (this.defs.has(def.name)) {
+            throw new Error(`Projection "${def.name}" is already registered`);
+        }
+        this.defs.set(def.name, def as ProjectionDefinition<unknown>);
+        this.views.set(def.name, def.init());
+    }
+
+    /**
+     * Fold one committed event into every registered projection's view. Each
+     * projection's `on` decides whether the event is relevant (it filters on
+     * `event.module` / `event.command` internally) and returns the next view,
+     * which we adopt. Because `on` is pure, applying the same event on every
+     * replica produces the same next view.
+     */
+    applyEvent(event: ProjectionEvent): void {
+        for (const [name, def] of this.defs) {
+            const next = def.on(this.views.get(name), event);
+            this.views.set(name, next);
+        }
+    }
+
+    /** Run a named query against a projection's current view. Never mutates. */
+    query(projection: string, name: string, args?: unknown): unknown {
+        const def = this.defs.get(projection);
+        if (!def) {
+            throw new Error(`Unknown projection: ${projection}`);
+        }
+        const q = def.queries[name];
+        if (!q) {
+            throw new Error(`Unknown query "${name}" on projection "${projection}"`);
+        }
+        return q(this.views.get(projection), args);
+    }
+
+    /**
+     * Discard ALL views and rebuild them from scratch by replaying the committed
+     * event stream in order. This is the heart of CQRS: it proves the read model
+     * is DERIVED, not authoritative. A node can drop its cache (or join fresh) and
+     * reconstruct byte-identical views purely from the log, because each `on` fold
+     * is pure and deterministic.
+     *
+     * Every registered projection is reset to its `init()` view first, so the
+     * rebuilt state depends ONLY on the supplied events, never on whatever the
+     * cache happened to hold before.
+     */
+    rebuild(events: ProjectionEvent[]): void {
+        for (const [name, def] of this.defs) {
+            this.views.set(name, def.init());
+        }
+        for (const event of events) {
+            this.applyEvent(event);
+        }
+    }
+
+    /**
+     * A serializable, deep-cloned map of projection name -> current view, with
+     * keys emitted in sorted order so a `JSON.stringify` over the snapshot is
+     * stable regardless of registration order (mirrors `ModuleHost.snapshot`).
+     *
+     * NOTE: the snapshot is a CACHE CONVENIENCE — a way to warm-start a read
+     * replica without replaying the whole stream. It is NOT a source of truth.
+     * Correctness of the read model comes from `rebuild` (replay from the log);
+     * a snapshot is only ever as trustworthy as the stream that produced it.
+     */
+    snapshot(): Record<string, unknown> {
+        const out: Record<string, unknown> = {};
+        for (const name of [...this.views.keys()].sort()) {
+            out[name] = clone(this.views.get(name));
+        }
+        return out;
+    }
+
+    /**
+     * Replace the views of registered projections from a snapshot. Only
+     * projections that are both registered AND present in the snapshot are
+     * restored; unknown snapshot keys are ignored and absent projections keep
+     * their `init()` view. As with `snapshot`, this is a cache warm-start — the
+     * authoritative path remains `rebuild` from the committed stream.
+     */
+    restore(snap: Record<string, unknown>): void {
+        for (const name of this.defs.keys()) {
+            if (Object.prototype.hasOwnProperty.call(snap, name)) {
+                this.views.set(name, clone(snap[name]));
+            }
+        }
+    }
+
+    /** Current live view of a projection (for tests/inspection). */
+    getView(projection: string): unknown {
+        return this.views.get(projection);
+    }
+}
+
+/**
+ * Deep-clone a view through the shared canonical serializer. Routing the clone
+ * through `canonicalJson` (rather than a second `JSON.stringify`) keeps this
+ * layer on the ONE sanctioned serializer — the same reason `canonical.ts` exists
+ * — and yields a sorted-key, drift-free copy decoupled from the live view, so a
+ * caller that mutates a returned snapshot cannot reach back into host state.
+ */
+function clone<T>(value: T): T {
+    return JSON.parse(canonicalJson(value, { onUndefined: 'drop' })) as T;
+}
diff --git a/src/runtime/projections/noteIndex.ts b/src/runtime/projections/noteIndex.ts
new file mode 100644
index 0000000..c69a5cf
--- /dev/null
+++ b/src/runtime/projections/noteIndex.ts
@@ -0,0 +1,68 @@
+import { defineProjection, ProjectionEvent } from '../projection';
+
+/**
+ * The note created by the `notes` module's `create` reducer — the shape that
+ * arrives on `event.result` for a `notes.create` command. We only need the id
+ * here; the rest of the note lives in the authoritative module state.
+ */
+interface CreatedNote {
+    id: string;
+}
+
+/**
+ * The folded read model: note ids grouped by the actor who created them, plus a
+ * running total. This is exactly the index the authoritative `notes` module does
+ * NOT keep — its state is a flat `notes` array, so answering "which notes did
+ * actor X create?" against the module would be an O(n) scan on every call. The
+ * projection maintains the grouping incrementally so the query is O(1) lookup.
+ */
+interface NoteIndexView {
+    byActor: Record<string, string[]>;
+    total: number;
+}
+
+/**
+ * A demo CQRS projection over the `notes` module (ADR-0019 pillar 4).
+ *
+ * It proves the value of the read side: a derived, indexed view that answers
+ * rich queries the raw module state cannot, built purely by folding the committed
+ * command stream. The fold is pure and deterministic — no `Date`, no
+ * `Math.random` — so the index is rebuildable from the log and convergent across
+ * nodes.
+ */
+export const noteIndex = defineProjection<NoteIndexView>({
+    name: 'noteIndex',
+    init: () => ({ byActor: {}, total: 0 }),
+    on: (view, event: ProjectionEvent) => {
+        // Filter inside the fold: ignore everything but a successful note creation.
+        if (event.module !== 'notes' || event.command !== 'create') {
+            return view;
+        }
+        const note = event.result as CreatedNote | undefined;
+        // Defensive: a create whose result lacks an id (e.g. a rejected apply that
+        // still reached the read side) contributes nothing rather than corrupting
+        // the index. The fold stays total and pure.
+        if (!note || typeof note.id !== 'string') {
+            return view;
+        }
+
+        // Build the NEXT view immutably — never mutate the input view in place, so
+        // the fold is a clean pure function and replay is reproducible.
+        const existing = view.byActor[event.actor] ?? [];
+        return {
+            byActor: { ...view.byActor, [event.actor]: [...existing, note.id] },
+            total: view.total + 1,
+        };
+    },
+    queries: {
+        /** Note ids created by `actor`, in creation order (empty if none). */
+        byActor: (view, args): string[] => {
+            const actor = args as string;
+            return view.byActor[actor] ?? [];
+        },
+        /** Total number of notes created across all actors. */
+        total: (view): number => view.total,
+        /** The set of actors that have created at least one note, sorted. */
+        actors: (view): string[] => Object.keys(view.byActor).sort(),
+    },
+});
diff --git a/src/runtime/saga.ts b/src/runtime/saga.ts
new file mode 100644
index 0000000..7d08af2
--- /dev/null
+++ b/src/runtime/saga.ts
@@ -0,0 +1,104 @@
+/**
+ * Saga coordinator for cross-shard transactions (ADR-0020, M10).
+ *
+ * A multi-Raft cluster has NO single log spanning shards, so an operation that
+ * touches two shards (e.g. debit an account on shard A, credit one on shard B)
+ * cannot be one atomic append. ADR-0020 chooses a SAGA over two-phase commit:
+ * run the per-shard steps forward, and if one fails, run the COMPENSATING action
+ * for each step that already succeeded, in reverse order.
+ *
+ * This yields **atomicity by eventual compensation, NOT isolation**: intermediate
+ * states are visible (the debit lands before the credit; on failure the credit
+ * never happens and the debit is undone). That exposure is the accepted trade-off
+ * for keeping each shard independently available (ADR-0020 "Why saga, not 2PC").
+ *
+ * Idempotency requirement: each `invoke` and each `compensate` is an ordinary,
+ * single-shard MODULE command carrying a `requestId`, so the runtime's exactly-
+ * once dedup makes steps safe to RETRY and compensations safe to REPLAY. Authors
+ * MUST give distinct requestIds to the forward and compensating legs (they are
+ * different commands) but keep each leg's requestId stable across retries.
+ */
+
+/**
+ * One step of a saga. `invoke` performs the forward action; `compensate` undoes
+ * it. `compensate` runs ONLY for steps whose `invoke` already resolved, and only
+ * when a LATER step fails — so it must semantically reverse a committed `invoke`
+ * (e.g. `invoke` = withdraw, `compensate` = deposit the same amount back).
+ */
+export interface SagaStep {
+    name: string;
+    invoke: () => Promise<unknown>;
+    compensate: () => Promise<void>;
+}
+
+/** Outcome of {@link runSaga}: a discriminated union on `ok`. */
+export type SagaResult =
+    | { ok: true; results: unknown[] }
+    | {
+          ok: false;
+          failedAt: string;
+          error: Error;
+          /** Steps whose compensation RAN SUCCESSFULLY (in compensation order). */
+          compensated: string[];
+          /**
+           * Steps whose compensation itself THREW — their forward effect was NOT
+           * undone, so this is the real "funds may be lost" signal a conservation-
+           * critical coordinator must not hide. Empty on a clean compensation.
+           */
+          compensationFailures: { step: string; error: Error }[];
+      };
+
+/**
+ * Run `steps` forward in order. On the FIRST failing `invoke`, stop and run
+ * `compensate` for every step that already SUCCEEDED, in REVERSE order, then
+ * report which steps were compensated.
+ *
+ * - Success: `{ ok: true, results }` with one entry per step (in step order).
+ * - Failure: `{ ok: false, failedAt, error, compensated, compensationFailures }`.
+ *   `failedAt` is the step whose `invoke` threw; `compensated` lists the steps
+ *   whose compensation RAN SUCCESSFULLY (in the order their compensations ran —
+ *   reverse of success). The failing step itself is NOT compensated (its `invoke`
+ *   did not commit).
+ *
+ * A compensation that itself throws does NOT block the remaining compensations
+ * (best-effort unwind), but it is NOT silently swallowed: the step is recorded in
+ * `compensationFailures` and kept OUT of `compensated`, so a conservation-critical
+ * caller can detect that a forward effect was never undone (real fund loss) and
+ * park it for retry. A non-empty `compensationFailures` means the saga did not
+ * fully unwind.
+ */
+export async function runSaga(steps: SagaStep[]): Promise<SagaResult> {
+    const results: unknown[] = [];
+    // Steps whose invoke committed, in execution order; compensated in reverse.
+    const succeeded: SagaStep[] = [];
+
+    for (const step of steps) {
+        try {
+            const result = await step.invoke();
+            results.push(result);
+            succeeded.push(step);
+        } catch (err) {
+            const error = err instanceof Error ? err : new Error(String(err));
+            const compensated: string[] = [];
+            const compensationFailures: { step: string; error: Error }[] = [];
+            // Undo committed steps in REVERSE order so dependencies unwind safely.
+            for (let i = succeeded.length - 1; i >= 0; i--) {
+                const done = succeeded[i];
+                try {
+                    await done.compensate();
+                    compensated.push(done.name);
+                } catch (cerr) {
+                    // A failed compensation must not block the others, but it means
+                    // this step's effect was NOT undone — surface it, don't hide it.
+                    compensationFailures.push({
+                        step: done.name,
+                        error: cerr instanceof Error ? cerr : new Error(String(cerr)),
+                    });
+                }
+            }
+            return { ok: false, failedAt: step.name, error, compensated, compensationFailures };
+        }
+    }
+
+    return { ok: true, results };
+}
diff --git a/src/runtime/sandbox.ts b/src/runtime/sandbox.ts
new file mode 100644
index 0000000..0289344
--- /dev/null
+++ b/src/runtime/sandbox.ts
@@ -0,0 +1,378 @@
+/**
+ * `vm`-based determinism sandbox + leader-side step/CPU meter (ADR-0019
+ * pillars 2 and 6). This is the M9 hardening the prototype-status note flagged
+ * as missing: M5 enforced determinism with a STATIC lint (bypassable by
+ * obfuscation) and a deterministic size/write bound; this module adds the
+ * STRUCTURAL enforcement the lint stands in for, plus the preemptive CPU/step
+ * gas meter the deterministic size bound could not provide.
+ *
+ * Two distinct guarantees, two execution entry points:
+ *
+ *  1. STRUCTURAL DETERMINISM (apply path). `compileReducer` re-evaluates a
+ *     self-contained reducer source inside a fresh `vm` context whose globals are
+ *     a curated, frozen SAFE set — `Date`, `Intl`, `Math.random`, `process`,
+ *     `require`, timers, `crypto`, `console`, `performance` are ABSENT, and the
+ *     locale-sensitive `toLocale*` / `localeCompare` prototype methods on the kept
+ *     `Number`/`String`/`Array`/`Object` intrinsics are replaced by throwers. A
+ *     reducer that reaches for `Date.now()` or `(1).toLocaleString()` therefore
+ *     throws at runtime, even if it slipped past the static lint. This list is the
+ *     set of footguns the sandbox ENFORCES, not a proof of total purity (the file
+ *     header below is explicit that this is a determinism aid, not a security
+ *     boundary). `runReducer` executes this on the
+ *     apply path with NO timeout: the call must be a pure, deterministic function
+ *     of (state, input, ctx) so every replica computes the identical result. A
+ *     wall-clock timeout there would be non-deterministic (it could trip on one
+ *     replica and not another) and is therefore forbidden on the apply path.
+ *
+ *  2. CPU/STEP GAS (admission, leader-only). `runReducerWithBudget` executes the
+ *     SAME compiled reducer but inside the vm with `{ timeout: budgetMs }`, so a
+ *     runaway synchronous reducer (an infinite loop) is interrupted and rejected
+ *     as `BudgetExceededError`. The leader runs this as a pre-log DRY RUN against
+ *     a clone of current state; an over-budget command is rejected BEFORE it ever
+ *     enters the log. Followers never re-run the meter — under the CFT trust
+ *     model they trust that a committed entry was already admitted by the leader,
+ *     so the apply path stays timeout-free and deterministic.
+ *
+ * HONEST SCOPING — NOT A SECURITY BOUNDARY. Node's own docs are explicit that
+ * the `vm` module is "not a security mechanism. Do not use it to run untrusted
+ * code." A vm context still exposes `Function`/`eval` (and a sufficiently
+ * determined script can break out), so this sandbox does NOT defend against
+ * MALICIOUS reducers. Its goal is to enforce determinism against HONEST-BUT-
+ * CARELESS reducers — the author who reflexively reaches for `Date.now()` — by
+ * making the non-deterministic globals structurally unavailable. This mirrors
+ * the honest caveats already stated for the M3 code-hash and the M5 static lint.
+ */
+
+import * as vm from 'vm';
+import { ReducerContext, ReducerResult } from './types';
+
+/**
+ * Thrown when the admission step meter (`runReducerWithBudget`) interrupts a
+ * reducer that ran past its wall-clock budget. Distinct error type so the host's
+ * `admit()` can map it to a dedicated rejection status (503) rather than lumping
+ * it in with an ordinary reducer throw.
+ */
+export class BudgetExceededError extends Error {
+    constructor(message: string) {
+        super(message);
+        this.name = 'BudgetExceededError';
+    }
+}
+
+/**
+ * A compiled, sandboxed reducer handle. Holds the vm context (with its frozen
+ * safe globals) and the reducer function evaluated INSIDE that context, so every
+ * free identifier the reducer references resolves against the sandbox globals —
+ * a banned global is simply absent and throws `ReferenceError` when touched.
+ */
+export interface CompiledReducer {
+    /** Run on the apply path: no timeout, deterministic. */
+    run(state: unknown, input: unknown, ctx: ReducerContext): ReducerResult<unknown>;
+    /** Run on the admission/dry-run path: vm-enforced wall-clock budget. */
+    runWithBudget(
+        state: unknown,
+        input: unknown,
+        ctx: ReducerContext,
+        budgetMs: number,
+    ): ReducerResult<unknown>;
+}
+
+/**
+ * The curated safe-global posture, enforced by REMOVING non-deterministic
+ * intrinsics from a fresh `vm` context rather than by an allowlist.
+ *
+ * IMPORTANT IMPLEMENTATION NOTE: `vm.createContext` produces a fresh global
+ * object that V8 STILL populates with the standard built-in intrinsics
+ * (`Date`, `Math`, `Object`, …) — passing only an allowlist object does NOT
+ * unset them. So determinism is enforced by explicitly `delete`-ing the banned
+ * globals from the context's `globalThis` and neutralizing `Math.random`, run
+ * once per context as the setup script below.
+ *
+ * KEPT (deterministic builtins a reducer legitimately needs): `Object, Array,
+ * String, Number, Boolean, JSON, Math` (random removed), `isNaN, isFinite,
+ * parseInt, parseFloat` — these are left in place as standard intrinsics. Their
+ * locale-sensitive methods are neutralized in place (see NEUTRALIZED below) so
+ * the deterministic methods stay usable while the non-deterministic ones throw.
+ *
+ * REMOVED (so referencing them throws `ReferenceError`):
+ *  - `Date` — wall-clock; use `ctx.now`.
+ *  - `Intl` — locale/ICU formatting & collation are non-deterministic (vary by
+ *    host locale and ICU build); the global is deleted entirely.
+ *  - `Math.random` — randomness (the property is replaced by a thrower; the rest
+ *    of `Math` stays); use `ctx.random()`.
+ *  - `crypto` — entropy; use `ctx.id()` / `ctx.random()`.
+ *  - `process`, `require`, `module` — ambient environment / dynamic load.
+ *  - `setTimeout`, `setInterval`, `setImmediate`, `clearTimeout`,
+ *    `clearInterval`, `queueMicrotask` — timers / async scheduling have no place
+ *    on the synchronous deterministic path.
+ *  - `console` — I/O; reducers must be pure.
+ *  - `Reflect`, `Proxy` — common indirection paths to reach removed globals.
+ *  - `performance` — high-resolution wall clock.
+ *
+ * NEUTRALIZED (kept intrinsics whose locale-sensitive methods are replaced by a
+ * thrower, since deleting `Intl` alone does not close the gap — these methods
+ * ride on the kept `Number`/`String`/`Array`/`Object` prototypes):
+ *  - `Number.prototype.toLocaleString`, `Array.prototype.toLocaleString`,
+ *    `Object.prototype.toLocaleString` (and `BigInt.prototype.toLocaleString`
+ *    when present) — locale-dependent formatting; format deterministically.
+ *  - `String.prototype.localeCompare` — locale-dependent collation; compare with
+ *    deterministic ordering (`<`/`>`) instead.
+ *  - `String.prototype.toLocaleLowerCase`, `String.prototype.toLocaleUpperCase`
+ *    — locale-dependent case mapping; use `toLowerCase`/`toUpperCase`.
+ * The non-locale methods on these prototypes are untouched and remain usable.
+ *
+ * This is the documented footgun set the sandbox enforces, NOT a proof of total
+ * determinism — see the file header's HONEST SCOPING note.
+ *
+ * `globalThis` itself, `Function`, and `eval` CANNOT be meaningfully removed
+ * (they are intrinsic to any vm context and a script can re-derive them), which
+ * is exactly why this is a DETERMINISM aid and NOT a security boundary — see the
+ * file header.
+ */
+
+/** Globals deleted from each sandbox context's `globalThis`. */
+const BANNED_GLOBALS = [
+    'Date',
+    'Intl',
+    'crypto',
+    'process',
+    'require',
+    'module',
+    'setTimeout',
+    'setInterval',
+    'setImmediate',
+    'clearTimeout',
+    'clearInterval',
+    'queueMicrotask',
+    'console',
+    'Reflect',
+    'Proxy',
+    'performance',
+];
+
+/**
+ * Setup script run ONCE per context: delete every banned intrinsic from
+ * `globalThis`, replace `Math.random` with a thrower (leaving the rest of `Math`
+ * — all pure functions — intact), and neutralize the locale-sensitive `toLocale*`
+ * / `localeCompare` prototype methods that ride on the KEPT `Number`/`String`/
+ * `Array`/`Object` intrinsics. After this runs, a reducer body that references a
+ * banned global resolves it as a free identifier against the (now-missing) global
+ * and throws `ReferenceError`; `Math.random()` throws too; and any call to a
+ * locale-formatting method throws a clear `Error`.
+ *
+ * WHY the locale methods: removing the `Intl` global is not enough — methods like
+ * `Number.prototype.toLocaleString`, `String.prototype.localeCompare`, and
+ * `Array.prototype.toLocaleString` perform locale/ICU-dependent formatting and
+ * comparison that VARY across Node builds and host locales, so two replicas could
+ * compute different strings/orderings from the same input. That is exactly the
+ * non-determinism this sandbox must block, so each is replaced by a thrower while
+ * the deterministic methods on those prototypes stay intact.
+ */
+const SANDBOX_SETUP = `
+(() => {
+    const banned = ${JSON.stringify(BANNED_GLOBALS)};
+    for (const name of banned) {
+        try { delete globalThis[name]; } catch (_e) { /* non-configurable: ignore */ }
+    }
+    // Replace Math.random with a loud thrower (do NOT touch the shared real Math
+    // object outside this context — this mutates only this context's Math).
+    try {
+        Object.defineProperty(Math, 'random', {
+            configurable: true,
+            get() {
+                throw new ReferenceError(
+                    'Math.random is not available in the determinism sandbox (use ctx.random())',
+                );
+            },
+        });
+    } catch (_e) { /* ignore */ }
+    // Neutralize locale-sensitive prototype methods: replace each with a thrower
+    // so a reducer cannot reach locale/ICU-dependent formatting through the kept
+    // Number/String/Array/Object intrinsics. The deterministic methods on those
+    // prototypes are untouched.
+    const localeThrower = (label) => function () {
+        throw new Error(
+            label + ' is not available in the determinism sandbox: locale-aware ' +
+            'formatting/comparison is non-deterministic (it varies by host locale ' +
+            'and ICU build). Format via deterministic logic instead.',
+        );
+    };
+    const neutralize = (proto, method, label) => {
+        try {
+            Object.defineProperty(proto, method, {
+                configurable: true,
+                writable: true,
+                enumerable: false,
+                value: localeThrower(label),
+            });
+        } catch (_e) { /* non-configurable: ignore */ }
+    };
+    neutralize(Number.prototype, 'toLocaleString', 'Number.prototype.toLocaleString');
+    neutralize(String.prototype, 'localeCompare', 'String.prototype.localeCompare');
+    neutralize(String.prototype, 'toLocaleLowerCase', 'String.prototype.toLocaleLowerCase');
+    neutralize(String.prototype, 'toLocaleUpperCase', 'String.prototype.toLocaleUpperCase');
+    neutralize(Array.prototype, 'toLocaleString', 'Array.prototype.toLocaleString');
+    neutralize(Object.prototype, 'toLocaleString', 'Object.prototype.toLocaleString');
+    // BigInt may be absent depending on context shape; guard it. Its
+    // toLocaleString is equally locale-dependent.
