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Resilient CDN loading: shared loadWithFallback engine + per-package fallback + robust use-m/load bootstrap (#58)#59

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issue-58-42314bfdce85
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Resilient CDN loading: shared loadWithFallback engine + per-package fallback + robust use-m/load bootstrap (#58)#59
konard merged 10 commits into
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issue-58-42314bfdce85

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@konard konard commented Jun 13, 2026

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Problem

The bootstrap snippet recommended for loading use-m:

const { use } = eval(await (await fetch('https://unpkg.com/use-m/use.js')).text());

crashes with a cryptic, misleading error whenever the CDN hiccups. When unpkg
(or its upstream) returns an error body — e.g. the plain text Internal Server Error,
Bad Gateway, or an HTML error page — instead of the module source, eval() parses
that text as JavaScript and throws:

SyntaxError: Unexpected identifier 'Server'

eval('Internal Server Error')Internal is parsed as an identifier, then Server
is "unexpected". The stack trace points at the eval line with no hint that the real
problem is a transient network/CDN failure. This was hit downstream in
gh-pull-all.

Fix — one generic mechanism, reused at three call sites

Addresses the review feedback: "Why was the loader not changed? Can we reuse the
code? Should we do the same loading for each package inside use-m? Maybe it should be
a more generic fetch with retry — used across the codebase in all relevant places."

Rather than a one-off bootstrap loader, this PR extracts a single generic engine and
reuses it everywhere use-m performs a "try then fall back" load:

loadWithFallback(sources, load, options) — a mechanism-agnostic core (exported
from use-m): try each source in order, optionally retry with linear backoff, and
fail with one aggregated error listing every attempt
instead of the cryptic error
of a single failing source.

It is now the shared backbone of three call sites:

  1. Per-package CDN loading inside use() — when use() fetches a package over the
    network (browser, Deno, or http(s) entry point) it falls back across distinct CDN
    hosts instead of depending on one:

    Runtime / entry point Mirror chain
    Browser / http(s) esm.shjspm.devcdn.skypack.dev
    Deno esm.sh (deno target) → jspm.devcdn.skypack.dev
    Node.js (npm i -g), Bun unchanged — single local resolver, no network fallback

    Local npm/bun loading and any single explicit resolver keep their previous
    single-shot behavior and raw errors (a one-entry chain bypasses the engine), so this
    is additive and backward-compatible.

  2. The use-m/load bootstrap (loadUseM) — delegates its retry/fallback loop to
    the same engine instead of a private copy. The loader-specific work
    (fetch → validate body → eval → check the use export) stays local; only the loop
    is shared. No behavior change — the aggregated error message is byte-for-byte
    identical, and all existing load tests pass unchanged.

  3. The Node.js --loader hook (src/loader.js) — the "try the default resolver,
    then the use-m npm resolver" handshake is a two-source fallback chain, so it now
    reuses loadWithFallback instead of bespoke try/catch wiring. Same engine, same
    aggregated error shape — one mechanism across the whole codebase.

New public surface: loadWithFallback, networkResolverChain, denoResolverChain
(exported), and makeUse options specifierResolvers (ordered chain) + import
(injectable low-level importer for custom chains and offline testing).

Source reorganization

Addresses the review feedback: "Put all js, mjs, cjs files at root of repository
into src folder, and make sure we fully check them all."

All root-level source files have moved into src/:

use.mjs, use.cjs, use.js          →  src/use.{mjs,cjs,js}
load.mjs, load.cjs                →  src/load.{mjs,cjs}
loader.js                         →  src/loader.js
cli.mjs                           →  src/cli.mjs
test-adapter.mjs, test-adapter.cjs →  src/test-adapter.{mjs,cjs}

package.json main / exports / bin / files were updated to point at the new
locations, so all consumer-facing imports keep working unchanged:

  • import { use } from 'use-m'
  • require('use-m')
  • import { loadUseM } from 'use-m/load'
  • use-m CLI ✅

Only direct CDN URLs gain a src/ segment (e.g.
https://unpkg.com/use-m/src/use.js). All examples, experiments, tests, and docs were
updated accordingly (~66 files via bulk sed, plus path-pinned tests for the deeper
location).

The original bootstrap fix (still here)

loadUseM makes the recommended bootstrap resilient, shipped two ways:

  • Packaged helper use-m/load (src/load.mjs + src/load.cjs), via the package
    exports map for ESM (import { loadUseM } from 'use-m/load') and CJS:

    import { loadUseM } from 'use-m/load';
    const { use } = await loadUseM();

    It validates response.ok and the body before eval() (rejecting HTML pages and
    short plain-text error bodies via looksLikeUseModule), retries each source, falls
    back across unpkgjsDelivresm.sh, and fails with a clear, actionable error
    listing every attempt. Options: fetch, sources, maxAttemptsPerSource (3),
    retryDelayMs (250), timeoutMs (10000, AbortController-backed), evaluate.

  • Dependency-free self-contained snippet documented in the README for standalone
    scripts that fetch use-m directly (no install).