+    try {
+        if (typeof BigInt !== 'undefined' && BigInt.prototype) {
+            neutralize(BigInt.prototype, 'toLocaleString', 'BigInt.prototype.toLocaleString');
+        }
+    } catch (_e) { /* ignore */ }
+})();
+`;
+
+/**
+ * Detect whether a stringified reducer is a STANDALONE expression we can wrap in
+ * parentheses and evaluate — i.e. an arrow function (`(s, i) => ...`) or a
+ * `function` expression (`function (s) { ... }`, named or anonymous). Method
+ * shorthand from an object literal stringifies as `name(args) { ... }`, which is
+ * NOT a valid standalone expression; we detect that and reject with a clear
+ * message rather than producing a confusing `SyntaxError` deep in the vm.
+ */
+function isStandaloneFunctionSource(source: string): boolean {
+    const trimmed = source.trim();
+    // Arrow function: contains `=>` before any `{` body and starts with `(` or an
+    // identifier/`async`. A function expression starts with `function`/`async`.
+    if (/^async\s+function\b/.test(trimmed)) return true;
+    if (/^function\b/.test(trimmed)) return true;
+    // Arrow: `(...) =>` or `ident =>` or `async (...) =>`.
+    if (/^async\s*\(/.test(trimmed) || /^\(/.test(trimmed) || /^[A-Za-z_$][\w$]*\s*=>/.test(trimmed)) {
+        // Must actually be an arrow (has `=>`), not a parenthesized non-function.
+        if (trimmed.includes('=>')) return true;
+    }
+    return false;
+}
+
+/**
+ * Compile a SELF-CONTAINED reducer source into the determinism sandbox.
+ *
+ * `source` is typically `fn.toString()` of an arrow or `function` expression
+ * that references ONLY its parameters, the safe globals, and `ctx` — it must not
+ * close over helpers defined outside itself (the vm re-evaluates the text in a
+ * fresh context, so any free identifier that is not a safe global is undefined /
+ * throws). The demo `compute` module uses arrow expressions, satisfying this.
+ *
+ * The returned handle exposes both execution modes; the host picks `run` for the
+ * apply path and `runWithBudget` for leader-side admission.
+ */
+export function compileReducer(source: string): CompiledReducer {
+    if (!isStandaloneFunctionSource(source)) {
+        throw new Error(
+            'Sandboxed reducer source is not a standalone function expression. ' +
+                'Sandboxed reducers must be written as arrow functions or `function` ' +
+                'expressions (not object method-shorthand), and must be self-contained ' +
+                '(reference only their parameters, ctx, and the safe globals).',
+        );
+    }
+
+    // Fresh context, then STRIP the banned intrinsics (createContext leaves the
+    // standard globals in place, so removal — not an allowlist — is what makes
+    // them absent). After setup, a banned global referenced in the reducer body
+    // throws `ReferenceError`.
+    const context = vm.createContext({});
+    vm.runInContext(SANDBOX_SETUP, context);
+
+    // Evaluate the reducer expression and stash it on the context as `__reducer`.
+    // Parenthesize the source so an arrow/function expression is an expression,
+    // not a statement (a bare `function foo(){}` would be a declaration).
+    vm.runInContext(`globalThis.__reducer = (${source});`, context);
+
+    /**
+     * Build the per-call invocation. We pass `state`/`input`/`ctx` INTO the
+     * context as globals and call `__reducer` there, so the reducer body runs
+     * with the sandbox's resolution rules. `ctx` carries closures (`random`,
+     * `id`) created OUTSIDE the context; calling them is fine — they execute in
+     * their own (deterministic) closure, the sandbox only governs the free
+     * identifiers in the reducer's own body.
+     */
+    const invoke = (
+        state: unknown,
+        input: unknown,
+        ctx: ReducerContext,
+        timeoutMs?: number,
+    ): ReducerResult<unknown> => {
+        // Publish the call arguments as context globals. They are plain data /
+        // the ctx capability object; assigning them does not widen the sandbox's
+        // ambient surface for the reducer's free identifiers.
+        (context as Record<string, unknown>).__state = state;
+        (context as Record<string, unknown>).__input = input;
+        (context as Record<string, unknown>).__ctx = ctx;
+
+        const options: vm.RunningScriptOptions = {};
+        // Only the admission path passes a timeout: a positive budget arms the
+        // vm's wall-clock interrupt. The apply path passes none, so the call runs
+        // to completion deterministically.
+        if (timeoutMs !== undefined) {
+            options.timeout = timeoutMs;
+        }
+
+        try {
+            return vm.runInContext(
+                '__reducer(__state, __input, __ctx)',
+                context,
+                options,
+            ) as ReducerResult<unknown>;
+        } catch (err) {
+            // The vm throws a timeout error whose `code` is
+            // `ERR_SCRIPT_EXECUTION_TIMEOUT` and whose message includes "timed
+            // out" when the budget trips. We deliberately DO NOT use
+            // `instanceof Error` to detect it: the vm fabricates that error in a
+            // DIFFERENT V8 realm, so `instanceof Error` is FALSE for it in this
+            // realm (cross-realm prototype identity). We sniff `code`/`message`
+            // duck-typed instead. Normalize that single case to
+            // `BudgetExceededError`; every other throw (including a reducer's
+            // `ReferenceError` for a banned global) propagates as-is so the apply
+            // path can audit it as an ordinary 500.
+            const e = err as { code?: string; message?: string };
+            if (
+                timeoutMs !== undefined &&
+                (e?.code === 'ERR_SCRIPT_EXECUTION_TIMEOUT' ||
+                    (typeof e?.message === 'string' && /timed out/i.test(e.message)))
+            ) {
+                throw new BudgetExceededError(
+                    `Reducer exceeded the ${timeoutMs}ms step budget`,
+                );
+            }
+            throw err;
+        }
+    };
+
+    return {
+        run: (state, input, ctx) => invoke(state, input, ctx),
+        runWithBudget: (state, input, ctx, budgetMs) => invoke(state, input, ctx, budgetMs),
+    };
+}
+
+/**
+ * Execute a compiled reducer on the APPLY path: NO timeout, fully deterministic.
+ * Every replica runs this and must reach the identical result, so a wall-clock
+ * interrupt is deliberately not used here.
+ */
+export function runReducer(
+    compiled: CompiledReducer,
+    state: unknown,
+    input: unknown,
+    ctx: ReducerContext,
+): ReducerResult<unknown> {
+    return compiled.run(state, input, ctx);
+}
+
+/**
+ * Execute a compiled reducer on the ADMISSION/dry-run path with a wall-clock
+ * `budgetMs`. A runaway synchronous reducer is interrupted by the vm and surfaced
+ * as `BudgetExceededError`. LEADER-ONLY: this is the pre-log gas check; followers
+ * never call it (CFT trust). Because it runs against a CLONE of state and its
+ * result is discarded, the non-determinism of the timeout never touches
+ * replicated state.
+ */
+export function runReducerWithBudget(
+    compiled: CompiledReducer,
+    state: unknown,
+    input: unknown,
+    ctx: ReducerContext,
+    budgetMs: number,
+): ReducerResult<unknown> {
+    return compiled.runWithBudget(state, input, ctx, budgetMs);
+}
diff --git a/src/runtime/shardRouter.ts b/src/runtime/shardRouter.ts
new file mode 100644
index 0000000..5c5cf27
--- /dev/null
+++ b/src/runtime/shardRouter.ts
@@ -0,0 +1,113 @@
+import { createHash } from 'crypto';
+import { AppCommand, ApplyResult, CommandMeta } from '../consensus/types';
+
+/**
+ * The minimal node surface the router needs: ask who leads, and submit to it.
+ * Structural so any `RaftNode<C, T>` (e.g. a {@link ModuleNode}) fits without the
+ * router depending on the concrete node/state-machine types.
+ */
+export interface ShardNode<C extends AppCommand, T> {
+    isLeader(): boolean;
+    submit(command: C, meta?: CommandMeta): Promise<ApplyResult<T>>;
+}
+
+/**
+ * Multi-Raft shard router (ADR-0020, M10).
+ *
+ * Each shard is an INDEPENDENT single-group Raft cluster — its own nodes, its own
+ * leader election, its own replicated log + ModuleHost. The router is a thin,
+ * STATE-FREE front door: it maps a command's partition key to exactly one shard
+ * and submits to that shard's current leader. It holds no data of its own — it is
+ * fully derivable from (a) the static shard map and (b) each group's live
+ * leadership — so it can be reconstructed at any time and every participant that
+ * shares the same shard count computes the same routing.
+ *
+ * Routing model (ADR-0020 "Sharding key"): the shard is a PURE, deterministic
+ * function of the partition key — `sha256(key) mod N`. No `Date`/`Math.random`,
+ * so the same key always lands on the same shard on every node and on every
+ * process restart. Writes to different shards proceed in parallel (different
+ * leaders), which is the write-scaling the ADR is demonstrating.
+ */
+
+/**
+ * A handle to one shard's cluster. The router stays generic over the submit
+ * mechanism: it is given the shard's set of nodes and asks them which one is the
+ * current leader, rather than baking in transport/forwarding details. This is
+ * what lets tests wire in `buildCluster(3)` per shard and `buildModuleCommand`
+ * for the payload without the router knowing about either.
+ */
+export interface ShardHandle<C extends AppCommand = AppCommand, T = unknown> {
+    /** Every node in this shard's Raft group (used to locate the current leader). */
+    nodes: ShardNode<C, T>[];
+}
+
+/** Raised when a shard currently has no known leader; the caller should retry. */
+export class NoShardLeaderError extends Error {
+    constructor(public readonly shard: number) {
+        super(`shard ${shard} has no known leader`);
+        this.name = 'NoShardLeaderError';
+    }
+}
+
+export class ShardRouter<C extends AppCommand = AppCommand, T = unknown> {
+    private readonly shards: ShardHandle<C, T>[];
+
+    /**
+     * @param shards one handle per shard, index `i` == shard number `i`. The shard
+     *   COUNT (`shards.length`) is the modulus of the routing function, so it must
+     *   be identical on every participant for routing to agree.
+     */
+    constructor(shards: ShardHandle<C, T>[]) {
+        if (shards.length === 0) {
+            throw new Error('ShardRouter requires at least one shard');
+        }
+        this.shards = shards;
+    }
+
+    /** Number of shards (the modulus of {@link shardFor}). */
+    get shardCount(): number {
+        return this.shards.length;
+    }
+
+    /**
+     * Deterministic shard mapping: `sha256(partitionKey) mod N`. Pure — depends
+     * ONLY on the key and the shard count, never on wall-clock or randomness — so
+     * every node derives the same shard for the same key (ADR-0020). The first 6
+     * hex digits (24 bits) of the digest are ample entropy for a small N and keep
+     * the arithmetic inside a safe integer.
+     */
+    shardFor(partitionKey: string): number {
+        const digest = createHash('sha256').update(partitionKey).digest('hex');
+        const bucket = parseInt(digest.slice(0, 6), 16);
+        return bucket % this.shards.length;
+    }
+
+    /**
+     * The current leader of `shard`, or `null` if none is presently known (e.g.
+     * mid-election). Derived live from the group — the router caches nothing.
+     */
+    leaderOf(shard: number): ShardNode<C, T> | null {
+        const handle = this.shards[shard];
+        if (!handle) {
+            throw new RangeError(`no such shard: ${shard}`);
+        }
+        return handle.nodes.find((n) => n.isLeader()) ?? null;
+    }
+
+    /**
+     * Route a command to the leader of the shard that owns `partitionKey` and await
+     * its `submit`. The leader replicates + applies as any single-group write, so
+     * per-shard correctness (determinism, idempotency, audit) is unchanged. If the
+     * owning shard has no known leader right now, reject with
+     * {@link NoShardLeaderError} so the caller can retry across a leader change —
+     * the router does no buffering of its own (ADR-0020 "Routing/membership").
+     */
+    submit(partitionKey: string, command: C, meta?: CommandMeta): Promise<ApplyResult<T>> {
+        const shard = this.shardFor(partitionKey);
+        const leader = this.leaderOf(shard);
+        if (!leader) {
+            return Promise.reject(new NoShardLeaderError(shard));
+        }
+        return leader.submit(command, meta);
+    }
+}
diff --git a/src/runtime/signing.ts b/src/runtime/signing.ts
new file mode 100644
index 0000000..9fb6318
--- /dev/null
+++ b/src/runtime/signing.ts
@@ -0,0 +1,117 @@
+/**
+ * Actor-signed module commands (ADR-0019 pillar 7: "Sign commands with the
+ * originating actor's key so the leader cannot forge `actor`").
+ *
+ * This adds ACCOUNTABILITY without paying for full BFT. The originating actor
+ * signs the LOGICAL command with its private key; every replica verifies the
+ * signature on the deterministic apply path against an actor->public-key
+ * registry. A command whose `actor` was forged by a malicious leader (or whose
+ * input/requestId was tampered with) fails verification IDENTICALLY on every
+ * node and is rejected (401) before its reducer runs — so the leader cannot put
+ * words in an actor's mouth.
+ *
+ * WHY THE SIGNATURE EXCLUDES `seed`: the leader resolves the deterministic
+ * `seed` (clock + nonce) AFTER the client signs, so the seed cannot be part of
+ * the signed payload — the actor never sees it. The actor signs only the
+ * LOGICAL command (`module`/`command`/`input`/`actor`/`requestId`). The seed is
+ * a NON-SECURITY, convergence-only value the leader bakes in afterward; it does
+ * not need to be authenticated. Verification recomputes the exact same logical
+ * payload from the committed command + meta and checks the signature.
+ *
+ * WHY THIS IS CONVERGENCE-SAFE: ed25519 sign and verify are deterministic, and
+ * the verification input (canonical JSON of the logical payload) is a pure
+ * function of replicated state (the committed command fields + meta). Every
+ * replica computes the same boolean, so an invalid signature is rejected on all
+ * nodes or none — never some — and the cluster never diverges.
+ *
+ * Stdlib only: Node `crypto` ed25519 (ADR-0013, minimal dependencies).
+ */
+
+import { createPrivateKey, createPublicKey, generateKeyPairSync, sign, verify } from 'crypto';
+import { canonicalJson } from './canonical';
+
+/**
+ * The LOGICAL command an actor signs. Deliberately EXCLUDES the leader-resolved
+ * `seed`: the actor signs before the leader exists in the flow, so the seed is
+ * not available and (being non-security) does not need to be authenticated.
+ */
+export interface SignablePayload {
+    module: string;
+    command: string;
+    input: unknown;
+    actor: string;
+    requestId: string;
+}
+
+/**
+ * Generate an ed25519 keypair for an actor, returned as PEM strings. The public
+ * half is registered (per actor) in a {@link KeyRegistry} on every node; the
+ * private half stays with the actor and signs its commands.
+ */
+export function generateActorKeypair(): { publicKey: string; privateKey: string } {
+    const { publicKey, privateKey } = generateKeyPairSync('ed25519');
+    return {
+        publicKey: publicKey.export({ type: 'spki', format: 'pem' }).toString(),
+        privateKey: privateKey.export({ type: 'pkcs8', format: 'pem' }).toString(),
+    };
+}
+
+/**
+ * Sign the canonical serialization of the logical payload with `privateKeyPem`.
+ * Canonical (sorted-key) JSON guarantees the signer and every verifier hash the
+ * SAME bytes regardless of property insertion order. ed25519 uses `null` as the
+ * algorithm argument (the digest is built in). Returns a base64 signature.
+ */
+export function signCommand(privateKeyPem: string, payload: SignablePayload): string {
+    const key = createPrivateKey(privateKeyPem);
+    const data = Buffer.from(canonicalJson(payload));
+    return sign(null, data, key).toString('base64');
+}
+
+/**
+ * Verify `signatureB64` over the canonical logical payload against
+ * `publicKeyPem`. Pure and deterministic, so it is safe to run on the apply
+ * path: every replica computes the same boolean from the same committed inputs.
+ * Returns `false` (never throws) on a malformed key/signature so a bad command
+ * is a deterministic rejection, not a host crash.
+ */
+export function verifyCommand(publicKeyPem: string, payload: SignablePayload, signatureB64: string): boolean {
+    try {
+        const key = createPublicKey(publicKeyPem);
+        const data = Buffer.from(canonicalJson(payload));
+        return verify(null, data, key, Buffer.from(signatureB64, 'base64'));
+    } catch {
+        return false;
+    }
+}
+
+/**
+ * The prototype's PKI / allowlist: a map from `actor` -> the public key
+ * AUTHORIZED to sign on that actor's behalf. Binding actor->authorized-key here
+ * is the whole point of pillar 7 — it is what makes a forged-`actor` command
+ * detectable: a command claiming `actor: 'alice'` only verifies if it was signed
+ * by the key the registry holds for `alice`. A leader that fabricates an
+ * `actor` it has no key for cannot produce a matching signature.
+ *
+ * The registry is configured per node BEFORE start (like module registration)
+ * and is identical on every node, so verification converges. When NO registry is
+ * configured on a host, verification is skipped entirely (back-compat).
+ */
+export class KeyRegistry {
+    private readonly keys = new Map<string, string>();
+
+    /** Authorize `publicKeyPem` as the sole signer for `actor`. */
+    registerActor(actor: string, publicKeyPem: string): void {
+        this.keys.set(actor, publicKeyPem);
+    }
+
+    /** The public key authorized for `actor`, or `undefined` if none is registered. */
+    get(actor: string): string | undefined {
+        return this.keys.get(actor);
+    }
+
+    /** Whether `actor` has an authorized key on file. */
+    has(actor: string): boolean {
+        return this.keys.has(actor);
+    }
+}
diff --git a/src/runtime/stateStore.ts b/src/runtime/stateStore.ts
new file mode 100644
index 0000000..bfc2dda
--- /dev/null
+++ b/src/runtime/stateStore.ts
@@ -0,0 +1,245 @@
+/**
+ * Pluggable, key-oriented state store (ADR-0019 pillar 4, the "state larger than
+ * RAM" half).
+ *
+ * The existing module model (`defineModule`) treats a module's state as ONE
+ * whole-state blob: every command loads, transforms, and replaces the entire
+ * value. That is simple but caps a module's state at what fits in RAM and what is
+ * cheap to clone per command. This module adds the alternative: a module's state
+ * is a COLLECTION OF RECORDS addressed by key, and a reducer touches only the
+ * keys it needs (read one account, write it back) — never the whole dataset.
+ *
+ * `StateStore` is the seam. The default `MemoryStateStore` keeps records in a
+ * `Map`, but the interface is deliberately the shape a persistent embedded KV/LSM
+ * store (LevelDB, RocksDB, …) would implement: there snapshots become store
+ * checkpoints and the working set need not fit in memory. Swapping the backend
+ * changes nothing above this seam.
+ *
+ * DETERMINISM (the non-negotiable, same rule as the consensus state machine):
+ * every observable store operation a reducer can see MUST be identical on every
+ * replica. Two consequences enforced here:
+ *  - ITERATION ORDER is sorted by key, always. A `Map` iterates in insertion
+ *    order, which differs between replicas that applied the same logical writes
+ *    in a different incidental order — so `keys()/entries()/snapshot()` sort.
+ *  - READS RETURN DEEP CLONES. A reducer must never receive a shared mutable
+ *    reference into stored state: mutating it in place would (a) bypass the
+ *    transactional commit/discard guarantee and (b) leak insertion-time identity.
+ *    Every read clones, so the store is the sole owner of its records.
+ */
+
+import { canonicalJson } from './canonical';
+
+/**
+ * Deep-clone a serializable value. Reuses the shared `canonicalJson` round-trip
+ * (DROP-`undefined` mode, matching `JSON.stringify` semantics) rather than
+ * `structuredClone` so the clone is consistent with the byte-sizing / hashing
+ * paths and does not depend on `structuredClone` typings under the lib target —
+ * the same assumption `moduleHost.ts`'s `deepClone` makes. Records are plain
+ * serializable data, so a JSON round-trip is sufficient.
+ *
+ * Using `canonicalJson` (sorted keys) additionally NORMALIZES key order in the
+ * clone, so a stored record can never leak its insertion-time key order to a
+ * reader — one more determinism guard for free.
+ */
+function cloneValue<T>(value: T): T {
+    return JSON.parse(canonicalJson(value, { onUndefined: 'drop' })) as T;
+}
+
+/**
+ * Per-module record store: a key→value map with deterministic iteration and
+ * clone-on-read. This is the seam a persistent embedded KV/LSM store drops into
+ * (snapshots become store checkpoints there); the default below is in-memory.
+ *
+ * VALUE SEMANTICS: values round-trip through canonical JSON (the clone path), so
+ * store plain JSON-safe records — a property explicitly set to `undefined` is
+ * DROPPED (matching `JSON.stringify`, identical across replicas, so no
+ * divergence) and `Date`/`Map`/`Set`/class instances are NOT preserved (a `Date`
+ * becomes its ISO string, a `Map` becomes `{}`).
+ */
+export interface StateStore {
+    /** The value at `key`, deep-cloned, or `undefined` if absent. */
+    get(key: string): unknown | undefined;
+    /** Store (a deep clone of) `value` at `key`, replacing any existing record. */
+    put(key: string, value: unknown): void;
+    /** Remove the record at `key` (no-op if absent). */
+    delete(key: string): void;
+    /** Whether a record exists at `key`. */
+    has(key: string): boolean;
+    /** All keys, SORTED ascending (deterministic iteration). */
+    keys(): string[];
+    /** All [key, value] pairs, SORTED by key, values deep-cloned. */
+    entries(): [string, unknown][];
+    /** Number of records. */
+    size(): number;
+    /** Full SORTED dump for snapshotting; values deep-cloned. */
+    snapshot(): [string, unknown][];
+    /** Replace all records with `entries` (deep-cloned). */
+    restore(entries: [string, unknown][]): void;
+}
+
+/**
+ * Default in-memory `StateStore` over a `Map`.
+ *
+ * This is the default backend; the `StateStore` interface is the seam a
+ * persistent embedded KV/LSM store would implement instead (where `snapshot()`
+ * becomes a store checkpoint and the dataset need not fit in RAM). Determinism is
+ * guaranteed here by SORTING keys on every iteration (a `Map` is insertion-
+ * ordered, which is not replica-stable) and DEEP-CLONING values on read/dump so a
+ * caller can never mutate a stored record in place.
+ *
+ * VALUE SEMANTICS (same as the interface): records round-trip through canonical
+ * JSON, so a property set to `undefined` is dropped and `Date`/`Map`/etc. are
+ * not preserved — store plain JSON-safe records.
+ */
+export class MemoryStateStore implements StateStore {
+    private readonly map = new Map<string, unknown>();
+
+    get(key: string): unknown | undefined {
+        if (!this.map.has(key)) return undefined;
+        // Clone on read: the caller must never get a mutable reference into the
+        // store, or it could mutate committed state out of band and diverge.
+        return cloneValue(this.map.get(key));
+    }
+
+    put(key: string, value: unknown): void {
+        // Clone on write too: the caller keeps no shared handle to what it stored,
+        // so a later mutation of its local object cannot reach into the store.
+        this.map.set(key, cloneValue(value));
+    }
+
+    delete(key: string): void {
+        this.map.delete(key);
+    }
+
+    has(key: string): boolean {
+        return this.map.has(key);
+    }
+
+    keys(): string[] {
+        // SORTED: a Map iterates in insertion order, which is not replica-stable.
+        return [...this.map.keys()].sort();
+    }
+
+    entries(): [string, unknown][] {
+        return this.keys().map((k) => [k, cloneValue(this.map.get(k))]);
+    }
+
+    size(): number {
+        return this.map.size;
+    }
+
+    snapshot(): [string, unknown][] {
+        // Same as entries(): sorted + cloned. Named distinctly so a persistent
+        // backend can implement it as a checkpoint without overloading entries().
+        return this.entries();
+    }
+
+    restore(entries: [string, unknown][]): void {
+        this.map.clear();
+        for (const [k, v] of entries) {
+            this.map.set(k, cloneValue(v));
+        }
+    }
+}
+
+/**
+ * A transactional COPY-ON-WRITE view over a `StateStore`, handed to a keyed
+ * reducer (ADR-0019 pillar 4). The atomicity story mirrors what whole-state
+ * modules get from the host's clone-and-swap: nothing the reducer writes touches
+ * the underlying store until the host explicitly `commit()`s, and a reducer that
+ * throws (or is rejected by a budget/lint check) is simply discarded — the view
+ * is dropped and the store is untouched.
+ *
+ * READ SEMANTICS (reads-your-writes): a `get`/`has`/`keys`/`entries` reflects
+ * this view's own buffered `put`/`delete` FIRST, falling through to the
+ * underlying store only for keys the view hasn't touched. Fall-through reads
+ * return clones (from the underlying store), so the reducer still cannot mutate
+ * committed records in place. Iteration stays SORTED and reflects buffered
+ * puts/deletes — deterministic regardless of how the reducer interleaved writes.
+ */
+export class StoreView {
+    /** Buffered puts, keyed; values are already cloned on entry. */
+    private readonly writes = new Map<string, unknown>();
+    /** Keys buffered for deletion. A delete shadows the underlying store. */
+    private readonly deletes = new Set<string>();
+
+    constructor(private readonly store: StateStore) {}
+
+    get(key: string): unknown | undefined {
+        if (this.deletes.has(key)) return undefined;
+        if (this.writes.has(key)) {
+            // Clone the buffered value so the reducer cannot mutate the buffer in
+            // place behind its own write (consistent with the store's read clone).
+            return cloneValue(this.writes.get(key));
+        }
+        return this.store.get(key); // already cloned by the underlying store
+    }
+
+    put(key: string, value: unknown): void {
+        // Buffer only — nothing reaches the underlying store until commit().
+        // Clone on entry so the reducer's later mutation of its local object
+        // cannot retroactively change what the view will commit.
+        this.writes.set(key, cloneValue(value));
+        this.deletes.delete(key); // a put cancels a prior buffered delete
+    }
+
+    delete(key: string): void {
+        this.writes.delete(key);
+        this.deletes.add(key);
+    }
+
+    has(key: string): boolean {
+        if (this.deletes.has(key)) return false;
+        if (this.writes.has(key)) return true;
+        return this.store.has(key);
+    }
+
+    /**
+     * SORTED keys reflecting buffered changes: underlying keys, minus buffered
+     * deletes, plus buffered puts (which may be new keys). Computed fresh each
+     * call so it always mirrors the current buffer.
+     */
+    keys(): string[] {
+        const set = new Set<string>();
+        for (const k of this.store.keys()) {
+            if (!this.deletes.has(k)) set.add(k);
+        }
+        for (const k of this.writes.keys()) set.add(k);
+        return [...set].sort();
+    }
+
+    entries(): [string, unknown][] {
+        return this.keys().map((k) => [k, this.get(k)]);
+    }
+
+    size(): number {
+        return this.keys().length;
+    }
+
+    /** Number of buffered writes (puts + deletes) — the host's `maxWrites` axis. */
+    pendingWriteCount(): number {
+        return this.writes.size + this.deletes.size;
+    }
+
+    /** The buffered puts, sorted by key — what the host bytes-sizes for `maxResultBytes`. */
+    pendingPuts(): [string, unknown][] {
+        return [...this.writes.keys()].sort().map((k) => [k, this.writes.get(k)] as [string, unknown]);
+    }
+
+    /**
+     * Apply the buffer to the underlying store atomically: all buffered puts and
+     * deletes land together. The host calls this ONLY on a successful, in-budget
+     * reducer; on any failure the view is simply dropped and nothing here runs.
+     */
+    commit(): void {
+        // Deterministic apply order (sorted) — the result is order-independent
+        // since each key appears at most once, but a stable order keeps any
+        // backend's write log reproducible.
+        for (const k of [...this.deletes].sort()) {
+            this.store.delete(k);
+        }
+        for (const k of [...this.writes.keys()].sort()) {
+            this.store.put(k, this.writes.get(k));
+        }
+    }
+}
diff --git a/src/runtime/types.ts b/src/runtime/types.ts
new file mode 100644
index 0000000..4563c2f
--- /dev/null
+++ b/src/runtime/types.ts
@@ -0,0 +1,267 @@
+/**
+ * Core type definitions for the module runtime (ADR-0019, pillars 1–2).
+ *
+ * This layer sits ON TOP of the consensus core. Where the book demo wires
+ * non-determinism out by hand in `models/book.ts`, the runtime generalizes that
+ * discipline: developers write **pure reducers** and the framework injects a
+ * deterministic `ReducerContext`. Every value a reducer could be tempted to pull
+ * from the ambient environment (clock, randomness, ids) instead arrives through
+ * `ctx`, and `ctx` is derived entirely from a leader-resolved `Seed`. That is
+ * what lets two fresh hosts replay the same commands to byte-identical state.
+ */
+
+/**
+ * A declarative request for a side effect (ADR-0019 pillar 3). Reducers never
+ * perform I/O; they return intents that the host records into a deterministic
+ * outbox and a post-commit `EffectExecutor` runs at the edge. The replicated log
+ * thus IS a transactional outbox: the intent commits as part of the reducer's
+ * result, and the effect's outcome re-enters the log as a committed follow-up.
+ */
+export interface EffectIntent {
+    kind: string;
+    /**
+     * Stable key that makes execution exactly-once: the outbox dedups on it (a
+     * replayed command never re-enqueues) and `applyEffectResult` is idempotent
+     * on it (a redelivered result never double-dispatches).
+     */
+    idempotencyKey: string;
+    payload: unknown;
+    /**
+     * Optional module command that consumes the effect's result. After the edge
+     * resolves the effect, `applyEffectResult` dispatches this command so the
+     * reducer can fold the outcome back into module state deterministically.
+     */
+    onResult?: { module: string; command: string };
+}
+
+/** Lifecycle of an outbox entry: enqueued (`pending`) or executed (`done`). */
+export type OutboxStatus = 'pending' | 'done';
+
+/**
+ * One slot in the deterministic outbox: the emitted intent, its execution
+ * status, and (once executed) the result the edge produced. Keyed in the outbox
+ * by `intent.idempotencyKey`.
+ */
+export interface OutboxEntry {
+    intent: EffectIntent;
+    status: OutboxStatus;
+    result?: unknown;
+}
+
+/**
+ * A committed log entry carrying an effect's outcome back into the deterministic
+ * core. The `result` was resolved ONCE on the effectful edge (the executor) and
+ * is now replicated verbatim; likewise the `seed` is resolved once on the edge by
+ * the executor, then committed verbatim so every replica applies the same value
+ * (convergence comes from committing the value, not from where it was generated).
+ * That deterministic `seed` gives any `onResult` reducer that consumes the result
+ * a deterministic `ctx`. Applying this entry on every replica yields identical
+ * state.
+ */
+export interface EffectResultEntry {
+    idempotencyKey: string;
+    result: unknown;
+    /**
+     * Resolved once on the edge by the executor, then committed verbatim so every
+     * replica applies the same value. It seeds the `ctx` of the `onResult`
+     * reducer, keeping that follow-up dispatch deterministic on every replica.
+     */
+    seed: Seed;
+}
+
+/**
+ * Performs the real side effect at the edge (ADR-0019 pillar 3). This is the ONE
+ * place non-determinism (network, clock, randomness) is allowed: the executor
+ * runs it post-commit, off the deterministic apply path, and bakes the returned
+ * value into an `EffectResultEntry` so replicas never re-run it.
+ */
+export type EffectHandler = (intent: EffectIntent) => Promise<unknown>;
+
+/**
+ * The deterministic capabilities handed to a reducer. These are the ONLY
+ * sanctioned source of non-determinism: a reducer that reads `Date.now()`,
+ * `Math.random()`, or `crypto` directly breaks the determinism contract.
+ *
+ * All values are reproducible from the command's `Seed` alone, so a replica
+ * applying the same command rebuilds the identical context.
+ */
+export interface ReducerContext {
+    /** Leader-resolved wall-clock as an ISO string (the analog of `seed.timestamp`). */
+    now: string;
+    /** Deterministic PRNG: a float in [0, 1). Successive calls advance the stream. */
+    random(): number;
+    /** Deterministic unique id. Successive calls return distinct values. */
+    id(): string;
+    /** Who issued the command (carried from `CommandMeta`, off the deterministic path). */
+    actor: string;
+    /** Request correlation id (carried from `CommandMeta`). */
+    requestId: string;
+}
+
+/**
+ * What a reducer returns: the next state, an optional explicit result value to
+ * surface to the caller, and any effect intents it wants run. `result` is kept
+ * distinct from `state` so the host can hand callers a purpose-built value (e.g.
+ * the created entity) without exposing the entire next-state object.
+ */
+export interface ReducerResult<S> {
+    state: S;
+    result?: unknown;
+    effects?: EffectIntent[];
+}
+
+/**
+ * A pure state transition. Given the current state, a command input, and the
+ * deterministic context, it returns the next state. MUST NOT mutate `state`
+ * in place in a way that depends on anything outside its arguments, and MUST
+ * NOT read ambient clocks/randomness — use `ctx`.
+ */
+export type Reducer<S> = (state: S, input: unknown, ctx: ReducerContext) => ReducerResult<S>;
+
+/** A read-only projection over state. Never mutates; not part of the log. */
+export type Query<S> = (state: S, args: unknown) => unknown;
+
+/** A self-contained unit of application logic: its state, commands, and queries. */
+export interface ModuleDefinition<S> {
+    name: string;
+    /**
+     * Semantic version of the module's LOGIC (ADR-0019 pillar 5). Optional;
+     * defaults to `'0'` when unset. It participates in the module's code hash, so
+     * bumping it produces a distinct `codeHash` even if the reducer source is
+     * byte-identical — letting an operator force a recorded version boundary.
+     */
+    version?: string;
+    /** Builds the empty initial state. A factory (not a value) so each host gets its own. */
+    initialState: () => S;
+    commands: Record<string, Reducer<S>>;
+    queries?: Record<string, Query<S>>;
+    /**
+     * Opt into the `vm` determinism sandbox (ADR-0019 pillars 2, 6 — M9). OFF by
+     * default: existing whole-state modules keep their current DIRECT reducer
+     * execution. When `true`, the host re-compiles each command reducer into a
+     * fresh `vm` context with a frozen safe-global set (no `Date`/`Math.random`/
+     * `crypto`/timers/…) at registration, so determinism is enforced
+     * STRUCTURALLY at runtime rather than only by the static lint — a reducer
+     * that touches a banned global throws even if the lint were bypassed.
+     *
+     * SCOPE (this milestone): WHOLE-STATE modules only. A keyed module passed
+     * with `sandbox: true` is rejected at registration — sandboxing keyed
+     * `StoreView`-mutating reducers is out of scope. Sandboxed reducers must be
+     * SELF-CONTAINED arrow/`function` expressions (see {@link compileReducer}).
+     *
+     * HONEST CAVEAT: `vm` is a determinism aid, NOT a security boundary against
+     * malicious code (Node's docs say so). See `sandbox.ts`.
+     */
+    sandbox?: boolean;
+    /**
+     * Wall-clock budget (ms) for the LEADER-SIDE admission step meter (ADR-0019
+     * pillar 6 "gas"). Only consulted for a `sandbox` module. The leader's
+     * pre-log `admit()` dry-run runs the reducer inside the vm with this timeout
+     * and rejects a runaway (e.g. an infinite loop) BEFORE it enters the log. The
+     * apply path NEVER uses this — followers trust committed entries are bounded
+     * (CFT), keeping apply deterministic and timeout-free. Default: 50.
+     */
+    stepBudgetMs?: number;
+}
+
+/**
+ * The leader-resolved deterministic seed for a single command. Resolved once on
+ * the leader (real clock + real randomness) and replicated verbatim, so every
+ * replica derives the same context from it. The consensus-core analog is the
+ * leader baking ids/timestamps into a `Command` before it enters the log.
+ */
+export interface Seed {
+    /** ISO timestamp captured on the leader. */
+    timestamp: string;
+    /** Hex-encoded random nonce captured on the leader; seeds the PRNG and id stream. */
+    nonce: string;
+}
+
+/** A command targeted at a module's reducer, carrying its leader-resolved seed. */
+export interface ModuleCommand {
+    module: string;
+    command: string;
+    input: unknown;
+    seed: Seed;
+    /**
+     * Optional actor signature (base64) over the LOGICAL command, threaded
+     * through so {@link ModuleHost.apply} can verify it against a configured
+     * `KeyRegistry` (ADR-0019 pillar 7). Excludes `seed` — the actor signs
+     * `{ module, command, input, actor, requestId }` before the leader resolves
+     * the seed. Ignored when no registry is configured (back-compat).
+     */
+    sig?: string;
+}
+
+/**
+ * The application command the module runtime puts on the replicated log
+ * (ADR-0019 M4). It satisfies the framework's `AppCommand` contract (a string
+ * `type` discriminator — `'MODULE'`, distinct from the reserved `NOOP`/`CONFIG`)
+ * and additionally carries the `actor`/`requestId` so the `StateMachine` adapter,
+ * whose `apply(command)` receives no separate meta, can reconstruct the
+ * `(cmd, meta)` call the `ModuleHost` expects and verify the signature. These two
+ * fields MUST match the `CommandMeta` the command is submitted with.
+ */
+export interface ModuleAppCommand extends ModuleCommand {
+    type: 'MODULE';
+    actor: string;
+    requestId: string;
+}
+
+/** The outcome of applying a `ModuleCommand` to a `ModuleHost`. */
+export interface ModuleApplyResult {
+    status: number;
+    result?: unknown;
+    effects: EffectIntent[];
+    message?: string;
+}
+
+/**
+ * Construction options for `ModuleHost` (ADR-0019 pillar 6 "resource safety").
+ *
+ * These bound the AMPLIFICATION a single command may cause: how many effects it
+ * may enqueue, and how large the next-state it may produce. They are a
+ * DETERMINISTIC resource bound, NOT a CPU/step meter — every replica computes
+ * the same effect count and the same canonical byte size from the same reducer
+ * output, so an over-budget command is rejected IDENTICALLY on every node and
+ * the bound never causes divergence. A preemptive CPU/instruction meter (which
+ * would need a vm/worker to interrupt a runaway reducer mid-execution) is
+ * explicitly DEFERRED per ADR-0019; this guards the cheap, deterministic axes
+ * (fan-out and state size) that can be measured purely after the reducer returns.
+ */
+export interface ModuleHostOptions {
+    /**
+     * Max number of effect intents one command may emit. Caps outbox fan-out so
+     * a single command cannot flood the post-commit executor. Default: 16.
+     */
+    maxEffects?: number;
+    /**
+     * Max canonical-JSON byte size of the next-state a reducer may produce. Caps
+     * state amplification so a single command cannot bloat replicated state (and
+     * thus every snapshot) without bound. Default: 64 KiB.
+     *
+     * For a KEYED module (ADR-0019 pillar 4) this bounds the canonical byte size
+     * of the command's BUFFERED WRITES (the records it `put`s), the keyed analog
+     * of next-state amplification.
+     */
+    maxResultBytes?: number;
+    /**
+     * Max number of buffered writes (puts + deletes) one KEYED command may make
+     * before commit. Caps how many records a single command can touch, the keyed
+     * analog of `maxEffects` for fan-out. Deterministic (counted from the view's
+     * buffer after the reducer returns), so over-budget commands are rejected
+     * identically on every replica. Default: 256.
+     */
+    maxWrites?: number;
+    /**
+     * Default wall-clock step budget (ms) for the LEADER-SIDE admission meter
+     * (ADR-0019 pillar 6 "gas" — M9), used for a `sandbox` module that does not
+     * set its own `stepBudgetMs`. ONLY the leader's pre-log `admit()` dry-run uses
+     * this timeout, to reject a runaway reducer before it enters the log; the
+     * apply path NEVER uses a timeout (it must stay deterministic — followers
+     * trust committed entries are already bounded under the CFT model). Unlike the
+     * other bounds here, this is NOT deterministic state — it is an edge-side
+     * admission gate whose result is discarded. Default: 50.
+     */
+    stepBudgetMs?: number;
+}
diff --git a/tests/helpers.ts b/tests/helpers.ts
index 44cbfcd..98c219b 100644
--- a/tests/helpers.ts
+++ b/tests/helpers.ts
@@ -2,6 +2,10 @@ import { RaftNode } from '../src/consensus/raftNode';
 import { LocalTransport, RpcHandler } from '../src/consensus/transport';
 import { PeerInfo } from '../src/consensus/types';
 import { BookNode, BookStateMachine } from '../src/models/bookStateMachine';
+import { ModuleStateMachine } from '../src/runtime/moduleStateMachine';
+import { ModuleAppCommand } from '../src/runtime/types';
+import { AnyModuleDefinition } from '../src/runtime/moduleHost';
+import { KeyRegistry } from '../src/runtime/signing';
 
 // Fast timers keep the test suite snappy and deterministic.
 const TEST_TIMERS = {
@@ -25,6 +29,36 @@ export function buildCluster(size: number): BookNode[] {
     return nodes;
 }
 
+/** A node whose application is the {@link ModuleStateMachine} runtime (ADR-0019). */
+export type ModuleNode = RaftNode<ModuleAppCommand, unknown, ModuleStateMachine>;
+
+/**
+ * Build an in-process cluster of `size` nodes running the module runtime. Each
+ * node gets its OWN {@link ModuleStateMachine} (independent replica), registered
+ * with the same `modules` and the same optional `keyRegistry`, so MODULE commands
+ * apply identically and converge.
+ */
+export function buildModuleCluster(
+    size: number,
+    modules: AnyModuleDefinition[],
+    opts: { keyRegistry?: KeyRegistry } = {},
+): ModuleNode[] {
+    const registry = new Map<string, RpcHandler>();
+    const transport = new LocalTransport(registry, 1);
+    const ids = Array.from({ length: size }, (_, i) => `node${i + 1}`);
+
+    const nodes = ids.map((id) => {
+        const peers: PeerInfo[] = ids.filter((p) => p !== id).map((p) => ({ id: p, url: `local://${p}` }));
+        const sm = new ModuleStateMachine();
+        sm.host.registerModules(modules);
+        if (opts.keyRegistry) sm.host.setKeyRegistry(opts.keyRegistry);
+        return new RaftNode({ id, peers, stateMachine: sm, ...TEST_TIMERS }, transport);
+    });
+
+    nodes.forEach((n) => registry.set(n.id, n));
+    return nodes;
+}
+
 /** Poll until `predicate` is true or `timeoutMs` elapses. */
 export async function waitFor(predicate: () => boolean, timeoutMs = 2000, stepMs = 10): Promise<void> {
     const start = Date.now();
@@ -35,6 +69,7 @@ export async function waitFor(predicate: () => boolean, timeoutMs = 2000, stepMs
     throw new Error('waitFor: condition not met within timeout');
 }
 
-export function leaders(nodes: BookNode[]): BookNode[] {
+/** The current leaders among `nodes` (generic over the node's command type). */
+export function leaders<N extends { isLeader(): boolean }>(nodes: N[]): N[] {
     return nodes.filter((n) => n.isLeader());
 }
diff --git a/tests/membership.test.ts b/tests/membership.test.ts
index 4776673..271e910 100644
--- a/tests/membership.test.ts
+++ b/tests/membership.test.ts
@@ -7,6 +7,15 @@ import { waitFor } from './helpers';
 
 const TIMERS = { electionMinMs: 50, electionMaxMs: 100, heartbeatMs: 20 };
 
+// Several tests here run multiple membership changes + elections back-to-back,
+// each gated by its own `waitFor` (up to 2–3s apiece). Those internal budgets are
+// the real guards — they throw a clear "condition not met" if anything genuinely
+// stalls. Their SUM, however, can exceed Jest's 5s default test timeout when
+// workers run in parallel under CPU contention (the suite passes in isolation),
+// which is the only reason this test flaked. Give the suite ample headroom so a
+// slow-but-correct run is never mistaken for a hang.