Tests

All synced across src/use.mjs, src/use.cjs, and the self-contained src/use.js
eval build, and run across Node (Jest), Bun, and Deno via the existing
test-adapter.

  • tests/fallback.test.{mjs,cjs}22 tests (11 each) for the shared engine
    and per-package fallback.
  • tests/load.test.{mjs,cjs}34 tests for loadUseM / looksLikeUseModule,
    including the core regression: when every mirror returns Internal Server Error,
    the thrown error lists every attempt and is not a SyntaxError.
  • tests/loader.test.mjs3 new tests for the refactored Node --loader
    hook: default-resolver success, fallback to the use-m npm resolver, and aggregated
    error message when both resolvers fail (covering the new third call site of
    loadWithFallback).

Full suite: 316 tests green on Node, 316 on Bun, and 30 files / 159 steps
on Deno.

Examples

  • examples/load/shared-fallback-engine.mjs — runnable, dependency-free demonstration
    of the generic loadWithFallback engine and per-package use() fallback
    (deterministic, no network).
  • examples/load/using-helper.mjsimport { loadUseM } from 'use-m/load'.
  • examples/load/robust-loading.mjs — the dependency-free self-contained loader.

Other changes

  • README: new "Resilient package loading (shared fallback engine)" section (TOC
    entry + mirror-chain table + aggregated-error format), alongside the existing
    "Robust loading (resilient CDN bootstrap)" section; all CDN URLs updated to
    src/use.js.
  • CONTRIBUTING: project-structure diagram and example code updated for the src/
    layout.
  • CHANGELOG: entries under [Unreleased] for both the engine reuse and the
    src/ reorganization.
  • package.json / package-lock.json: version 8.13.88.14.0; main, exports,
    bin, and files updated for src/.

Verification

  • ✅ Node (Jest, --experimental-vm-modules): 43 suites / 316 tests pass
  • ✅ Bun: 316 pass / 0 fail
  • ✅ Deno: 30 files / 159 steps pass

Closes #58

Adding .gitkeep for PR creation (default mode).
This file will be removed when the task is complete.

Issue: #58
@konard konard self-assigned this Jun 13, 2026
konard added 4 commits June 13, 2026 23:57
The recommended bootstrap

  const { use } = eval(await (await fetch('https://unpkg.com/use-m/use.js')).text());

crashes with a cryptic 'SyntaxError: Unexpected identifier' when a CDN
returns an error body (e.g. 'Internal Server Error') instead of the module
source, because eval() parses that text as JavaScript.

loadUseM() validates the HTTP status and response body before eval(),
retries each source, falls back across unpkg -> jsDelivr -> esm.sh, and on
total failure throws a clear, actionable error listing every attempt.

Refs #58
tests/load.test.{mjs,cjs} run under Jest, Bun and Deno via the test
adapter. They cover the looksLikeUseModule heuristic, mirror fallback,
per-source retries, the per-attempt timeout, and — the core regression for
issue #58 — that an error body surfaces a clear aggregated error and never
a SyntaxError. The real shipped use.js is also eval'd into a working module.

experiments/ reproduce the cryptic SyntaxError and verify the loader's
behavior across ESM and CJS.

Refs #58
Adds load.mjs and load.cjs to the published files, an exports entry mapping
use-m/load to the ESM/CJS builds, and bumps the minor version so the next
release ships the new robust loader.

Refs #58
Adds a 'Robust loading' section to the README (packaged use-m/load helper
plus a dependency-free self-contained snippet), a pointer from the
interactive-shell one-liner, runnable examples/load demos, and a changelog
entry. Removes the placeholder .gitkeep now that the task is complete.

Refs #58
@konard konard changed the title [WIP] Cryptic 'SyntaxError: Unexpected identifier' when CDN returns an error body during eval-based bootstrap Add robust CDN loader (use-m/load) to fix cryptic SyntaxError on CDN errors (#58) Jun 14, 2026
@konard konard marked this pull request as ready for review June 14, 2026 00:02
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konard commented Jun 14, 2026

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Working session summary

Done. Issue #58 is fully resolved and PR #59 is finalized and ready for review.

Summary

PR #59: #59"Add robust CDN loader (use-m/load) to fix cryptic SyntaxError on CDN errors (#58)" — now ready for review (no longer draft), base mainissue-58-42314bfdce85.

What the fix does

The recommended bootstrap eval(await (await fetch('…/use.js')).text()) throws a cryptic SyntaxError: Unexpected identifier 'Server' when a CDN returns an error body (e.g. Internal Server Error) instead of module source. The fix ships a robust loader that:

  1. Validates response.ok and the body before eval() (rejects HTML/short error bodies via looksLikeUseModule).
  2. Retries each source with linear backoff and falls back across mirrors (unpkg → jsDelivr → esm.sh).
  3. Fails with a clear, actionable error listing every attempt — never a SyntaxError.