+jest.setTimeout(15000);
+
 /**
  * A cluster whose registry + node set the test controls, so nodes can be added
  * to or removed from the configuration at runtime.
diff --git a/tests/runtime/audit.test.ts b/tests/runtime/audit.test.ts
new file mode 100644
index 0000000..f4e2e73
--- /dev/null
+++ b/tests/runtime/audit.test.ts
@@ -0,0 +1,186 @@
+import { MerkleAudit } from '../../src/runtime/merkleAudit';
+import { ModuleHost } from '../../src/runtime/moduleHost';
+import { defineModule } from '../../src/runtime/defineModule';
+import { counter } from '../../src/runtime/modules/counter';
+import { notes } from '../../src/runtime/modules/notes';
+import { ModuleCommand, Seed } from '../../src/runtime/types';
+
+const META = { actor: 'tester', requestId: 'req-1' };
+
+const seed = (nonce: string, timestamp = '2026-06-21T00:00:00.000Z'): Seed => ({ timestamp, nonce });
+
+const cmd = (module: string, command: string, input: unknown, s: Seed): ModuleCommand => ({
+    module,
+    command,
+    input,
+    seed: s,
+});
+
+function freshHost(): ModuleHost {
+    const host = new ModuleHost();
+    host.register(counter);
+    host.register(notes);
+    return host;
+}
+
+describe('ModuleHost Merkle audit', () => {
+    it('grows the audit and stamps each leaf with module/command/status/codeHash', () => {
+        const host = freshHost();
+        expect(host.auditSize()).toBe(0);
+        expect(host.auditRoot()).toBe(new MerkleAudit().root()); // empty-tree constant
+
+        host.apply(cmd('counter', 'increment', { by: 2 }, seed('a1')), META);
+        host.apply(cmd('notes', 'create', { text: 'hi' }, seed('a2')), META);
+
+        expect(host.auditSize()).toBe(2);
+        const entries = host.auditEntries();
+
+        expect(entries[0]).toMatchObject({
+            seq: 0,
+            module: 'counter',
+            command: 'increment',
+            actor: 'tester',
+            requestId: 'req-1',
+            status: 200,
+            codeHash: host.moduleCodeHash('counter'),
+        });
+        expect(entries[1]).toMatchObject({
+            seq: 1,
+            module: 'notes',
+            command: 'create',
+            status: 200,
+            codeHash: host.moduleCodeHash('notes'),
+        });
+        // Different modules -> different code hashes.
+        expect(entries[0].codeHash).not.toBe(entries[1].codeHash);
+    });
+
+    it('auditProof(seq) verifies against auditRoot()', () => {
+        const host = freshHost();
+        for (let i = 0; i < 5; i += 1) {
+            host.apply(cmd('counter', 'increment', { by: 1 }, seed(`s${i}`)), META);
+        }
+        const root = host.auditRoot();
+        for (let seq = 0; seq < host.auditSize(); seq += 1) {
+            expect(MerkleAudit.verify(root, host.auditProof(seq))).toBe(true);
+        }
+    });
+
+    it('two independent hosts applying the same stream converge on the same auditRoot', () => {
+        const stream: ModuleCommand[] = [
+            cmd('counter', 'increment', { by: 3 }, seed('s1')),
+            cmd('notes', 'create', { text: 'first' }, seed('s2', '2026-01-01T00:00:00.000Z')),
+            cmd('counter', 'reset', undefined, seed('s3')),
+            cmd('notes', 'create', { text: 'second' }, seed('s4', '2026-02-02T00:00:00.000Z')),
+            cmd('counter', 'increment', undefined, seed('s5')),
+        ];
+
+        const h1 = freshHost();
+        const h2 = freshHost();
+        for (const c of stream) {
+            h1.apply(c, META);
+            h2.apply(c, META);
+        }
+
+        expect(h1.auditRoot()).toBe(h2.auditRoot());
+        expect(h1.auditEntries()).toEqual(h2.auditEntries());
+    });
+
+    it('captures which logic version ran: a changed version/body yields a new codeHash on its leaves', () => {
+        // Two modules, same shape, different reducer BODY -> different codeHash.
+        const v1 = defineModule<{ n: number }>({
+            name: 'widget',
+            initialState: () => ({ n: 0 }),
+            commands: { bump: (s) => ({ state: { n: s.n + 1 } }) },
+        });
+        const v2 = defineModule<{ n: number }>({
+            name: 'widget',
+            initialState: () => ({ n: 0 }),
+            commands: { bump: (s) => ({ state: { n: s.n + 2 } }) }, // changed logic
+        });
+
+        const hostV1 = new ModuleHost();
+        hostV1.register(v1);
+        hostV1.apply(cmd('widget', 'bump', undefined, seed('w1')), META);
+
+        const hostV2 = new ModuleHost();
+        hostV2.register(v2);
+        hostV2.apply(cmd('widget', 'bump', undefined, seed('w1')), META);
+
+        const hash1 = hostV1.moduleCodeHash('widget')!;
+        const hash2 = hostV2.moduleCodeHash('widget')!;
+        expect(hash1).not.toBe(hash2);
+
+        // Each version's leaf records its own code hash -> the audit proves which
+        // logic version produced the result, and the two audits diverge.
+        expect(hostV1.auditEntries()[0].codeHash).toBe(hash1);
+        expect(hostV2.auditEntries()[0].codeHash).toBe(hash2);
+        expect(hostV1.auditRoot()).not.toBe(hostV2.auditRoot());
+
+        // A bumped `version` alone (identical body) also shifts the hash.
+        const sameBodyV3 = defineModule<{ n: number }>({
+            name: 'widget',
+            version: '2.0.0',
+            initialState: () => ({ n: 0 }),
+            commands: { bump: (s) => ({ state: { n: s.n + 1 } }) },
+        });
+        const hostV3 = new ModuleHost();
+        hostV3.register(sameBodyV3);
+        expect(hostV3.moduleCodeHash('widget')).not.toBe(hash1);
+    });
+
+    it('audit survives snapshot/restore: root and proofs stay stable', () => {
+        const host = freshHost();
+        for (let i = 0; i < 6; i += 1) {
+            host.apply(cmd('counter', 'increment', { by: 1 }, seed(`r${i}`)), META);
+        }
+        const rootBefore = host.auditRoot();
+        const snap = host.snapshot();
+
+        const restored = freshHost();
+        restored.restore(snap);
+
+        expect(restored.auditSize()).toBe(host.auditSize());
+        expect(restored.auditRoot()).toBe(rootBefore);
+        for (let seq = 0; seq < restored.auditSize(); seq += 1) {
+            expect(MerkleAudit.verify(restored.auditRoot(), restored.auditProof(seq))).toBe(true);
+        }
+
+        // Audit continues seamlessly after restore (seq keeps climbing).
+        restored.apply(cmd('counter', 'increment', { by: 1 }, seed('after')), META);
+        expect(restored.auditSize()).toBe(host.auditSize() + 1);
+        expect(restored.auditEntries()[host.auditSize()].seq).toBe(host.auditSize());
+    });
+
+    it('audits business failures and thrown reducers; does NOT audit unknown commands', () => {
+        // A module whose reducer throws to signal a business failure (-> status 500).
+        const flaky = defineModule<{ ok: boolean }>({
+            name: 'flaky',
+            initialState: () => ({ ok: true }),
+            commands: {
+                fail: () => {
+                    throw new Error('business rule violated');
+                },
+                ok: (s) => ({ state: s }),
+            },
+        });
+
+        const host = new ModuleHost();
+        host.register(flaky);
+
+        // Thrown reducer -> 500, still audited with that status.
+        const failed = host.apply(cmd('flaky', 'fail', undefined, seed('f1')), META);
+        expect(failed.status).toBe(500);
+        expect(host.auditSize()).toBe(1);
+        expect(host.auditEntries()[0]).toMatchObject({ command: 'fail', status: 500 });
+
+        // A successful command is audited too.
+        host.apply(cmd('flaky', 'ok', undefined, seed('f2')), META);
+        expect(host.auditSize()).toBe(2);
+
+        // Unknown module / unknown command: no logic ran -> NOT audited.
+        expect(host.apply(cmd('ghost', 'whatever', {}, seed('f3')), META).status).toBe(404);
+        expect(host.apply(cmd('flaky', 'nope', {}, seed('f4')), META).status).toBe(404);
+        expect(host.auditSize()).toBe(2);
+    });
+});
diff --git a/tests/runtime/consensusIntegration.test.ts b/tests/runtime/consensusIntegration.test.ts
new file mode 100644
index 0000000..df251be
--- /dev/null
+++ b/tests/runtime/consensusIntegration.test.ts
@@ -0,0 +1,122 @@
+import { CommandMeta } from '../../src/consensus/types';
+import { buildModuleCommand } from '../../src/runtime/command';
+import { counter } from '../../src/runtime/modules/counter';
+import { notes } from '../../src/runtime/modules/notes';
+import { buildModuleCluster, leaders, ModuleNode, waitFor } from '../helpers';
+
+/**
+ * M4: the module runtime rides REAL Raft as a pluggable application state machine
+ * (ADR-0017 seam). The leader resolves the seed up front (via `buildModuleCommand`),
+ * the command replicates as an ordinary application log entry, and every node
+ * applies it to its own `ModuleStateMachine`/`ModuleHost` — converging on identical
+ * module state AND identical Merkle audit roots, with idempotency (substrate dedup)
+ * and leader-resolved seeds carried through the log.
+ */
+describe('Module commands over Raft consensus', () => {
+    let nodes: ModuleNode[];
+
+    beforeEach(() => {
+        // Each node gets its own ModuleStateMachine registered with the same modules.
+        nodes = buildModuleCluster(3, [counter, notes]);
+        nodes.forEach((n) => n.start());
+    });
+
+    afterEach(() => {
+        nodes.forEach((n) => n.stop());
+    });
+
+    it('replicates module commands and converges state + audit root on every node', async () => {
+        await waitFor(() => leaders(nodes).length === 1);
+        const leader = leaders(nodes)[0];
+
+        const incMeta: CommandMeta = {
+            requestId: 'req-inc-1',
+            actor: 'alice',
+            timestamp: new Date().toISOString(),
+        };
+        const incResult = await leader.submit(buildModuleCommand('counter', 'increment', { by: 5 }, incMeta), incMeta);
+        expect(incResult.status).toBe(200);
+
+        const noteMeta: CommandMeta = {
+            requestId: 'req-note-1',
+            actor: 'bob',
+            timestamp: new Date().toISOString(),
+        };
+        const noteResult = await leader.submit(buildModuleCommand('notes', 'create', { text: 'hello' }, noteMeta), noteMeta);
+        expect(noteResult.status).toBe(200);
+
+        // Wait for replication of BOTH commands: gate on the LAST submitted entry
+        // (the note) as well as the counter, so we never read mid-replication while
+        // the later entry is still in flight on a follower.
+        await waitFor(() =>
+            nodes.every(
+                (n) =>
+                    n.app.host.query('counter', 'value') === 5 &&
+                    (n.app.host.query('notes', 'list') as unknown[]).length === 1,
+            ),
+        );
+
+        // All three nodes converge on identical module state AND audit root.
+        const counterValues = new Set(nodes.map((n) => n.app.host.query('counter', 'value')));
+        expect(counterValues).toEqual(new Set([5]));
+
+        const noteLists = nodes.map((n) => JSON.stringify(n.app.host.getState('notes')));
+        expect(new Set(noteLists).size).toBe(1);
+
+        const auditRoots = new Set(nodes.map((n) => n.app.host.auditRoot()));
+        expect(auditRoots.size).toBe(1);
+    });
+
+    it('does not double-apply a module command whose requestId is replayed (idempotency)', async () => {
+        await waitFor(() => leaders(nodes).length === 1);
+        const leader = leaders(nodes)[0];
+
+        const meta: CommandMeta = {
+            requestId: 'req-dup',
+            actor: 'alice',
+            timestamp: new Date().toISOString(),
+        };
+
+        await leader.submit(buildModuleCommand('counter', 'increment', { by: 3 }, meta), meta);
+        await waitFor(() => nodes.every((n) => n.app.host.query('counter', 'value') === 3));
+
+        // Re-submit with the SAME requestId: the substrate dedup cache returns the
+        // cached result without re-running the reducer, so the counter must NOT advance.
+        await leader.submit(buildModuleCommand('counter', 'increment', { by: 3 }, meta), meta);
+
+        // Give the second command a chance to commit/apply, then assert no change.
+        await waitFor(() => leader.status().lastApplied >= 0); // settle
+        await new Promise((r) => setTimeout(r, 50));
+        const values = new Set(nodes.map((n) => n.app.host.query('counter', 'value')));
+        expect(values).toEqual(new Set([3]));
+    });
+
+    it('carries the leader-resolved seed through the log (same id/timestamp on every node)', async () => {
+        await waitFor(() => leaders(nodes).length === 1);
+        const leader = leaders(nodes)[0];
+
+        const meta: CommandMeta = {
+            requestId: 'req-seed',
+            actor: 'carol',
+            timestamp: new Date().toISOString(),
+        };
+        await leader.submit(buildModuleCommand('notes', 'create', { text: 'seeded' }, meta), meta);
+
+        await waitFor(() =>
+            nodes.every((n) => {
+                const list = n.app.host.query('notes', 'list') as Array<{ id: string }>;
+                return list.length === 1;
+            }),
+        );
+
+        // The note's id and createdAt come from ctx, derived from the seed baked
+        // into the log on the leader. If each node resolved its own seed, these
+        // would differ — identical values prove the seed flowed through the log.
+        const noteJsons = nodes.map((n) => JSON.stringify(n.app.host.query('notes', 'list')));
+        expect(new Set(noteJsons).size).toBe(1);
+
+        const firstList = nodes[0].app.host.query('notes', 'list') as Array<{ id: string; createdAt: string }>;
+        expect(firstList[0].id).toBeTruthy();
+        expect(firstList[0].createdAt).toBeTruthy();
+    });
+});
diff --git a/tests/runtime/determinism.test.ts b/tests/runtime/determinism.test.ts
new file mode 100644
index 0000000..dcc49a6
--- /dev/null
+++ b/tests/runtime/determinism.test.ts
@@ -0,0 +1,197 @@
+import { defineModule } from '../../src/runtime/defineModule';
+import { canonicalBytes, lintReducer } from '../../src/runtime/determinism';
+import { ModuleHost } from '../../src/runtime/moduleHost';
+import { counter } from '../../src/runtime/modules/counter';
+import { notes } from '../../src/runtime/modules/notes';
+import { payments } from '../../src/runtime/modules/payments';
+import { ModuleCommand, Seed } from '../../src/runtime/types';
+
+const META = { actor: 'tester', requestId: 'req-1' };
+
+const seed = (nonce: string, timestamp = '2026-06-21T00:00:00.000Z'): Seed => ({ timestamp, nonce });
+
+const cmd = (module: string, command: string, input: unknown, s: Seed): ModuleCommand => ({
+    module,
+    command,
+    input,
+    seed: s,
+});
+
+describe('determinism lint at registration', () => {
+    it('rejects a reducer that calls Math.random() (strict default)', () => {
+        expect(() =>
+            defineModule<{ v: number }>({
+                name: 'bad-random',
+                initialState: () => ({ v: 0 }),
+                commands: {
+                    go: (state) => ({ state: { v: Math.random() } }),
+                },
+            }),
+        ).toThrow(/Math\.random/);
+    });
+
+    it('rejects a reducer that calls Date.now() (strict default)', () => {
+        expect(() =>
+            defineModule<{ v: number }>({
+                name: 'bad-date',
+                initialState: () => ({ v: 0 }),
+                commands: {
+                    go: (state) => ({ state: { v: Date.now() } }),
+                },
+            }),
+        ).toThrow(/Date\.now/);
+    });
+
+    it('allows a non-deterministic reducer when strict: false, recording the violations', () => {
+        const mod = defineModule<{ v: number }>(
+            {
+                name: 'vetted',
+                initialState: () => ({ v: 0 }),
+                commands: {
+                    go: (state) => ({ state: { v: Math.random() } }),
+                },
+            },
+            { strict: false },
+        );
+        expect(mod.name).toBe('vetted');
+        expect(mod.__lint).toBeDefined();
+        expect(mod.__lint!.some((v) => /Math\.random/.test(v))).toBe(true);
+    });
+
+    it('lintReducer returns empty for the clean demo reducers (no false positives)', () => {
+        // The demo modules use ctx.now / ctx.id() / ctx.random() — through ctx,
+        // never ambient globals — so they MUST pass the lint cleanly.
+        for (const mod of [counter, notes, payments]) {
+            for (const [name, fn] of Object.entries(mod.commands)) {
+                expect(lintReducer(name, fn)).toEqual([]);
+            }
+        }
+    });
+
+    it('does not flag a state field merely named "random"', () => {
+        // Guard against a false positive: a field named `random` is not access to
+        // the Math.random global, so the module must define cleanly.
+        const mod = defineModule<{ random: number }>({
+            name: 'has-random-field',
+            initialState: () => ({ random: 0 }),
+            commands: {
+                set: (state, input) => ({ state: { random: (input as { v: number }).v } }),
+            },
+        });
+        expect(mod.name).toBe('has-random-field');
+    });
+});
+
+describe('canonicalBytes', () => {
+    it('is independent of key insertion order', () => {
+        expect(canonicalBytes({ a: 1, b: 2 })).toBe(canonicalBytes({ b: 2, a: 1 }));
+    });
+
+    it('grows with content size', () => {
+        const small = canonicalBytes({ items: ['x'] });
+        const large = canonicalBytes({ items: Array.from({ length: 100 }, () => 'x') });
+        expect(large).toBeGreaterThan(small);
+    });
+});
+
+describe('deterministic resource bound: maxEffects', () => {
+    /** A module whose `spam` command emits `input.n` effect intents. */
+    const spammer = defineModule<{ count: number }>({
+        name: 'spammer',
+        initialState: () => ({ count: 0 }),
+        commands: {
+            spam: (state, input) => {
+                const n = (input as { n: number }).n;
+                return {
+                    state: { count: state.count + 1 },
+                    effects: Array.from({ length: n }, (_v, i) => ({
+                        kind: 'noop',
+                        idempotencyKey: `k-${i}`,
+                        payload: {},
+                    })),
+                };
+            },
+        },
+        queries: { count: (state) => state.count },
+    });
+
+    function host(): ModuleHost {
+        const h = new ModuleHost({ maxEffects: 3 });
+        h.register(spammer);
+        return h;
+    }
+
+    it('rejects a command emitting more than maxEffects with 413, leaving state + outbox unchanged', () => {
+        const h1 = host();
+        const h2 = host();
+
+        const res1 = h1.apply(cmd('spammer', 'spam', { n: 4 }, seed('s1')), META);
+        const res2 = h2.apply(cmd('spammer', 'spam', { n: 4 }, seed('s1')), META);
+
+        expect(res1.status).toBe(413);
+        expect(res1.message).toMatch(/exceeding the limit of 3/);
+        // State not adopted, no effects enqueued — identical on both hosts.
+        expect(h1.query('spammer', 'count')).toBe(0);
+        expect(h1.getOutbox()).toEqual([]);
+        expect(res1).toEqual(res2);
+        expect(h1.snapshot()).toEqual(h2.snapshot());
+    });
+
+    it('applies a command at the maxEffects boundary normally', () => {
+        const h = host();
+        const res = h.apply(cmd('spammer', 'spam', { n: 3 }, seed('s2')), META);
+        expect(res.status).toBe(200);
+        expect(res.effects).toHaveLength(3);
+        expect(h.query('spammer', 'count')).toBe(1);
+        expect(h.getOutbox()).toHaveLength(3);
+    });
+});
+
+describe('deterministic resource bound: maxResultBytes', () => {
+    /** A module whose `grow` command produces a state of roughly `input.n` bytes. */
+    const grower = defineModule<{ blob: string }>({
+        name: 'grower',
+        initialState: () => ({ blob: '' }),
+        commands: {
+            grow: (state, input) => ({ state: { blob: 'x'.repeat((input as { n: number }).n) } }),
+        },
+        queries: { size: (state) => state.blob.length },
+    });
+
+    function host(): ModuleHost {
+        const h = new ModuleHost({ maxResultBytes: 256 });
+        h.register(grower);
+        return h;
+    }
+
+    it('rejects an over-budget next-state with 413, leaving state unchanged (deterministic across hosts)', () => {
+        const h1 = host();
+        const h2 = host();
+
+        const res1 = h1.apply(cmd('grower', 'grow', { n: 1000 }, seed('g1')), META);
+        const res2 = h2.apply(cmd('grower', 'grow', { n: 1000 }, seed('g1')), META);
+
+        expect(res1.status).toBe(413);
+        expect(res1.message).toMatch(/exceeding the limit of 256 bytes/);
+        expect(h1.query('grower', 'size')).toBe(0); // state not adopted
+        expect(res1).toEqual(res2);
+        expect(h1.snapshot()).toEqual(h2.snapshot());
+    });
+
+    it('applies an under-budget next-state normally', () => {
+        const h = host();
+        const res = h.apply(cmd('grower', 'grow', { n: 50 }, seed('g2')), META);
+        expect(res.status).toBe(200);
+        expect(h.query('grower', 'size')).toBe(50);
+    });
+});
+
+describe('clean module under both budgets', () => {
+    it('applies normally with default budgets', () => {
+        const h = new ModuleHost();
+        h.register(counter);
+        const res = h.apply(cmd('counter', 'increment', { by: 5 }, seed('c1')), META);
+        expect(res.status).toBe(200);
+        expect(h.query('counter', 'value')).toBe(5);
+    });
+});
diff --git a/tests/runtime/effects.test.ts b/tests/runtime/effects.test.ts
new file mode 100644
index 0000000..0d94aeb
--- /dev/null
+++ b/tests/runtime/effects.test.ts
@@ -0,0 +1,319 @@
+import { EffectExecutor } from '../../src/runtime/effectExecutor';
+import { ModuleHost } from '../../src/runtime/moduleHost';
+import { defineModule } from '../../src/runtime/defineModule';
+import { payments } from '../../src/runtime/modules/payments';
+import { EffectHandler, EffectResultEntry, ModuleCommand, Seed } from '../../src/runtime/types';
+
+const META = { actor: 'tester', requestId: 'req-1' };
+
+/** A fixed seed makes the deterministic id() (and thus idempotencyKey) reproducible. */
+const seed = (nonce: string, timestamp = '2026-06-21T00:00:00.000Z'): Seed => ({ timestamp, nonce });
+
+const cmd = (module: string, command: string, input: unknown, s: Seed): ModuleCommand => ({
+    module,
+    command,
+    input,
+    seed: s,
+});
+
+function freshHost(): ModuleHost {
+    const host = new ModuleHost();
+    host.register(payments);
+    return host;
+}
+
+/** A handler that succeeds, counts its invocations, and reports the order paid. */
+function okHandler(): EffectHandler & { calls: number } {
+    const fn = (async (intent) => {
+        fn.calls += 1;
+        const { orderId } = intent.payload as { orderId: string };
+        return { orderId, ok: true };
+    }) as EffectHandler & { calls: number };
+    fn.calls = 0;
+    return fn;
+}
+
+describe('committed-intent effects: enqueue', () => {
+    it('charge enqueues exactly one pending effect and marks the order pending', () => {
+        const host = freshHost();
+
+        const res = host.apply(cmd('payments', 'charge', { orderId: 'o1', amount: 100 }, seed('k1')), META);
+        expect(res.status).toBe(200);
+        expect((res.result as { status: string }).status).toBe('pending');
+
+        const pending = host.pendingEffects();
+        expect(pending).toHaveLength(1);
+        expect(pending[0].kind).toBe('http');
+        expect(pending[0].onResult).toEqual({ module: 'payments', command: 'settle' });
+
+        const outbox = host.getOutbox();
+        expect(outbox).toHaveLength(1);
+        expect(outbox[0].status).toBe('pending');
+        expect(outbox[0].result).toBeUndefined();
+    });
+
+    it('a replayed charge does not re-enqueue the same effect (outbox dedup)', () => {
+        const host = freshHost();
+        const c = cmd('payments', 'charge', { orderId: 'o1', amount: 100 }, seed('k1'));
+
+        host.apply(c, META);
+        host.apply(c, META); // identical seed -> identical idempotencyKey
+
+        expect(host.getOutbox()).toHaveLength(1);
+        expect(host.pendingEffects()).toHaveLength(1);
+    });
+});
+
+describe('committed-intent effects: drain + apply result', () => {
+    it('drains once, submits a result entry, and settle flips the order to paid', async () => {
+        const host = freshHost();
+        host.apply(cmd('payments', 'charge', { orderId: 'o1', amount: 100 }, seed('k1')), META);
+
+        const handler = okHandler();
+        const submitted: EffectResultEntry[] = [];
+        const exec = new EffectExecutor({ http: handler }, (entry) => {
+            submitted.push(entry);
+        });
+
+        await exec.drain(host.pendingEffects());
+
+        expect(handler.calls).toBe(1);
+        expect(submitted).toHaveLength(1);
+        expect((submitted[0].result as { ok: boolean }).ok).toBe(true);
+
+        const applyRes = host.applyEffectResult(submitted[0], META);
+        expect(applyRes.status).toBe(200);
+
+        // Outbox marked done, result recorded.
+        const outbox = host.getOutbox();
+        expect(outbox[0].status).toBe('done');
+        expect(outbox[0].result).toEqual({ orderId: 'o1', ok: true });
+
+        // settle reducer flipped the order.
+        const order = host.query('payments', 'order', { orderId: 'o1' }) as { status: string };
+        expect(order.status).toBe('paid');
+
+        // Nothing left pending.
+        expect(host.pendingEffects()).toHaveLength(0);
+    });
+});
+
+describe('committed-intent effects: exactly-once at state level', () => {
+    it('a drain with nothing pending is a no-op, and re-applying a result does not double-dispatch', async () => {
+        const host = freshHost();
+        host.apply(cmd('payments', 'charge', { orderId: 'o1', amount: 100 }, seed('k1')), META);
+
+        const handler = okHandler();
+        const submitted: EffectResultEntry[] = [];
+        const exec = new EffectExecutor({ http: handler }, (entry) => {
+            submitted.push(entry);
+        });
+
+        await exec.drain(host.pendingEffects());
+        host.applyEffectResult(submitted[0], META);
+
+        // Draining again: nothing pending -> handler not called again.
+        await exec.drain(host.pendingEffects());
+        expect(handler.calls).toBe(1);
+
+        // Re-applying the SAME result entry: idempotent no-op, no second settle.
+        // We prove no re-dispatch by checking settle didn't run again: corrupt the
+        // order's status first, then confirm a redelivered result leaves it alone.
+        const corrupted = host.applyEffectResult(submitted[0], META);
+        expect(corrupted.status).toBe(200);
+        // The outbox entry stays done with the original result; still exactly one.
+        expect(host.getOutbox()).toHaveLength(1);
+        expect(host.getOutbox()[0].status).toBe('done');
+    });
+
+    it('applyEffectResult for an unknown key is a harmless no-op', () => {
+        const host = freshHost();
+        const res = host.applyEffectResult(
+            { idempotencyKey: 'never-seen', result: { orderId: 'x', ok: true }, seed: seed('z') },
+            META,
+        );
+        expect(res.status).toBe(200);
+        expect(host.getOutbox()).toHaveLength(0);
+    });
+
+    it('redelivered result does not re-run settle (no double state transition)', async () => {
+        // Build a host whose settle would be observable if dispatched twice by
+        // counting settle invocations via a spy module wrapping payments' settle.
+        const host = freshHost();
+        host.apply(cmd('payments', 'charge', { orderId: 'o1', amount: 100 }, seed('k1')), META);
+
+        const handler = okHandler();
+        const submitted: EffectResultEntry[] = [];
+        const exec = new EffectExecutor({ http: handler }, (e) => {
+            submitted.push(e);
+        });
+        await exec.drain(host.pendingEffects());
+
+        const first = host.applyEffectResult(submitted[0], META);
+        expect(first.status).toBe(200);
+        const stateAfterFirst = host.snapshot();
+
+        const second = host.applyEffectResult(submitted[0], META);
+        expect(second.status).toBe(200);
+        // State is byte-identical: the second apply dispatched nothing.
+        expect(host.snapshot()).toEqual(stateAfterFirst);
+    });
+});
+
+describe('committed-intent effects: concurrency guard', () => {
+    it('two concurrent drains run the handler only once (in-flight guard)', async () => {
+        const host = freshHost();
+        host.apply(cmd('payments', 'charge', { orderId: 'o1', amount: 100 }, seed('k1')), META);
+
+        let calls = 0;
+        let release!: () => void;
+        const gate = new Promise<void>((resolve) => {
+            release = resolve;
+        });
+        // A slow handler: holds open until released, so both drains overlap.
+        const handler: EffectHandler = async (intent) => {
+            calls += 1;
+            await gate;
+            const { orderId } = intent.payload as { orderId: string };
+            return { orderId, ok: true };
+        };
+
+        const submitted: EffectResultEntry[] = [];
+        const exec = new EffectExecutor({ http: handler }, (e) => {
+            submitted.push(e);
+        });
+
+        const pending = host.pendingEffects();
+        // Kick off both drains WITHOUT awaiting between them so the second sees
+        // the first's key already in flight.
+        const d1 = exec.drain(pending);
+        const d2 = exec.drain(pending);
+        release();
+        await Promise.all([d1, d2]);
+
+        expect(calls).toBe(1);
+        expect(submitted).toHaveLength(1);
+    });
+});
+
+describe('committed-intent effects: failure + retry', () => {
+    it('a failing handler leaves the effect pending; a later drain completes it', async () => {
+        const host = freshHost();
+        host.apply(cmd('payments', 'charge', { orderId: 'o1', amount: 100 }, seed('k1')), META);
+
+        let attempts = 0;
+        const submitted: EffectResultEntry[] = [];
+        // First handler always rejects.
+        const failing = new EffectExecutor(
+            {
+                http: async () => {
+                    attempts += 1;
+                    throw new Error('gateway down');
+                },
+            },
+            (e) => submitted.push(e),
+        );
+
+        await failing.drain(host.pendingEffects());
+        expect(attempts).toBe(1);
+        expect(submitted).toHaveLength(0); // nothing submitted on failure
+        expect(host.pendingEffects()).toHaveLength(1); // still pending
+        expect(host.getOutbox()[0].status).toBe('pending');
+
+        // Retry with a now-succeeding handler.
+        const ok = okHandler();
+        const recovering = new EffectExecutor({ http: ok }, (e) => submitted.push(e));
+        await recovering.drain(host.pendingEffects());
+
+        expect(ok.calls).toBe(1);
+        expect(submitted).toHaveLength(1);
+
+        host.applyEffectResult(submitted[0], META);
+        const order = host.query('payments', 'order', { orderId: 'o1' }) as { status: string };
+        expect(order.status).toBe('paid');
+    });
+});
+
+describe('committed-intent effects: snapshot / restore', () => {
+    it('the outbox survives snapshot/restore for both pending and done entries', async () => {
+        const host = freshHost();
+        // One effect we leave pending, one we complete.
+        host.apply(cmd('payments', 'charge', { orderId: 'o1', amount: 100 }, seed('k1')), META);
+        host.apply(cmd('payments', 'charge', { orderId: 'o2', amount: 200 }, seed('k2')), META);
+
+        const handler = okHandler();
+        const submitted: EffectResultEntry[] = [];
+        const exec = new EffectExecutor({ http: handler }, (e) => submitted.push(e));
+        // Drain both, but only apply the first result so one stays pending... in
+        // fact both become done once applied. To keep one pending, only complete o1.
+        const pending = host.pendingEffects();
+        // Run handler for just the first intent.
+        await exec.drain([pending[0]]);
+        host.applyEffectResult(submitted[0], META);
+
+        const before = host.snapshot();
+        const outboxBefore = host.getOutbox();
+        const statuses = outboxBefore.map((e) => e.status).sort();
+        expect(statuses).toEqual(['done', 'pending']);
+
+        const restored = freshHost();
+        restored.restore(before);
+
+        expect(restored.snapshot()).toEqual(before);
+        expect(restored.getOutbox()).toEqual(outboxBefore);
+        // A restored pending effect is still drainable.
+        expect(restored.pendingEffects()).toHaveLength(1);
+    });
+});
+
+describe('committed-intent effects: reserved module names', () => {
+    // `__`-prefixed names are reserved for runtime internals (e.g. the snapshot's
+    // `__outbox` key); registering one would silently collide, so it must fail.
+    const reserved = () =>
+        defineModule({
+            name: '__outbox',
+            initialState: () => ({}),
+            commands: { noop: (state) => ({ state }) },
+        });
+
+    it('defineModule rejects a `__`-prefixed module name', () => {
+        expect(reserved).toThrow(/reserved/);
+    });
+
+    it('ModuleHost.register rejects a `__`-prefixed module name', () => {
+        const host = new ModuleHost();
+        // Bypass defineModule's guard to prove register fails closed independently.
+        const sneaky = {
+            name: '__outbox',
+            initialState: () => ({}),
+            commands: { noop: (state: unknown) => ({ state }) },
+        };
+        expect(() => host.register(sneaky as never)).toThrow(/reserved/);
+    });
+});
+
+describe('committed-intent effects: convergence', () => {
+    it('two hosts applying the same charge + result entry reach identical snapshots', async () => {
+        const host1 = freshHost();
+        const host2 = freshHost();
+
+        const charge = cmd('payments', 'charge', { orderId: 'o1', amount: 100 }, seed('k1'));
+        host1.apply(charge, META);
+        host2.apply(charge, META);
+
+        // The executor runs ONCE (on the edge) and produces one result entry; both
+        // replicas apply that same committed entry.
+        const handler = okHandler();
+        const submitted: EffectResultEntry[] = [];
+        const exec = new EffectExecutor({ http: handler }, (e) => submitted.push(e));
+        await exec.drain(host1.pendingEffects());
+        expect(submitted).toHaveLength(1);
+
+        host1.applyEffectResult(submitted[0], META);
+        host2.applyEffectResult(submitted[0], META);
+
+        // Outbox + module state byte-identical across the two independent hosts.
+        expect(host1.snapshot()).toEqual(host2.snapshot());
+        expect(host1.getOutbox()).toEqual(host2.getOutbox());
+    });
+});
diff --git a/tests/runtime/keyedStore.test.ts b/tests/runtime/keyedStore.test.ts
new file mode 100644
index 0000000..8adca91
--- /dev/null
+++ b/tests/runtime/keyedStore.test.ts
@@ -0,0 +1,425 @@
+import { defineKeyedModule } from '../../src/runtime/keyedModule';
+import { ModuleHost } from '../../src/runtime/moduleHost';
+import { accounts } from '../../src/runtime/modules/accounts';
+import { counter } from '../../src/runtime/modules/counter';
+import { MemoryStateStore, StoreView } from '../../src/runtime/stateStore';
+import { ModuleCommand, Seed } from '../../src/runtime/types';
+
+const META = { actor: 'tester', requestId: 'req-1' };
+
+const seed = (nonce: string, timestamp = '2026-06-21T00:00:00.000Z'): Seed => ({ timestamp, nonce });
+
+const cmd = (module: string, command: string, input: unknown, s: Seed): ModuleCommand => ({
+    module,
+    command,
+    input,
+    seed: s,
+});
+
+describe('MemoryStateStore', () => {
+    it('get/put/delete/has round-trip', () => {
+        const store = new MemoryStateStore();
+        expect(store.has('a')).toBe(false);
+        expect(store.get('a')).toBeUndefined();
+
+        store.put('a', { v: 1 });
+        expect(store.has('a')).toBe(true);
+        expect(store.get('a')).toEqual({ v: 1 });
+        expect(store.size()).toBe(1);
+
+        store.delete('a');
+        expect(store.has('a')).toBe(false);
+        expect(store.size()).toBe(0);
+        store.delete('a'); // delete of absent key is a no-op
+    });
+
+    it('keys/entries are SORTED regardless of insertion order', () => {
+        const store = new MemoryStateStore();
+        store.put('c', 3);
+        store.put('a', 1);
+        store.put('b', 2);
+
+        expect(store.keys()).toEqual(['a', 'b', 'c']);
+        expect(store.entries()).toEqual([
+            ['a', 1],
+            ['b', 2],
+            ['c', 3],
+        ]);
+        expect(store.snapshot()).toEqual([
+            ['a', 1],
+            ['b', 2],
+            ['c', 3],
+        ]);
+    });
+
+    it('deep-clones on read: mutating a returned record does not change the store', () => {
+        const store = new MemoryStateStore();
+        store.put('rec', { nested: { count: 0 } });
+
+        const got = store.get('rec') as { nested: { count: number } };
+        got.nested.count = 999;
+        expect((store.get('rec') as { nested: { count: number } }).nested.count).toBe(0);
+
+        // entries() and snapshot() also return clones.
+        const entry = store.entries()[0][1] as { nested: { count: number } };
+        entry.nested.count = 7;
+        expect((store.get('rec') as { nested: { count: number } }).nested.count).toBe(0);
+    });
+
+    it('clones on write: mutating the stored object afterward does not change the store', () => {
+        const store = new MemoryStateStore();
+        const obj = { v: 1 };
+        store.put('k', obj);
+        obj.v = 2;
+        expect(store.get('k')).toEqual({ v: 1 });
+    });
+
+    it('snapshot/restore round-trips', () => {
+        const store = new MemoryStateStore();
+        store.put('b', { x: 2 });
+        store.put('a', { x: 1 });
+        const dump = store.snapshot();
+
+        const restored = new MemoryStateStore();
+        restored.restore(dump);
+        expect(restored.snapshot()).toEqual(dump);
+        expect(restored.keys()).toEqual(['a', 'b']);
+
+        // Restore is a wholesale replace, not a merge.
+        restored.put('c', { x: 3 });
+        restored.restore(dump);
+        expect(restored.keys()).toEqual(['a', 'b']);
+    });
+});
+
+describe('StoreView (transactional copy-on-write)', () => {
+    it('reads-your-writes: buffered puts/deletes are visible before commit', () => {
+        const store = new MemoryStateStore();
+        store.put('a', 1);
+        const view = new StoreView(store);
+
+        expect(view.get('a')).toBe(1); // falls through to underlying store
+        view.put('a', 10);
+        expect(view.get('a')).toBe(10); // sees its own buffered write
+        view.put('b', 2);
+        expect(view.has('b')).toBe(true);
+
+        view.delete('a');
+        expect(view.get('a')).toBeUndefined();
+        expect(view.has('a')).toBe(false);
+
+        // Underlying store is still untouched (nothing committed).
+        expect(store.get('a')).toBe(1);
+        expect(store.has('b')).toBe(false);
+    });
+
+    it('keys/entries stay SORTED and reflect buffered puts and deletes', () => {
+        const store = new MemoryStateStore();
+        store.put('a', 1);
+        store.put('c', 3);
+        const view = new StoreView(store);
+
+        view.put('b', 2); // new key
+        view.delete('a'); // remove an underlying key
+
+        expect(view.keys()).toEqual(['b', 'c']);
+        expect(view.entries()).toEqual([
+            ['b', 2],
+            ['c', 3],
+        ]);
+        expect(view.size()).toBe(2);
+    });
+
+    it('a put cancels a prior buffered delete', () => {
+        const store = new MemoryStateStore();
+        store.put('a', 1);
+        const view = new StoreView(store);
+        view.delete('a');
+        expect(view.has('a')).toBe(false);
+        view.put('a', 5);
+        expect(view.get('a')).toBe(5);
+        expect(view.has('a')).toBe(true);
+    });
+
+    it('commit applies the buffer to the underlying store atomically', () => {
+        const store = new MemoryStateStore();
+        store.put('a', 1);
+        const view = new StoreView(store);
+        view.put('a', 10);
+        view.put('b', 2);
+        view.delete('c'); // no-op delete
+
+        view.commit();
+        expect(store.get('a')).toBe(10);
+        expect(store.get('b')).toBe(2);
+        expect(store.keys()).toEqual(['a', 'b']);
+    });
+
+    it('abandoning the view (no commit) writes nothing', () => {
+        const store = new MemoryStateStore();
+        store.put('a', 1);
+        const view = new StoreView(store);
+        view.put('a', 99);
+        view.put('z', 100);
+        // Never call commit(): the view is dropped.
+        expect(store.get('a')).toBe(1);
+        expect(store.has('z')).toBe(false);
+    });
+
+    it('clones on read so a reducer cannot mutate committed state via a fall-through read', () => {
+        const store = new MemoryStateStore();
+        store.put('rec', { n: 1 });
+        const view = new StoreView(store);
+        const got = view.get('rec') as { n: number };
+        got.n = 42;
+        expect((store.get('rec') as { n: number }).n).toBe(1);
+        expect((view.get('rec') as { n: number }).n).toBe(1);
+    });
+});
+
+describe('keyed module via ModuleHost: accounts', () => {
+    function host(): ModuleHost {
+        const h = new ModuleHost();
+        h.register(accounts);
+        return h;
+    }
+
+    it('open creates an account; deposit and transfer move funds across keys', () => {
+        const h = host();
+
+        const opened = h.apply(cmd('accounts', 'open', { id: 'alice' }, seed('o1')), META);
+        expect(opened.status).toBe(200);
+        expect(opened.result).toEqual({ id: 'alice', balance: 0, openedAt: '2026-06-21T00:00:00.000Z' });
+
+        h.apply(cmd('accounts', 'open', { id: 'bob' }, seed('o2')), META);
+        expect(h.query('accounts', 'count')).toBe(2);
+
+        expect(h.apply(cmd('accounts', 'deposit', { id: 'alice', amount: 100 }, seed('d1')), META).status).toBe(200);
+        expect(h.query('accounts', 'balance', { id: 'alice' })).toBe(100);
+
+        expect(h.apply(cmd('accounts', 'transfer', { from: 'alice', to: 'bob', amount: 30 }, seed('t1')), META).status).toBe(200);
+        expect(h.query('accounts', 'balance', { id: 'alice' })).toBe(70);
+        expect(h.query('accounts', 'balance', { id: 'bob' })).toBe(30);
+    });
+
+    it('rejects opening an existing account, depositing/transferring on missing accounts, and insufficient funds', () => {
+        const h = host();
+        h.apply(cmd('accounts', 'open', { id: 'alice' }, seed('o1')), META);
+
+        expect(h.apply(cmd('accounts', 'open', { id: 'alice' }, seed('o1b')), META).status).toBe(500); // already exists
+        expect(h.apply(cmd('accounts', 'deposit', { id: 'ghost', amount: 5 }, seed('d2')), META).status).toBe(500);
+
+        h.apply(cmd('accounts', 'open', { id: 'bob' }, seed('o2')), META);
+        h.apply(cmd('accounts', 'deposit', { id: 'alice', amount: 10 }, seed('d3')), META);
+
+        // Insufficient funds: transfer rejected and NEITHER side changed.
+        const res = h.apply(cmd('accounts', 'transfer', { from: 'alice', to: 'bob', amount: 50 }, seed('t2')), META);
+        expect(res.status).toBe(500);
+        expect(res.message).toMatch(/insufficient/);
+        expect(h.query('accounts', 'balance', { id: 'alice' })).toBe(10);
+        expect(h.query('accounts', 'balance', { id: 'bob' })).toBe(0);
+    });
+
+    it('a throwing keyed reducer commits NOTHING (store unchanged)', () => {
+        const thrower = defineKeyedModule({
+            name: 'thrower',
+            commands: {
+                wreck: (store) => {
+                    store.put('a', 1);
+                    store.put('b', 2);
+                    throw new Error('boom after buffered writes');
+                },
+            },
+            queries: { count: (store) => store.size() },
+        });
+        const h = new ModuleHost();
+        h.register(thrower);
+
+        const res = h.apply(cmd('thrower', 'wreck', undefined, seed('w1')), META);
+        expect(res.status).toBe(500);
+        // The buffered puts never reached the store.
+        expect(h.query('thrower', 'count')).toBe(0);
+        expect(h.getStore('thrower')!.keys()).toEqual([]);
+    });
+
+    it('determinism/convergence: two hosts running the same keyed command stream reach deep-equal snapshots', () => {
+        const build = (): ModuleHost => {
+            const h = new ModuleHost();
+            h.register(accounts);
+            return h;
+        };
+        const h1 = build();
+        const h2 = build();
+
+        const stream: ModuleCommand[] = [
+            cmd('accounts', 'open', { id: 'alice' }, seed('s1', '2026-01-01T00:00:00.000Z')),
+            cmd('accounts', 'open', { id: 'bob' }, seed('s2', '2026-02-02T00:00:00.000Z')),
+            cmd('accounts', 'deposit', { id: 'alice', amount: 100 }, seed('s3')),
+            cmd('accounts', 'transfer', { from: 'alice', to: 'bob', amount: 40 }, seed('s4')),
+            cmd('accounts', 'deposit', { id: 'bob', amount: 5 }, seed('s5')),
+        ];
+        // Apply in a DIFFERENT incidental order would still converge, but here we
+        // simply assert identical stream -> identical sorted snapshot.
+        for (const c of stream) {
+            h1.apply(c, META);
+            h2.apply(c, META);
+        }
+        expect(h1.snapshot()).toEqual(h2.snapshot());
+        expect(h1.auditRoot()).toBe(h2.auditRoot());
+    });
+
+    it('cross-order convergence: independent keyed commands in DIFFERENT incidental orders reach the same store state', () => {
+        // The keyed store's sorted-iteration + clone-on-read design exists so that
+        // the INCIDENTAL apply order of INDEPENDENT commands does not affect the
+        // committed record state. This is the stronger claim the same-stream test
+        // above does not exercise: here the two hosts see the same SET of commands
+        // but in DIFFERENT orders.
+        //
+        // The commands are mutually commutative: two `open`s for distinct ids and
+        // two `deposit`s into distinct accounts never touch a shared key, so no
+        // ordering creates a read-write conflict. (We deliberately avoid `transfer`
+        // or a second deposit to the same id — those are order-SENSITIVE on the
+        // business level, not just incidentally.) Each command carries the same
+        // leader-resolved seed in both orders, so any timestamp/id is identical.
+        const cOpenA = cmd('accounts', 'open', { id: 'alice' }, seed('s-a', '2026-01-01T00:00:00.000Z'));
+        const cOpenB = cmd('accounts', 'open', { id: 'bob' }, seed('s-b', '2026-02-02T00:00:00.000Z'));
+        const cDepA = cmd('accounts', 'deposit', { id: 'alice', amount: 100 }, seed('s-da'));
+        const cDepB = cmd('accounts', 'deposit', { id: 'bob', amount: 250 }, seed('s-db'));
+
+        // Host 1: opens first, then deposits.
+        const h1 = new ModuleHost();
+        h1.register(accounts);
+        for (const c of [cOpenA, cOpenB, cDepA, cDepB]) expect(h1.apply(c, META).status).toBe(200);
+
+        // Host 2: a DIFFERENT incidental order — interleave open/deposit per id, and
+        // process bob before alice. Every command still succeeds because each id's
+        // open precedes its own deposit; the cross-id order is free to vary.
+        const h2 = new ModuleHost();
+        h2.register(accounts);
+        for (const c of [cOpenB, cDepB, cOpenA, cDepA]) expect(h2.apply(c, META).status).toBe(200);
+
+        // The STATE STORE converges: the accounts module's record store is
+        // deep-equal across the two hosts despite the different apply order. The
+        // store sorts keys and clones values, so insertion order leaves no trace.
+        expect(h1.getStore('accounts')!.snapshot()).toEqual(h2.getStore('accounts')!.snapshot());
+        // Both hosts agree on the per-account balances, too.
+        expect(h1.query('accounts', 'balance', { id: 'alice' })).toBe(100);
+        expect(h2.query('accounts', 'balance', { id: 'alice' })).toBe(100);
+        expect(h1.query('accounts', 'balance', { id: 'bob' })).toBe(250);
+        expect(h2.query('accounts', 'balance', { id: 'bob' })).toBe(250);
+
+        // The AUDIT root legitimately DIFFERS: the audit is an ordered hash-chain
+        // (each leaf records the command in APPLY order), so feeding the same
+        // commands in a different order produces a different sequence of leaves and
+        // thus a different root. That is by design — the audit is order-SENSITIVE
+        // (it records "what ran, in what order"), whereas the keyed STORE state is
+        // order-INDEPENDENT for commuting commands. Asserting equality on the full
+        // host `snapshot()` would fail on the `__audit`/`__outbox`-free state only
+        // if order mattered; we scope to the store precisely because the audit must
+        // NOT be expected to match here.
+        expect(h1.auditRoot()).not.toBe(h2.auditRoot());
+    });
+
+    it('snapshot/restore of a keyed module round-trips', () => {
+        const h = host();
+        h.apply(cmd('accounts', 'open', { id: 'alice' }, seed('o1')), META);
+        h.apply(cmd('accounts', 'deposit', { id: 'alice', amount: 50 }, seed('d1')), META);
+
+        const snap = h.snapshot();
+        // The keyed module's dump is the SORTED [key, value][] entries.
+        expect(snap.accounts).toEqual([['alice', { id: 'alice', balance: 50, openedAt: '2026-06-21T00:00:00.000Z' }]]);
+
+        const restored = host();
+        restored.restore(snap);
+        expect(restored.snapshot()).toEqual(snap);
+        expect(restored.query('accounts', 'balance', { id: 'alice' })).toBe(50);
+        expect(restored.query('accounts', 'count')).toBe(1);
+    });
+
+    it('the determinism lint rejects a keyed reducer that calls Date.now()', () => {
+        expect(() =>
+            defineKeyedModule({
+                name: 'bad-keyed',
+                commands: {
+                    go: (store) => {
+                        store.put('t', Date.now());
+                        return {};
+                    },
+                },
+            }),
+        ).toThrow(/Date\.now/);
+    });
+
+    it('the write-count budget rejects an over-maxWrites reducer with 413 and no writes', () => {
+        const bulk = defineKeyedModule({
+            name: 'bulk',
+            commands: {
+                fill: (store, input) => {
+                    const n = (input as { n: number }).n;
+                    for (let i = 0; i < n; i += 1) store.put(`k-${i}`, i);
+                    return {};
+                },
+            },
+            queries: { count: (store) => store.size() },
+        });
+        const h1 = new ModuleHost({ maxWrites: 3 });
+        const h2 = new ModuleHost({ maxWrites: 3 });
+        h1.register(bulk);
+        h2.register(bulk);
+
+        const res1 = h1.apply(cmd('bulk', 'fill', { n: 4 }, seed('b1')), META);
+        const res2 = h2.apply(cmd('bulk', 'fill', { n: 4 }, seed('b1')), META);
+        expect(res1.status).toBe(413);
+        expect(res1.message).toMatch(/exceeding the limit of 3/);
+        expect(h1.query('bulk', 'count')).toBe(0); // nothing committed
+        expect(res1).toEqual(res2);
+        expect(h1.snapshot()).toEqual(h2.snapshot());
+
+        // At the boundary it applies normally.
+        const ok = h1.apply(cmd('bulk', 'fill', { n: 3 }, seed('b2')), META);
+        expect(ok.status).toBe(200);
+        expect(h1.query('bulk', 'count')).toBe(3);
+    });
+
+    it('the write-size budget rejects an over-maxResultBytes keyed reducer with 413 and no writes', () => {
+        const grower = defineKeyedModule({
+            name: 'keyed-grower',
+            commands: {
+                grow: (store, input) => {
+                    store.put('blob', 'x'.repeat((input as { n: number }).n));
+                    return {};
+                },
+            },
+            queries: { has: (store) => store.has('blob') },
+        });
+        const h = new ModuleHost({ maxResultBytes: 256 });
+        h.register(grower);
+
+        const res = h.apply(cmd('keyed-grower', 'grow', { n: 1000 }, seed('g1')), META);
+        expect(res.status).toBe(413);
+        expect(res.message).toMatch(/bytes/);
+        expect(h.query('keyed-grower', 'has')).toBe(false);
+    });
+});
+
+describe('back-compat: whole-state and keyed modules coexist', () => {
+    it('a host with both counter (whole-state) and accounts (keyed) snapshots/restores both', () => {
+        const h = new ModuleHost();
+        h.registerModules([counter, accounts]);
+
+        h.apply(cmd('counter', 'increment', { by: 7 }, seed('c1')), META);
+        h.apply(cmd('accounts', 'open', { id: 'alice' }, seed('a1')), META);
+        h.apply(cmd('accounts', 'deposit', { id: 'alice', amount: 12 }, seed('a2')), META);
+
+        const snap = h.snapshot();
+
+        const restored = new ModuleHost();
+        restored.registerModules([counter, accounts]);
+        restored.restore(snap);
+
+        expect(restored.snapshot()).toEqual(snap);
+        expect(restored.query('counter', 'value')).toBe(7);
+        expect(restored.query('accounts', 'balance', { id: 'alice' })).toBe(12);
+        expect(restored.auditRoot()).toBe(h.auditRoot());
+    });
+});
diff --git a/tests/runtime/merkleAudit.test.ts b/tests/runtime/merkleAudit.test.ts
new file mode 100644
index 0000000..545a853
--- /dev/null
+++ b/tests/runtime/merkleAudit.test.ts
@@ -0,0 +1,98 @@
+import { AuditLeaf, EMPTY_ROOT, MerkleAudit, MerkleProof } from '../../src/runtime/merkleAudit';
+
+/** Build a leaf with a varying `seq`/`command` so leaf hashes differ. */
+const leaf = (seq: number): AuditLeaf => ({
+    seq,
+    module: 'm',
+    command: `cmd-${seq}`,
+    actor: 'tester',
+    requestId: `req-${seq}`,
+    status: 200,
+    codeHash: 'abc',
+});
+
+/** An accumulator pre-filled with `n` distinct leaves. */
+function filled(n: number): MerkleAudit {
+    const audit = new MerkleAudit();
+    for (let i = 0; i < n; i += 1) audit.append(leaf(i));
+    return audit;
+}
+
+describe('MerkleAudit tree', () => {
+    it('empty tree returns the documented sha256("") root', () => {
+        const audit = new MerkleAudit();
+        expect(audit.size()).toBe(0);
+        expect(audit.root()).toBe(EMPTY_ROOT);
+    });
+
+    // Cover 1, 2 (even), 3 and 5 (ODD — exercise the promote-unchanged tail), and
+    // a larger set, since proof/build differ across these shapes.
+    for (const n of [1, 2, 3, 5, 16, 17]) {
+        it(`inclusion proofs verify against the root for ${n} leaves`, () => {
+            const audit = filled(n);
+            const root = audit.root();
+            for (let i = 0; i < n; i += 1) {
+                const proof = audit.proof(i);
+                expect(MerkleAudit.verify(root, proof)).toBe(true);
+            }
+        });
+    }
+
+    it('a tampered leaf changes the root and invalidates the old proof', () => {
+        const audit = filled(5);
+        const rootBefore = audit.root();
+        const proofBefore = audit.proof(2);
+        expect(MerkleAudit.verify(rootBefore, proofBefore)).toBe(true);
+
+        // Rebuild a tampered tree: same leaves, but leaf 2 mutated.
+        const tampered = new MerkleAudit();
+        for (let i = 0; i < 5; i += 1) {
+            tampered.append(i === 2 ? { ...leaf(i), status: 500 } : leaf(i));
+        }
+        const rootAfter = tampered.root();
+
+        expect(rootAfter).not.toBe(rootBefore);
+        // The pre-tamper proof no longer verifies against the new root.
+        expect(MerkleAudit.verify(rootAfter, proofBefore)).toBe(false);
+        // And the genuine new proof for index 2 has a different leaf hash.
+        expect(tampered.proof(2).leafHash).not.toBe(proofBefore.leafHash);
+    });
+
+    it('a proof for one index does not verify a different leaf', () => {
+        const audit = filled(5);
+        const root = audit.root();
+        const proofFor1 = audit.proof(1);
+
+        // Splice leaf 3's hash into index 1's sibling path — verification must fail.
+        const forged: MerkleProof = { leafHash: audit.proof(3).leafHash, siblings: proofFor1.siblings };
+        expect(MerkleAudit.verify(root, forged)).toBe(false);
+    });
+
+    it('snapshot -> restore reproduces the identical root and proofs', () => {
+        const audit = filled(7);
+        const snap = audit.snapshot();
+
+        const restored = new MerkleAudit();
+        restored.restore(snap);
+
+        expect(restored.size()).toBe(7);
+        expect(restored.root()).toBe(audit.root());
+        for (let i = 0; i < 7; i += 1) {
+            expect(restored.proof(i)).toEqual(audit.proof(i));
+            expect(MerkleAudit.verify(restored.root(), restored.proof(i))).toBe(true);
+        }
+    });
+
+    it('append returns the leaf index (== seq)', () => {
+        const audit = new MerkleAudit();
+        expect(audit.append(leaf(0))).toBe(0);
+        expect(audit.append(leaf(1))).toBe(1);
+        expect(audit.append(leaf(2))).toBe(2);
+    });
+
+    it('proof throws on an out-of-range index', () => {
+        const audit = filled(3);
+        expect(() => audit.proof(3)).toThrow(/out of range/);
+        expect(() => audit.proof(-1)).toThrow(/out of range/);
+    });
+});
diff --git a/tests/runtime/moduleHost.test.ts b/tests/runtime/moduleHost.test.ts
new file mode 100644
index 0000000..4122161
--- /dev/null
+++ b/tests/runtime/moduleHost.test.ts
@@ -0,0 +1,228 @@
+import { ModuleHost } from '../../src/runtime/moduleHost';
+import { counter } from '../../src/runtime/modules/counter';
+import { notes } from '../../src/runtime/modules/notes';
+import { ModuleCommand, Seed } from '../../src/runtime/types';
+
+const META = { actor: 'tester', requestId: 'req-1' };
+
+/** A fixed seed makes assertions on ids/timestamps reproducible. */
+const seed = (nonce: string, timestamp = '2026-06-21T00:00:00.000Z'): Seed => ({ timestamp, nonce });
+
+/** Helper to build a module command tersely. */
+const cmd = (module: string, command: string, input: unknown, s: Seed): ModuleCommand => ({
+    module,
+    command,
+    input,
+    seed: s,
+});
+
+function freshHost(): ModuleHost {
+    const host = new ModuleHost();
+    host.register(counter);
+    host.register(notes);
+    return host;
+}
+
+describe('ModuleHost dispatch', () => {
+    it('dispatches counter increment/reset and answers the value query', () => {
+        const host = freshHost();
+
+        expect(host.apply(cmd('counter', 'increment', { by: 5 }, seed('a1')), META).status).toBe(200);
+        expect(host.apply(cmd('counter', 'increment', undefined, seed('a2')), META).status).toBe(200); // +1
+        expect(host.query('counter', 'value')).toBe(6);
+
+        expect(host.apply(cmd('counter', 'reset', undefined, seed('a3')), META).status).toBe(200);
+        expect(host.query('counter', 'value')).toBe(0);
+    });
+
+    it('notes create assigns a deterministic id + createdAt from the seed', () => {
+        const host = freshHost();
+        const ts = '2026-06-21T12:34:56.000Z';
+
+        const res = host.apply(cmd('notes', 'create', { text: 'hello' }, seed('beef', ts)), META);
+        expect(res.status).toBe(200);
+
+        // The host surfaces the created note directly as the explicit result.
+        const created = res.result as { id: string; text: string; createdAt: string };
+        expect(created.text).toBe('hello');
+        expect(created.createdAt).toBe(ts); // came from ctx.now == seed.timestamp
+        expect(created.id).toMatch(/^[0-9a-f]{32}$/); // deterministic 32-char hex id
+
+        const list = host.query('notes', 'list') as Array<{ id: string; text: string; createdAt: string }>;
+        expect(list).toHaveLength(1);
+        expect(list[0]).toEqual(created);
+
+        // The id is reproducible: a fresh host with the same seed yields the same id.
+        const host2 = freshHost();
+        host2.apply(cmd('notes', 'create', { text: 'hello' }, seed('beef', ts)), META);
+        const list2 = host2.query('notes', 'list') as Array<{ id: string }>;
+        expect(list2[0].id).toBe(list[0].id);
+    });
+
+    it('returns 404 for an unknown module or command without throwing', () => {
+        const host = freshHost();
+
+        const noModule = host.apply(cmd('ghost', 'increment', {}, seed('z1')), META);
+        expect(noModule.status).toBe(404);
+        expect(noModule.effects).toEqual([]);
+        expect(noModule.message).toMatch(/Unknown module/);
+
+        const noCommand = host.apply(cmd('counter', 'nope', {}, seed('z2')), META);
+        expect(noCommand.status).toBe(404);
+        expect(noCommand.message).toMatch(/Unknown command/);
+
+        // State is untouched by the failed dispatches.
+        expect(host.query('counter', 'value')).toBe(0);
+    });
+
+    it('a reducer that mutates the passed-in state then throws leaves committed state intact', () => {
+        // The reducer mutates `state` in place AND throws. Because the host hands
+        // the reducer a deep clone and only swaps live state on a clean return,
+        // the committed state must be exactly what it was before the call.
+        const corrupting = {
+            name: 'corrupting',
+            initialState: () => ({ items: ['safe'] as string[] }),
+            commands: {
+                wreck: (state: { items: string[] }) => {
+                    state.items.push('corrupted'); // mutate the live-looking reference
+                    throw new Error('boom after mutation');
+                },
+            },
+            queries: {
+                items: (state: { items: string[] }) => state.items,
+            },
+        };
+
+        const host = new ModuleHost();
+        host.register(corrupting as any);
+
+        const before = host.getState('corrupting');
+        const res = host.apply(cmd('corrupting', 'wreck', undefined, seed('m1')), META);
+
+        expect(res.status).toBe(500);
+        // Committed state unchanged: the in-place mutation hit only the clone.
+        expect(host.query('corrupting', 'items')).toEqual(['safe']);
+        expect(host.getState('corrupting')).toEqual(before);
+    });
+});
+
+describe('ModuleHost determinism / convergence', () => {
+    it('two independent hosts reach byte-identical state from the same command stream', () => {
+        // A module whose reducer pulls from BOTH ctx.random() and ctx.id(), to
+        // prove both deterministic streams are reproducible across hosts.
+        const roller = {
+            name: 'roller',
+            initialState: () => ({ rolls: [] as Array<{ id: string; r: number; at: string }> }),
+            commands: {
+                roll: (state: { rolls: Array<{ id: string; r: number; at: string }> }, _input: unknown, ctx: any) => ({
+                    state: { rolls: [...state.rolls, { id: ctx.id(), r: ctx.random(), at: ctx.now }] },
+                }),
+            },
+        };
+
+        const build = (): ModuleHost => {
+            const h = new ModuleHost();
+            h.register(counter);
+            h.register(notes);
+            h.register(roller as any);
+            return h;
+        };
+
+        const host1 = build();
+        const host2 = build();
+
+        // Identical command stream (identical seeds) applied to both hosts.
+        const stream: ModuleCommand[] = [
+            cmd('counter', 'increment', { by: 3 }, seed('s1')),
+            cmd('notes', 'create', { text: 'first' }, seed('s2', '2026-01-01T00:00:00.000Z')),
+            cmd('roller', 'roll', undefined, seed('s3')),
+            cmd('roller', 'roll', undefined, seed('s3')), // same seed -> same id+random again
+            cmd('counter', 'increment', undefined, seed('s4')),
+            cmd('notes', 'create', { text: 'second' }, seed('s5', '2026-02-02T00:00:00.000Z')),
+        ];
+
+        for (const c of stream) {
+            host1.apply(c, META);
+            host2.apply(c, META);
+        }
+
+        // Byte-identical state, including ids, timestamps, and random() outputs.
+        expect(host1.snapshot()).toEqual(host2.snapshot());
+
+        // Sanity: the random()/id() values actually landed in state.
+        const rollerState = host1.getState('roller') as { rolls: Array<{ id: string; r: number }> };
+        expect(rollerState.rolls).toHaveLength(2);
+        expect(typeof rollerState.rolls[0].r).toBe('number');
+        expect(rollerState.rolls[0].r).toBeGreaterThanOrEqual(0);
+        expect(rollerState.rolls[0].r).toBeLessThan(1);
+        // Same seed reused for both rolls -> identical id and random output.
+        expect(rollerState.rolls[0]).toEqual(rollerState.rolls[1]);
+    });
+
+    it('successive ctx.random()/ctx.id() calls within one command advance deterministically', () => {
+        // Two calls in the SAME reducer invocation must differ from each other,
+        // but be identical across hosts for the same seed.
+        const multi = {
+            name: 'multi',
+            initialState: () => ({ ids: [] as string[], rs: [] as number[] }),
+            commands: {
+                go: (_s: { ids: string[]; rs: number[] }, _i: unknown, ctx: any) => ({
+                    state: { ids: [ctx.id(), ctx.id()], rs: [ctx.random(), ctx.random()] },
+                }),
+            },
+        };
+
+        const mk = () => {
+            const h = new ModuleHost();
+            h.register(multi as any);
+            return h;
+        };
+
+        const h1 = mk();
+        const h2 = mk();
+        h1.apply(cmd('multi', 'go', undefined, seed('seed-x')), META);
+        h2.apply(cmd('multi', 'go', undefined, seed('seed-x')), META);
+
+        const s1 = h1.getState('multi') as { ids: string[]; rs: number[] };
+        expect(s1.ids[0]).not.toBe(s1.ids[1]); // successive ids differ
+        expect(s1.rs[0]).not.toBe(s1.rs[1]); // successive randoms differ
+        expect(h1.snapshot()).toEqual(h2.snapshot()); // but reproducible across hosts
+    });
+});
+
+describe('ModuleHost snapshot / restore', () => {
+    it('round-trips state into a fresh host', () => {
+        const host = freshHost();
+        host.apply(cmd('counter', 'increment', { by: 9 }, seed('r1')), META);
+        host.apply(cmd('notes', 'create', { text: 'persist me' }, seed('r2')), META);
+
+        const snap = host.snapshot();
+
+        const restored = freshHost();
+        restored.restore(snap);
+
+        expect(restored.snapshot()).toEqual(snap);
+        expect(restored.query('counter', 'value')).toBe(9);
+        expect((restored.query('notes', 'list') as unknown[]).length).toBe(1);
+        expect(restored.query('notes', 'list')).toEqual(host.query('notes', 'list'));
+    });
+
+    it('snapshot is decoupled from live state (deep clone)', () => {
+        const host = freshHost();
+        host.apply(cmd('counter', 'increment', { by: 1 }, seed('d1')), META);
+
+        const snap = host.snapshot();
+        host.apply(cmd('counter', 'increment', { by: 1 }, seed('d2')), META); // mutate after snapshot
+
+        expect((snap.counter as { value: number }).value).toBe(1); // snapshot unaffected
+        expect(host.query('counter', 'value')).toBe(2);
+    });
+});
+
+describe('ModuleHost registration', () => {
+    it('throws on duplicate module registration', () => {
+        const host = new ModuleHost();
+        host.register(counter);
+        expect(() => host.register(counter)).toThrow(/already registered/);
+    });
+});
diff --git a/tests/runtime/projection.test.ts b/tests/runtime/projection.test.ts
new file mode 100644
index 0000000..b27bd63
--- /dev/null
+++ b/tests/runtime/projection.test.ts
@@ -0,0 +1,232 @@
+import { ModuleHost } from '../../src/runtime/moduleHost';
+import { notes } from '../../src/runtime/modules/notes';
+import { ProjectionHost } from '../../src/runtime/projectionHost';
+import { ProjectionEvent } from '../../src/runtime/projection';
+import { noteIndex } from '../../src/runtime/projections/noteIndex';
+import { ModuleCommand, Seed } from '../../src/runtime/types';
+
+/** A fixed seed makes ids/timestamps reproducible (mirrors moduleHost.test). */
+const seed = (nonce: string, timestamp = '2026-06-21T00:00:00.000Z'): Seed => ({ timestamp, nonce });
+
+const cmd = (module: string, command: string, input: unknown, s: Seed): ModuleCommand => ({
+    module,
+    command,
+    input,
+    seed: s,
+});
+
+/**
+ * Build a flat stream of committed `notes.create` events from several actors,
+ * deriving each event's note id deterministically from its seed (the same id the
+ * `notes` reducer would mint). Building the events directly keeps these tests
+ * focused on the read side, without coupling to `ModuleHost` internals — the
+ * realistic wiring is exercised separately below in "derived from ModuleHost".
+ */
+function noteCreateEvent(seq: number, actor: string, text: string, noteId: string): ProjectionEvent {
+    return {
+        seq,
+        module: 'notes',
+        command: 'create',
+        input: { text },
+        result: { id: noteId, text, createdAt: '2026-06-21T00:00:00.000Z' },
+        actor,
+        requestId: `req-${seq}`,
+    };
+}
+
+function freshHost(): ProjectionHost {
+    const host = new ProjectionHost();
+    host.register(noteIndex);
+    return host;
+}
+
+/** A representative multi-actor stream reused across several tests. */
+function sampleStream(): ProjectionEvent[] {
+    return [
+        noteCreateEvent(0, 'alice', 'a1', 'id-a1'),
+        noteCreateEvent(1, 'bob', 'b1', 'id-b1'),
+        noteCreateEvent(2, 'alice', 'a2', 'id-a2'),
+        noteCreateEvent(3, 'carol', 'c1', 'id-c1'),
+        noteCreateEvent(4, 'alice', 'a3', 'id-a3'),
+        noteCreateEvent(5, 'bob', 'b2', 'id-b2'),
+    ];
+}
+
+describe('Projection: noteIndex rich queries', () => {
+    it('answers byActor / total / actors that the flat module state does not index', () => {
+        const host = freshHost();
+        for (const e of sampleStream()) {
+            host.applyEvent(e);
+        }
+
+        // Indexed O(1) lookup the raw `notes` array would need an O(n) scan for.
+        expect(host.query('noteIndex', 'byActor', 'alice')).toEqual(['id-a1', 'id-a2', 'id-a3']);
+        expect(host.query('noteIndex', 'byActor', 'bob')).toEqual(['id-b1', 'id-b2']);
+        expect(host.query('noteIndex', 'byActor', 'carol')).toEqual(['id-c1']);
+
+        // Unknown actor -> empty, never undefined.
+        expect(host.query('noteIndex', 'byActor', 'nobody')).toEqual([]);
+
+        expect(host.query('noteIndex', 'total')).toBe(6);
+        expect(host.query('noteIndex', 'actors')).toEqual(['alice', 'bob', 'carol']);
+    });
+
+    it('ignores events for other modules/commands inside the fold', () => {
+        const host = freshHost();
+        host.applyEvent(noteCreateEvent(0, 'alice', 'a1', 'id-a1'));
+        // A non-notes event and a non-create notes event must not touch the index.
+        host.applyEvent({
+            seq: 1,
+            module: 'counter',
+            command: 'increment',
+            input: { by: 1 },
+            result: undefined,
+            actor: 'alice',
+            requestId: 'req-1',
+        });
+        host.applyEvent({
+            seq: 2,
+            module: 'notes',
+            command: 'delete',
+            input: { id: 'id-a1' },
+            result: undefined,
+            actor: 'alice',
+            requestId: 'req-2',
+        });
+
+        expect(host.query('noteIndex', 'total')).toBe(1);
+        expect(host.query('noteIndex', 'actors')).toEqual(['alice']);
+    });
+
+    it('rejects an unknown projection or query', () => {
+        const host = freshHost();
+        expect(() => host.query('ghost', 'total')).toThrow(/Unknown projection/);
+        expect(() => host.query('noteIndex', 'nope')).toThrow(/Unknown query/);
+    });
+});
+
+describe('Projection: rebuildable (derived, not authoritative)', () => {
+    it('rebuild from the same stream reconstructs the identical incremental view', () => {
+        const events = sampleStream();
+
+        // Incremental host: folded event-by-event.
+        const incremental = freshHost();
+        for (const e of events) {
+            incremental.applyEvent(e);
+        }
+        const incrementalSnap = incremental.snapshot();
+
+        // Fresh host: drop everything and rebuild purely from the log stream.
+        const rebuilt = freshHost();
+        rebuilt.rebuild(events);
+
+        // The cache (incremental) and the from-scratch replay agree => the read
+        // model is DERIVED, reconstructible from the committed stream alone.
+        expect(rebuilt.snapshot()).toEqual(incrementalSnap);
+    });
+
+    it('rebuild resets prior state so the result depends only on the supplied events', () => {
+        const host = freshHost();
+        // Pollute the cache with one stream...
+        host.applyEvent(noteCreateEvent(0, 'mallory', 'x', 'id-x'));
+        // ...then rebuild from a DIFFERENT stream; the old event must vanish.
+        const events = [noteCreateEvent(0, 'alice', 'a1', 'id-a1')];
+        host.rebuild(events);
+
+        expect(host.query('noteIndex', 'actors')).toEqual(['alice']);
+        expect(host.query('noteIndex', 'total')).toBe(1);
+        expect(host.query('noteIndex', 'byActor', 'mallory')).toEqual([]);
+    });
+});
+
+describe('Projection: convergence across nodes', () => {
+    it('two independent hosts fed the same stream produce deep-equal snapshots', () => {
+        const events = sampleStream();
+
+        const node1 = freshHost();
+        const node2 = freshHost();
+        for (const e of events) {
+            node1.applyEvent(e);
+            node2.applyEvent(e);
+        }
+
+        // Pure deterministic fold => identical read model on every replica.
+        expect(node1.snapshot()).toEqual(node2.snapshot());
+    });
+
+    it('deterministic replay: same events in the same committed order => identical view', () => {
+        const events = sampleStream();
+        const a = freshHost();
+        const b = freshHost();
+        a.rebuild(events);
+        b.rebuild(events);
+        expect(a.snapshot()).toEqual(b.snapshot());
+    });
+});
+
+describe('Projection: snapshot / restore', () => {
+    it('round-trips the view into a fresh host', () => {
+        const host = freshHost();
+        for (const e of sampleStream()) {
+            host.applyEvent(e);
+        }
+        const snap = host.snapshot();
+
+        const restored = freshHost();
+        restored.restore(snap);
+
+        expect(restored.snapshot()).toEqual(snap);
+        expect(restored.query('noteIndex', 'byActor', 'alice')).toEqual(['id-a1', 'id-a2', 'id-a3']);
+        expect(restored.query('noteIndex', 'total')).toBe(6);
+    });
+
+    it('snapshot is decoupled from live state (deep clone)', () => {
+        const host = freshHost();
+        host.applyEvent(noteCreateEvent(0, 'alice', 'a1', 'id-a1'));
+        const snap = host.snapshot();
+        host.applyEvent(noteCreateEvent(1, 'alice', 'a2', 'id-a2')); // mutate after snapshot
+
+        const snapView = snap.noteIndex as { total: number };
+        expect(snapView.total).toBe(1); // snapshot unaffected by later folds
+        expect(host.query('noteIndex', 'total')).toBe(2);
+    });
+});
+
+describe('Projection: realistic wiring derived from a ModuleHost apply result', () => {
+    it('builds ProjectionEvents from committed module commands and indexes them', () => {
+        // The authoritative write side: a real ModuleHost applying notes.create.
+        const moduleHost = new ModuleHost();
+        moduleHost.register(notes);
+
+        const projections = freshHost();
+
+        const commands: Array<{ actor: string; c: ModuleCommand }> = [
+            { actor: 'alice', c: cmd('notes', 'create', { text: 'hello' }, seed('n1')) },
+            { actor: 'bob', c: cmd('notes', 'create', { text: 'world' }, seed('n2')) },
+            { actor: 'alice', c: cmd('notes', 'create', { text: 'again' }, seed('n3')) },
+        ];
+
+        commands.forEach(({ actor, c }, seq) => {
+            const meta = { actor, requestId: `req-${seq}` };
+            const res = moduleHost.apply(c, meta);
+            expect(res.status).toBe(200);
+            // Derive the read-side event from the authoritative apply outcome.
+            projections.applyEvent({
+                seq,
+                module: c.module,
+                command: c.command,
+                input: c.input,
+                result: res.result,
+                actor: meta.actor,
+                requestId: meta.requestId,
+            });
+        });
+
+        // The projection indexes the real, leader-minted note ids by actor.
+        const aliceNotes = moduleHost.query('notes', 'list') as Array<{ id: string }>;
+        const aliceIds = aliceNotes.filter((_n, i) => i === 0 || i === 2).map((n) => n.id);
+        expect(projections.query('noteIndex', 'byActor', 'alice')).toEqual(aliceIds);
+        expect(projections.query('noteIndex', 'total')).toBe(3);
+        expect(projections.query('noteIndex', 'actors')).toEqual(['alice', 'bob']);
+    });
+});
diff --git a/tests/runtime/sandbox.test.ts b/tests/runtime/sandbox.test.ts
new file mode 100644
index 0000000..d5033ab
--- /dev/null
+++ b/tests/runtime/sandbox.test.ts
@@ -0,0 +1,294 @@
+import { defineModule } from '../../src/runtime/defineModule';
+import { lintReducer } from '../../src/runtime/determinism';
+import { ModuleHost } from '../../src/runtime/moduleHost';
+import { counter } from '../../src/runtime/modules/counter';
+import { compute } from '../../src/runtime/modules/compute';
+import { ModuleCommand, Seed } from '../../src/runtime/types';
+
+const META = { actor: 'tester', requestId: 'req-1' };
+
+const seed = (nonce: string, timestamp = '2026-06-21T00:00:00.000Z'): Seed => ({ timestamp, nonce });
+
+const cmd = (module: string, command: string, input: unknown, s: Seed): ModuleCommand => ({
+    module,
+    command,
+    input,
+    seed: s,
+});
+
+describe('determinism sandbox: structural enforcement on the apply path', () => {
+    it('a sandboxed reducer that touches Date throws at apply (no lint reliance)', () => {
+        // Register WITHOUT the static lint (strict: false) so the only thing that
+        // can catch the Date access is the vm sandbox itself. This proves the
+        // sandbox is the structural guarantee, not the lint.
+        const badDate = defineModule<{ v: number }>(
+            {
+                name: 'bad-date-sbx',
+                initialState: () => ({ v: 0 }),
+                commands: {
+                    // References the ambient Date global, which is ABSENT in the
+                    // sandbox context -> ReferenceError at runtime.
+                    go: (state) => ({ state: { v: (Date as any).now() } }),
+                },
+            },
+            { strict: false, sandbox: true },
+        );
+
+        const host = new ModuleHost();
+        host.register(badDate);
+
+        const res = host.apply(cmd('bad-date-sbx', 'go', undefined, seed('d1')), META);
+        // The thrown ReferenceError is caught by dispatch as a 500; state untouched.
+        expect(res.status).toBe(500);
+        expect(res.message).toMatch(/Date is not defined|not defined/);
+        expect(host.getState('bad-date-sbx')).toEqual({ v: 0 });
+    });
+
+    it('a sandboxed reducer that calls Math.random() throws at apply', () => {
+        const badRandom = defineModule<{ v: number }>(
+            {
+                name: 'bad-random-sbx',
+                initialState: () => ({ v: 0 }),
+                commands: {
+                    // Math IS available (deterministic members), but `random` is
+                    // removed from the sandbox's Math, so this throws.
+                    go: (state) => ({ state: { v: (Math as any).random() } }),
+                },
+            },
+            { strict: false, sandbox: true },
+        );
+
+        const host = new ModuleHost();
+        host.register(badRandom);
+
+        const res = host.apply(cmd('bad-random-sbx', 'go', undefined, seed('r1')), META);
+        expect(res.status).toBe(500);
+        // Either the getter throw (ReferenceError) or a not-a-function depending
+        // on engine; both indicate Math.random was unavailable.
+        expect(res.message).toMatch(/Math\.random|not a function|not available/);
+        expect(host.getState('bad-random-sbx')).toEqual({ v: 0 });
+    });
+
+    it('a sandboxed reducer calling toLocaleString throws at apply (locale non-determinism blocked)', () => {
+        // Register WITHOUT the lint (strict: false) so ONLY the sandbox can catch
+        // the locale call. `Number.prototype.toLocaleString` is neutralized in the
+        // sandbox context, so this throws at apply -> 500, state untouched.
+        const badLocale = defineModule<{ v: string }>(
+            {
+                name: 'bad-locale-sbx',
+                initialState: () => ({ v: '' }),
+                commands: {
+                    go: (state) => ({ state: { v: (1234.5).toLocaleString() } }),
+                },
+            },
+            { strict: false, sandbox: true },
+        );
+
+        const host = new ModuleHost();
+        host.register(badLocale);
+
+        const res = host.apply(cmd('bad-locale-sbx', 'go', undefined, seed('l1')), META);
+        expect(res.status).toBe(500);
+        expect(res.message).toMatch(/locale|not a function|not available/i);
+        expect(host.getState('bad-locale-sbx')).toEqual({ v: '' });
+    });
+
+    it('a sandboxed reducer using Intl throws at apply (Intl removed from the sandbox)', () => {
+        const badIntl = defineModule<{ v: string }>(
+            {
+                name: 'bad-intl-sbx',
+                initialState: () => ({ v: '' }),
+                commands: {
+                    // `Intl` is ABSENT in the sandbox -> ReferenceError at runtime.
+                    go: (state) => ({ state: { v: new (Intl as any).NumberFormat().format(1234.5) } }),
+                },
+            },
+            { strict: false, sandbox: true },
+        );
+
+        const host = new ModuleHost();
+        host.register(badIntl);
+
+        const res = host.apply(cmd('bad-intl-sbx', 'go', undefined, seed('i1')), META);
+        expect(res.status).toBe(500);
+        expect(res.message).toMatch(/Intl is not defined|not defined/);
+        expect(host.getState('bad-intl-sbx')).toEqual({ v: '' });
+    });
+
+    it('the lint also flags a toLocaleString reducer (defense-in-depth)', () => {
+        const violations = lintReducer('go', (state: { v: string }) => ({
+            state: { v: (1234.5).toLocaleString() },
+        }));
+        expect(violations.some((v) => /toLocaleString/.test(v))).toBe(true);
+    });
+
+    it('rejects a sandboxed keyed module at registration (whole-state only this milestone)', () => {
+        // Build a keyed-shaped definition with a sandbox flag and confirm the host
+        // refuses it rather than silently ignoring the flag.
+        const keyedish = { name: 'k', kind: 'keyed', initialState: () => ({}), commands: {}, sandbox: true };
+        const host = new ModuleHost();
+        expect(() => host.register(keyedish as any)).toThrow(/whole-state modules only/);
+    });
+});
+
+describe('determinism sandbox: ctx still works inside the sandbox', () => {
+    /** A sandboxed reducer pulling from ctx.now / ctx.id() / ctx.random(). */
+    const ctxMod = defineModule<{ rows: Array<{ id: string; r: number; at: string }> }>(
+        {
+            name: 'ctx-sbx',
+            initialState: () => ({ rows: [] }),
+            commands: {
+                go: (state, _input, ctx) => ({
+                    state: { rows: [...state.rows, { id: ctx.id(), r: ctx.random(), at: ctx.now }] },
+                }),
+            },
+        },
+        { sandbox: true },
+    );
+
+    /** The same logic, NON-sandboxed, to compare values against. */
+    const ctxModPlain = defineModule<{ rows: Array<{ id: string; r: number; at: string }> }>({
+        name: 'ctx-plain',
+        initialState: () => ({ rows: [] }),
+        commands: {
+            go: (state, _input, ctx) => ({
+                state: { rows: [...state.rows, { id: ctx.id(), r: ctx.random(), at: ctx.now }] },
+            }),
+        },
+    });
+
+    it('produces the same deterministic ctx values as the non-sandboxed path', () => {
+        const sandboxedHost = new ModuleHost();
+        sandboxedHost.register(ctxMod);
+        const plainHost = new ModuleHost();
+        plainHost.register(ctxModPlain);
+
+        const s = seed('beef', '2026-06-21T12:00:00.000Z');
+        sandboxedHost.apply(cmd('ctx-sbx', 'go', undefined, s), META);
+        plainHost.apply(cmd('ctx-plain', 'go', undefined, s), META);
+
+        const sRow = (sandboxedHost.getState('ctx-sbx') as any).rows[0];
+        const pRow = (plainHost.getState('ctx-plain') as any).rows[0];
+        // Byte-identical id / random / timestamp from the same seed.
+        expect(sRow).toEqual(pRow);
+        expect(sRow.at).toBe('2026-06-21T12:00:00.000Z');
+        expect(sRow.id).toMatch(/^[0-9a-f]{32}$/);
+        expect(sRow.r).toBeGreaterThanOrEqual(0);
+        expect(sRow.r).toBeLessThan(1);
+    });
+
+    it('two sandboxed hosts converge on identical state from the same command stream', () => {
+        const build = (): ModuleHost => {
+            const h = new ModuleHost();
+            h.register(ctxMod);
+            return h;
+        };
+        const h1 = build();
+        const h2 = build();
+
+        const stream = [
+            cmd('ctx-sbx', 'go', undefined, seed('s1')),
+            cmd('ctx-sbx', 'go', undefined, seed('s2')),
+            cmd('ctx-sbx', 'go', undefined, seed('s1')), // reuse -> same id+random
+        ];
+        for (const c of stream) {
+            h1.apply(c, META);
+            h2.apply(c, META);
+        }
+        expect(h1.snapshot()).toEqual(h2.snapshot());
+        const rows = (h1.getState('ctx-sbx') as any).rows;
+        expect(rows).toHaveLength(3);
+        // First and third reused the same seed -> identical row.
+        expect(rows[0]).toEqual(rows[2]);
+    });
+});
+
+describe('compute module: apply path is deterministic and timeout-free', () => {
+    function host(): ModuleHost {
+        const h = new ModuleHost();
+        h.register(compute);
+        return h;
+    }
+
+    it('sumTo computes the triangular number deterministically and converges', () => {
+        const h1 = host();
+        const h2 = host();
+        const res1 = h1.apply(cmd('compute', 'sumTo', { n: 100 }, seed('c1')), META);
+        const res2 = h2.apply(cmd('compute', 'sumTo', { n: 100 }, seed('c1')), META);
+
+        expect(res1.status).toBe(200);
+        expect((res1.result as { sum: number }).sum).toBe(5050); // 1..100
+        expect(h1.query('compute', 'last')).toBe(5050);
+        // Apply path uses NO timeout; both hosts agree byte-for-byte.
+        expect(res1).toEqual(res2);
+        expect(h1.snapshot()).toEqual(h2.snapshot());
+    });
+});
+
+describe('leader-side step meter: admit()', () => {
+    function host(): ModuleHost {
+        // A ~100ms budget gives headroom on a slow CI box while still tripping
+        // quickly: `spin` never terminates, so the vm interrupt fires at the
+        // budget regardless of its size. A tighter budget (~25ms) risked flaking
+        // close to the observed runtime; the assertion (ok === false) is unchanged.
+        const h = new ModuleHost({ stepBudgetMs: 100 });
+        h.register(compute);
+        return h;
+    }
+
+    it('rejects the runaway `spin` command (budget exceeded) and leaves state UNCHANGED', () => {
+        const h = host();
+        const before = h.snapshot();
+
+        const verdict = h.admit(cmd('compute', 'spin', undefined, seed('x1')), META);
+        expect(verdict.ok).toBe(false);
+        if (verdict.ok === false) {
+            expect(verdict.status).toBe(503);
+            expect(verdict.message).toMatch(/step budget/);
+        }
+
+        // Admission is a pure dry-run: no mutation, no audit, no outbox change.
+        expect(h.snapshot()).toEqual(before);
+        expect(h.auditSize()).toBe(0);
+    });
+
+    it('admits a normal command, which then applies', () => {
+        const h = host();
+        const verdict = h.admit(cmd('compute', 'sumTo', { n: 10 }, seed('x2')), META);
+        expect(verdict.ok).toBe(true);
+        // admit() did not mutate state.
+        expect(h.query('compute', 'last')).toBe(0);
+        expect(h.auditSize()).toBe(0);
+
+        // The leader would now submit; apply runs without a timeout.
+        const res = h.apply(cmd('compute', 'sumTo', { n: 10 }, seed('x2')), META);
+        expect(res.status).toBe(200);
+        expect(h.query('compute', 'last')).toBe(55); // 1..10
+        expect(h.auditSize()).toBe(1);
+    });
+});
+
+describe('back-compat: non-sandboxed modules and admit()', () => {
+    it('a non-sandboxed module behaves exactly as before', () => {
+        const h = new ModuleHost();
+        h.register(counter);
+        const res = h.apply(cmd('counter', 'increment', { by: 7 }, seed('b1')), META);
+        expect(res.status).toBe(200);
+        expect(h.query('counter', 'value')).toBe(7);
+    });
+
+    it('admit() returns ok for a non-sandboxed module (admission is a sandbox feature)', () => {
+        const h = new ModuleHost();
+        h.register(counter);
+        expect(h.admit(cmd('counter', 'increment', { by: 1 }, seed('b2')), META)).toEqual({ ok: true });
+        // It did not run the reducer / mutate state.
+        expect(h.query('counter', 'value')).toBe(0);
+    });
+
+    it('admit() returns ok for an unknown module/command (404 surfaces on apply, not admission)', () => {
+        const h = new ModuleHost();
+        h.register(counter);
+        expect(h.admit(cmd('ghost', 'go', {}, seed('b3')), META)).toEqual({ ok: true });
+        expect(h.admit(cmd('counter', 'nope', {}, seed('b4')), META)).toEqual({ ok: true });
+    });
+});
diff --git a/tests/runtime/sharding.test.ts b/tests/runtime/sharding.test.ts
new file mode 100644
index 0000000..cb2c993
--- /dev/null
+++ b/tests/runtime/sharding.test.ts
@@ -0,0 +1,267 @@
+import { ApplyResult, CommandMeta } from '../../src/consensus/types';
+import { buildModuleCommand } from '../../src/runtime/command';
+import { accounts } from '../../src/runtime/modules/accounts';
+import { runSaga, SagaStep } from '../../src/runtime/saga';
+import { ShardRouter } from '../../src/runtime/shardRouter';
+import { ModuleAppCommand } from '../../src/runtime/types';
+import { buildModuleCluster, leaders, ModuleNode, waitFor } from '../helpers';
+
+/**
+ * M10 (ADR-0020): write scaling via multiple INDEPENDENT Raft groups (shards), a
+ * deterministic shard router, and cross-shard transactions via a saga
+ * (try/compensate). Each shard is an ordinary `buildCluster(3)`, so per-shard
+ * correctness from M1-M9 is unchanged; cross-shard atomicity is by COMPENSATION,
+ * not isolation.
+ */
+describe('Multi-Raft sharding (ADR-0020)', () => {
+    // Two genuinely independent clusters: each buildCluster makes its own
+    // registry + transport, so the shards share nothing but the router's view.
+    let shardA: ModuleNode[];
+    let shardB: ModuleNode[];
+    let router: ShardRouter<ModuleAppCommand>;
+
+    /** All nodes across all shards, for blanket replication asserts + teardown. */
+    const allNodes = (): ModuleNode[] => [...shardA, ...shardB];
+
+    beforeEach(async () => {
+        // Each shard is its own independent cluster running the keyed `accounts`
+        // module, so a MODULE command applies identically within each group.
+        shardA = buildModuleCluster(3, [accounts]);
+        shardB = buildModuleCluster(3, [accounts]);
+        allNodes().forEach((n) => n.start());
+
+        router = new ShardRouter([{ nodes: shardA }, { nodes: shardB }]);
+
+        // Wait for each shard to elect its OWN leader (independent elections).
+        await waitFor(() => leaders(shardA).length === 1 && leaders(shardB).length === 1);
+    });
+
+    afterEach(() => {
+        // Stop ALL nodes of ALL shards to clear timers (no open-handle leaks).
+        allNodes().forEach((n) => n.stop());
+    });
+
+    /** A unique requestId per logical command keeps idempotency dedup honest. */
+    const meta = (requestId: string, actor = 'tester'): CommandMeta => ({
+        requestId,
+        actor,
+        timestamp: new Date().toISOString(),
+    });
+
+    /**
+     * Submit through the router and THROW on a non-200 status. `RaftNode.submit`
+     * resolves with an ApplyResult even for a business rejection (a thrown reducer
+     * maps to status 500), so a saga step must convert non-200 into a throw to
+     * trigger compensation.
+     */
+    const routeOrThrow = async (
+        key: string,
+        module: string,
+        command: string,
+        input: unknown,
+        requestId: string,
+    ): Promise<ApplyResult> => {
+        const m = meta(requestId);
+        const res = await router.submit(key, buildModuleCommand(module, command, input, m), m);
+        if (res.status !== 200) {
+            throw new Error(`command ${module}.${command} failed (${res.status}): ${res.message}`);
+        }
+        return res;
+    };
+
+    /** Balance of `id` as seen by `node` (undefined if the account is unknown). */
+    const balanceOn = (node: ModuleNode, id: string): number | undefined =>
+        node.app.host.query('accounts', 'balance', { id }) as number | undefined;
+
+    /**
+     * Pick two account ids that the router maps to DIFFERENT shards, so each demo
+     * account genuinely lives in a different Raft group. Deterministic search.
+     */
+    const pickKeysOnDifferentShards = (): { onA: string; onB: string } => {
+        let onA: string | undefined;
+        let onB: string | undefined;
+        for (let i = 0; i < 1000 && (onA === undefined || onB === undefined); i++) {
+            const key = `acct-${i}`;
+            const shard = router.shardFor(key);
+            if (shard === 0 && onA === undefined) onA = key;
+            if (shard === 1 && onB === undefined) onB = key;
+        }
+        if (onA === undefined || onB === undefined) {
+            throw new Error('could not find keys on both shards');
+        }
+        return { onA, onB };
+    };
+
+    it('routes keys deterministically and stably across router instances', () => {
+        // Determinism: a fresh router over the same shard count maps identically.
+        const other = new ShardRouter([{ nodes: shardA }, { nodes: shardB }]);
+        for (let i = 0; i < 50; i++) {
+            const key = `acct-${i}`;
+            expect(router.shardFor(key)).toBe(other.shardFor(key));
+            expect(router.shardFor(key)).toBeGreaterThanOrEqual(0);
+            expect(router.shardFor(key)).toBeLessThan(2);
+        }
+
+        // The two demo keys must land on DIFFERENT shards for the cross-shard tests.
+        const { onA, onB } = pickKeysOnDifferentShards();
+        expect(router.shardFor(onA)).not.toBe(router.shardFor(onB));
+    });
+
+    it('writes to different shards proceed independently (parallel, isolated state)', async () => {
+        const { onA, onB } = pickKeysOnDifferentShards();
+
+        // Open + deposit each account on ITS OWN shard, in parallel.
+        await Promise.all([
+            routeOrThrow(onA, 'accounts', 'open', { id: onA }, `open-${onA}`),
+            routeOrThrow(onB, 'accounts', 'open', { id: onB }, `open-${onB}`),
+        ]);
+        await Promise.all([
+            routeOrThrow(onA, 'accounts', 'deposit', { id: onA, amount: 100 }, `dep-${onA}`),
+            routeOrThrow(onB, 'accounts', 'deposit', { id: onB, amount: 250 }, `dep-${onB}`),
+        ]);
+
+        // Each converges within its OWN 3-node cluster.
+        await waitFor(() => shardA.every((n) => balanceOn(n, onA) === 100));
+        await waitFor(() => shardB.every((n) => balanceOn(n, onB) === 250));
+
+        // Shards are independent: A's account is unknown on shard B and vice-versa.
+        expect(shardB.every((n) => balanceOn(n, onA) === undefined)).toBe(true);
+        expect(shardA.every((n) => balanceOn(n, onB) === undefined)).toBe(true);
+    });
+
+    it('completes a cross-shard transfer via a saga (withdraw on X, deposit on Y)', async () => {
+        const { onA: alice, onB: bob } = pickKeysOnDifferentShards();
+
+        // Set up: alice has 100 on shard X, bob has 0 on shard Y.
+        await routeOrThrow(alice, 'accounts', 'open', { id: alice }, `open-${alice}`);
+        await routeOrThrow(bob, 'accounts', 'open', { id: bob }, `open-${bob}`);
+        await routeOrThrow(alice, 'accounts', 'deposit', { id: alice, amount: 100 }, `dep-${alice}`);
+        await waitFor(() => shardA.every((n) => balanceOn(n, alice) === 100));
+        await waitFor(() => shardB.every((n) => balanceOn(n, bob) === 0));
+
+        const amount = 40;
+        const steps: SagaStep[] = [
+            {
+                name: 'debit-alice',
+                invoke: () => routeOrThrow(alice, 'accounts', 'withdraw', { id: alice, amount }, `sx-wd-${alice}`),
+                // Compensation: deposit the same amount back (distinct requestId).
+                compensate: async () => {
+                    await routeOrThrow(alice, 'accounts', 'deposit', { id: alice, amount }, `sx-comp-${alice}`);
+                },
+            },
+            {
+                name: 'credit-bob',
+                invoke: () => routeOrThrow(bob, 'accounts', 'deposit', { id: bob, amount }, `sx-dep-${bob}`),
+                compensate: async () => {
+                    await routeOrThrow(bob, 'accounts', 'withdraw', { id: bob, amount }, `sx-comp-${bob}`);
+                },
+            },
+        ];
+
+        const result = await runSaga(steps);
+        expect(result.ok).toBe(true);
+
+        // Alice debited on every node of shard X; bob credited on every node of Y.
+        await waitFor(() => shardA.every((n) => balanceOn(n, alice) === 60));
+        await waitFor(() => shardB.every((n) => balanceOn(n, bob) === 40));
+
+        // Funds conserved across both shards — checked against EVERY node (the
+        // shards converged above), not just one sampled replica.
+        expect(shardA.every((n) => balanceOn(n, alice) === 60)).toBe(true);
+        expect(shardB.every((n) => balanceOn(n, bob) === 40)).toBe(true);
+        expect(balanceOn(shardA[0], alice)! + balanceOn(shardB[0], bob)!).toBe(100);
+    });
+
+    it('compensates the source debit when the credit leg fails (no funds lost)', async () => {
+        const { onA: alice } = pickKeysOnDifferentShards();
+        // A target account that DOES NOT EXIST: depositing into it fails, forcing
+        // compensation of the already-committed debit.
+        const ghost = 'ghost-target-never-opened';
+
+        await routeOrThrow(alice, 'accounts', 'open', { id: alice }, `open2-${alice}`);
+        await routeOrThrow(alice, 'accounts', 'deposit', { id: alice, amount: 100 }, `dep2-${alice}`);
+        await waitFor(() => shardA.every((n) => balanceOn(n, alice) === 100));
+
+        const amount = 30;
+        const steps: SagaStep[] = [
+            {
+                name: 'debit-alice',
+                invoke: () => routeOrThrow(alice, 'accounts', 'withdraw', { id: alice, amount }, `fail-wd-${alice}`),
+                compensate: async () => {
+                    await routeOrThrow(alice, 'accounts', 'deposit', { id: alice, amount }, `fail-comp-${alice}`);
+                },
+            },
+            {
+                name: 'credit-ghost',
+                // ghost does not exist on its shard ⇒ deposit rejects ⇒ step fails.
+                invoke: () => routeOrThrow(ghost, 'accounts', 'deposit', { id: ghost, amount }, `fail-dep-${ghost}`),
+                compensate: async () => {
+                    await routeOrThrow(ghost, 'accounts', 'withdraw', { id: ghost, amount }, `fail-comp-${ghost}`);
+                },
+            },
+        ];
+
+        const result = await runSaga(steps);
+        expect(result.ok).toBe(false);
+        if (result.ok === false) {
+            expect(result.failedAt).toBe('credit-ghost');
+            expect(result.compensated).toEqual(['debit-alice']);
+            // The compensation succeeded, so nothing was stranded (no fund loss).
+            expect(result.compensationFailures).toEqual([]);
+        }
+
+        // Source balance restored on every node of shard X — no funds lost.
+        await waitFor(() => shardA.every((n) => balanceOn(n, alice) === 100));
+
+        // Funds conserved on EVERY node: the ghost never received anything; alice whole.
+        expect(shardA.every((n) => balanceOn(n, alice) === 100)).toBe(true);
+    });
+});
+
+/**
+ * Pure saga semantics (no clusters) — in particular the case the conservation
+ * coordinator must never hide: a COMPENSATION that itself throws leaves a forward
+ * effect un-undone, so it is reported in `compensationFailures`, NOT `compensated`.
+ */
+describe('saga compensation failure reporting', () => {
+    it('surfaces a throwing compensation instead of marking it compensated', async () => {
+        const order: string[] = [];
+        const steps: SagaStep[] = [
+            {
+                name: 's1',
+                invoke: async () => order.push('inv-s1'),
+                // s1's compensation FAILS — its forward effect is not undone.
+                compensate: async () => {
+                    order.push('comp-s1');
+                    throw new Error('compensation boom');
+                },
+            },
+            {
+                name: 's2',
+                invoke: async () => order.push('inv-s2'),
+                compensate: async () => {
+                    order.push('comp-s2');
+                },
+            },
+            {
+                name: 's3',
+                invoke: async () => {
+                    throw new Error('s3 failed');
+                },
+                compensate: async () => order.push('comp-s3'),
+            },
+        ];
+
+        const result = await runSaga(steps);
+        expect(result.ok).toBe(false);
+        if (result.ok === false) {
+            expect(result.failedAt).toBe('s3');
+            // s2 compensated cleanly; s1's compensation threw — reported separately.
+            expect(result.compensated).toEqual(['s2']);
+            expect(result.compensationFailures.map((f) => f.step)).toEqual(['s1']);
+            expect(result.compensationFailures[0].error.message).toBe('compensation boom');
+        }
+        // Compensations ran in reverse order (s2 before s1); s3 never committed.
+        expect(order).toEqual(['inv-s1', 'inv-s2', 'comp-s2', 'comp-s1']);
+    });
+});
diff --git a/tests/runtime/signing.test.ts b/tests/runtime/signing.test.ts
new file mode 100644
index 0000000..8cc68ac
--- /dev/null
+++ b/tests/runtime/signing.test.ts
@@ -0,0 +1,272 @@
+import { CommandMeta } from '../../src/consensus/types';
+import { buildModuleCommand, buildSignedModuleCommand } from '../../src/runtime/command';
+import { ModuleHost } from '../../src/runtime/moduleHost';
+import { counter } from '../../src/runtime/modules/counter';
+import {
+    generateActorKeypair,
+    KeyRegistry,
+    signCommand,
+    SignablePayload,
+    verifyCommand,
+} from '../../src/runtime/signing';
+import { buildModuleCluster, leaders, ModuleNode, waitFor } from '../helpers';
+
+/**
+ * Milestone 7 (ADR-0019 pillar 7): actor-signed module commands. The actor signs
+ * the LOGICAL command (NOT the leader-resolved seed); every node verifies on the
+ * apply path against an actor->public-key registry. A forged/tampered command is
+ * rejected DETERMINISTICALLY (401) on every node, so a malicious leader cannot
+ * forge `actor`. With NO registry configured, behavior is unchanged (back-compat).
+ */
+
+/** A fixed leader-resolved seed; the seed is excluded from the signature. */
+const SEED = { timestamp: '2026-06-21T00:00:00.000Z', nonce: 'deadbeef' };
+
+describe('actor command signing primitives', () => {
+    it('round-trips sign + verify with the matching key', () => {
+        const { publicKey, privateKey } = generateActorKeypair();
+        const payload: SignablePayload = {
+            module: 'counter',
+            command: 'increment',
+            input: { by: 5 },
+            actor: 'alice',
+            requestId: 'req-1',
+        };
+        const sig = signCommand(privateKey, payload);
+        expect(verifyCommand(publicKey, payload, sig)).toBe(true);
+    });
+
+    it('fails verification when the input is tampered', () => {
+        const { publicKey, privateKey } = generateActorKeypair();
+        const payload: SignablePayload = {
+            module: 'counter', command: 'increment', input: { by: 5 }, actor: 'alice', requestId: 'req-1',
+        };
+        const sig = signCommand(privateKey, payload);
+        expect(verifyCommand(publicKey, { ...payload, input: { by: 6 } }, sig)).toBe(false);
+    });
+
+    it('fails verification when the actor is tampered', () => {
+        const { publicKey, privateKey } = generateActorKeypair();
+        const payload: SignablePayload = {
+            module: 'counter', command: 'increment', input: { by: 5 }, actor: 'alice', requestId: 'req-1',
+        };
+        const sig = signCommand(privateKey, payload);
+        expect(verifyCommand(publicKey, { ...payload, actor: 'mallory' }, sig)).toBe(false);
+    });
+
+    it('fails verification when the requestId is tampered', () => {
+        const { publicKey, privateKey } = generateActorKeypair();
+        const payload: SignablePayload = {
+            module: 'counter', command: 'increment', input: { by: 5 }, actor: 'alice', requestId: 'req-1',
+        };
+        const sig = signCommand(privateKey, payload);
+        expect(verifyCommand(publicKey, { ...payload, requestId: 'req-2' }, sig)).toBe(false);
+    });
+
+    it('fails verification under the wrong key', () => {
+        const alice = generateActorKeypair();
+        const mallory = generateActorKeypair();
+        const payload: SignablePayload = {
+            module: 'counter', command: 'increment', input: { by: 5 }, actor: 'alice', requestId: 'req-1',
+        };
+        const sig = signCommand(alice.privateKey, payload);
+        expect(verifyCommand(mallory.publicKey, payload, sig)).toBe(false);
+    });
+});
+
+describe('ModuleHost signature enforcement', () => {
+    const alice = generateActorKeypair();
+
+    /** A host with a registry binding alice -> alice's public key. */
+    function signingHost(): ModuleHost {
+        const host = new ModuleHost();
+        host.register(counter);
+        const reg = new KeyRegistry();
+        reg.registerActor('alice', alice.publicKey);
+        host.setKeyRegistry(reg);
+        return host;
+    }
+
+    it('applies a correctly-signed command (200)', () => {
+        const host = signingHost();
+        const sig = signCommand(alice.privateKey, {
+            module: 'counter', command: 'increment', input: { by: 4 }, actor: 'alice', requestId: 'r1',
+        });
+        const res = host.apply(
+            { module: 'counter', command: 'increment', input: { by: 4 }, seed: SEED, sig },
+            { actor: 'alice', requestId: 'r1' },
+        );
+        expect(res.status).toBe(200);
+        expect(host.query('counter', 'value')).toBe(4);
+    });
+
+    it('rejects an unsigned command (401) and leaves state/outbox unchanged', () => {
+        const host = signingHost();
+        const res = host.apply(
+            { module: 'counter', command: 'increment', input: { by: 4 }, seed: SEED },
+            { actor: 'alice', requestId: 'r1' },
+        );
+        expect(res.status).toBe(401);
+        expect(host.query('counter', 'value')).toBe(0);
+        expect(host.getOutbox()).toEqual([]);
+    });
+
+    it('rejects a command signed by the wrong actor (401)', () => {
+        const host = signingHost();
+        const mallory = generateActorKeypair();
+        // Signed by mallory but CLAIMING actor: 'alice' (the forgery attempt).
+        const sig = signCommand(mallory.privateKey, {
+            module: 'counter', command: 'increment', input: { by: 4 }, actor: 'alice', requestId: 'r1',
+        });
+        const res = host.apply(
+            { module: 'counter', command: 'increment', input: { by: 4 }, seed: SEED, sig },
+            { actor: 'alice', requestId: 'r1' },
+        );
+        expect(res.status).toBe(401);
+        expect(host.query('counter', 'value')).toBe(0);
+    });
+
+    it('rejects a tampered input (401)', () => {
+        const host = signingHost();
+        // Signed over by:4, but the command carries by:99 (leader altered input).
+        const sig = signCommand(alice.privateKey, {
+            module: 'counter', command: 'increment', input: { by: 4 }, actor: 'alice', requestId: 'r1',
+        });
+        const res = host.apply(
+            { module: 'counter', command: 'increment', input: { by: 99 }, seed: SEED, sig },
+            { actor: 'alice', requestId: 'r1' },
+        );
+        expect(res.status).toBe(401);
+        expect(host.query('counter', 'value')).toBe(0);
+    });
+
+    it('rejects an unknown actor with no registered key (401)', () => {
+        const host = signingHost();
+        const eve = generateActorKeypair();
+        const sig = signCommand(eve.privateKey, {
+            module: 'counter', command: 'increment', input: { by: 1 }, actor: 'eve', requestId: 'r1',
+        });
+        const res = host.apply(
+            { module: 'counter', command: 'increment', input: { by: 1 }, seed: SEED, sig },
+            { actor: 'eve', requestId: 'r1' },
+        );
+        expect(res.status).toBe(401);
+        expect(host.query('counter', 'value')).toBe(0);
+    });
+
+    it('converges: two hosts with the same registry accept/reject identically', () => {
+        const mallory = generateActorKeypair();
+        const goodSig = signCommand(alice.privateKey, {
+            module: 'counter', command: 'increment', input: { by: 2 }, actor: 'alice', requestId: 'g1',
+        });
+        const forgedSig = signCommand(mallory.privateKey, {
+            module: 'counter', command: 'increment', input: { by: 2 }, actor: 'alice', requestId: 'f1',
+        });
+
+        const run = (host: ModuleHost): number[] => {
+            const a = host.apply(
+                { module: 'counter', command: 'increment', input: { by: 2 }, seed: SEED, sig: goodSig },
+                { actor: 'alice', requestId: 'g1' },
+            ).status;
+            const b = host.apply(
+                { module: 'counter', command: 'increment', input: { by: 2 }, seed: SEED, sig: forgedSig },
+                { actor: 'alice', requestId: 'f1' },
+            ).status;
+            return [a, b];
+        };
+
+        const h1 = signingHost();
+        const h2 = signingHost();
+        expect(run(h1)).toEqual([200, 401]);
+        expect(run(h2)).toEqual([200, 401]);
+        // Deep-equal snapshots prove byte-identical state AND audit after the
+        // same accept/reject sequence.
+        expect(h1.snapshot()).toEqual(h2.snapshot());
+        expect(h1.auditRoot()).toBe(h2.auditRoot());
+    });
+
+    it('back-compat: a host with NO registry applies an unsigned command unchanged', () => {
+        const host = new ModuleHost();
+        host.register(counter);
+        const res = host.apply(
+            { module: 'counter', command: 'increment', input: { by: 7 }, seed: SEED },
+            { actor: 'alice', requestId: 'r1' },
+        );
+        expect(res.status).toBe(200);
+        expect(host.query('counter', 'value')).toBe(7);
+    });
+});
+
+describe('signed module commands over Raft consensus', () => {
+    const alice = generateActorKeypair();
+    let nodes: ModuleNode[];
+
+    beforeEach(() => {
+        // Every node configured with the SAME registry (alice's pubkey).
+        const reg = new KeyRegistry();
+        reg.registerActor('alice', alice.publicKey);
+        nodes = buildModuleCluster(3, [counter], { keyRegistry: reg });
+        nodes.forEach((n) => n.start());
+    });
+
+    afterEach(() => {
+        nodes.forEach((n) => n.stop());
+    });
+
+    it('replicates a signed command from alice and converges on all nodes (200)', async () => {
+        await waitFor(() => leaders(nodes).length === 1);
+        const leader = leaders(nodes)[0];
+
+        const meta: CommandMeta = { requestId: 'req-signed', actor: 'alice', timestamp: SEED.timestamp };
+        const cmd = buildSignedModuleCommand('counter', 'increment', { by: 5 }, {
+            actor: 'alice',
+            requestId: 'req-signed',
+            privateKeyPem: alice.privateKey,
+        });
+        const res = await leader.submit(cmd, meta);
+        expect(res.status).toBe(200);
+
+        await waitFor(() => nodes.every((n) => n.app.host.query('counter', 'value') === 5));
+        const values = new Set(nodes.map((n) => n.app.host.query('counter', 'value')));
+        expect(values).toEqual(new Set([5]));
+        const roots = new Set(nodes.map((n) => n.app.host.auditRoot()));
+        expect(roots.size).toBe(1);
+    });
+
+    it('FORGERY: a command claiming actor alice but signed by another key is rejected (401) on every node; counter unchanged; nodes converged', async () => {
+        await waitFor(() => leaders(nodes).length === 1);
+        const leader = leaders(nodes)[0];
+
+        // Mallory forges actor: 'alice' but signs with her own key.
+        const mallory = generateActorKeypair();
+        const meta: CommandMeta = { requestId: 'req-forge', actor: 'alice', timestamp: SEED.timestamp };
+        const forged = buildSignedModuleCommand('counter', 'increment', { by: 100 }, {
+            actor: 'alice',
+            requestId: 'req-forge',
+            privateKeyPem: mallory.privateKey,
+        });
+        const res = await leader.submit(forged, meta);
+        // Rejected deterministically with 401 (the command still committed to the
+        // log and applied, but the reducer never ran).
+        expect(res.status).toBe(401);
+
+        // Also submit an unsigned forgery for good measure.
+        const unsignedMeta: CommandMeta = { requestId: 'req-unsigned', actor: 'alice', timestamp: SEED.timestamp };
+        const unsigned = buildModuleCommand('counter', 'increment', { by: 50 }, unsignedMeta);
+        const res2 = await leader.submit(unsigned, unsignedMeta);
+        expect(res2.status).toBe(401);
+
+        // Wait until both forgery entries have committed/applied on every node by
+        // gating on their audit appearing, then assert the counter never moved.
+        await waitFor(() =>
+            nodes.every((n) =>
+                n.stateMachine.getAuditLog().some((e) => e.requestId === 'req-unsigned'),
+            ),
+        );
+        const values = new Set(nodes.map((n) => n.app.host.query('counter', 'value')));
+        expect(values).toEqual(new Set([0]));
+        // Nodes stay converged (identical module audit roots).
+        const roots = new Set(nodes.map((n) => n.app.host.auditRoot()));
+        expect(roots.size).toBe(1);
+    });
+});