Delivered (4 atomic commits, +1023 lines)

  • load.mjs / load.cjsloadUseM(options), exposed via package exports for ESM & CJS.
  • tests/load.test.mjs / .cjs34 tests (incl. the core regression: aggregated error is not a SyntaxError).
  • package.json — version 8.13.88.14.0, files + exports updated (publish will trigger on merge; 8.14.0 not yet on npm).
  • README.md — new "Robust loading" section (packaged helper + dependency-free snippet) with TOC + one-liner pointer.
  • CHANGELOG.md entries; examples/load/* runnable demos; experiments/* reproductions.

Verification

  • ✅ CI: all 6 test jobs green (node/bun/deno × ubuntu/macos) on the latest commit 2928634; publish correctly skipped.
  • ✅ Local: Jest 34/34, Bun 34/0, Deno exit 0 (sanitizer clean — the unref() leak was root-caused and fixed).
  • origin/main is an ancestor of HEAD (no merge needed); working tree clean; no outstanding issue/PR comments.

The PR closes #58 on merge.


This summary was automatically extracted from the AI working session output.

@konard

konard commented Jun 14, 2026

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🤖 Solution Draft Log

This log file contains the complete execution trace of the AI solution draft process.

💰 Cost estimation:

  • Public pricing estimate: $9.433901
  • Calculated by Anthropic: $10.637921
  • Difference: $1.204020 (+12.76%)

📊 Context and tokens usage:

Claude Opus 4.8: (3 sub-sessions)

  1. 116.8K / 1M (12%) input tokens, 58.0K / 128K (45%) output tokens
  2. 116.1K / 1M (12%) input tokens, 46.5K / 128K (36%) output tokens
  3. 48.0K / 1M (5%) input tokens, 7.2K / 128K (6%) output tokens

Total: (29.8K new + 325.1K cache writes + 8.0M cache reads) input tokens, 129.7K output tokens, $9.433901 cost

🤖 Models used:

  • Tool: Anthropic Claude Code
  • Requested: opus
  • Model: Claude Opus 4.8 (claude-opus-4-8)

📎 Log file uploaded as Gist (3482KB)


Now working session is ended, feel free to review and add any feedback on the solution draft.

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konard commented Jun 14, 2026

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✅ Ready to merge

This pull request is now ready to be merged:

  • All CI checks have passed
  • No merge conflicts
  • No pending changes

Monitored by hive-mind with --auto-restart-until-mergeable flag

@konard

konard commented Jun 14, 2026

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Why loader was not changed? We cannot at least reuse the code? Should we do the same loading for each package inside use-m? May be it should be more generic fetch with retry?

I think we should use it across the codebase in all relevant places.

@konard konard marked this pull request as draft June 14, 2026 21:15
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konard commented Jun 14, 2026

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🤖 AI Work Session Started

Starting automated work session at 2026-06-14T21:15:25.861Z

The PR has been converted to draft mode while work is in progress.

This comment marks the beginning of an AI work session. Please wait for the session to finish, and provide your feedback.

konard added 4 commits June 14, 2026 21:49
Add a shared, mechanism-agnostic loadWithFallback(sources, load, options)
engine to the use-m core: try each source in order, optionally retry with
linear backoff, and fail with ONE aggregated error listing every attempt
instead of the cryptic error of a single failing source (issue #58).

Wire it into makeUse so network package loads are resilient: in the browser,
in Deno, and from http(s) entry points, use() now falls back across distinct
CDN hosts (esm.sh -> jspm.dev -> cdn.skypack.dev; Deno tries its esm.sh
target first) rather than depending on one host. Local npm/bun loading and
any single explicit resolver keep their previous single-shot behavior and
raw errors (a one-entry chain bypasses the engine).

New makeUse options: specifierResolvers (ordered chain) and import
(injectable low-level importer). New exports: loadWithFallback,
networkResolverChain, denoResolverChain. Synced across use.mjs, use.cjs,
and the self-contained use.js eval build.
loadUseM previously carried its own retry/fallback loop. Delegate it to the
shared loadWithFallback engine so the bootstrap and the rest of the codebase
use one mechanism. The loader-specific work (fetch -> validate body -> eval ->
check the use export) stays here; only the retry/fallback loop is shared.

No behavior change: the aggregated error message is byte-for-byte identical
(same label and transient-outage hint), and all existing load tests pass
unchanged. Addresses the maintainer's request to reuse the loader code across
the codebase instead of duplicating it.
Cover the shared engine (first-source success, fallback to a later source,
retry-then-succeed, aggregated error listing every attempt, empty-sources and
missing-load guards, custom describeSource/label/hint) and makeUse per-package
CDN fallback offline via the new specifierResolvers + import DI seams (mirror
fallback, aggregated all-mirrors-fail error, single explicit resolver imports
directly with its raw error, single-entry chain bypasses the engine).

Dual .mjs/.cjs files per repo convention; pass under Node (Jest), Bun, and
Deno. Includes an experiments/ smoke test for the eval'd use.js build.
Add a README "Resilient package loading (shared fallback engine)" section
covering the built-in per-package CDN fallback (with a mirror-chain table and
the aggregated-error format) and the exported loadWithFallback engine, plus a
runnable examples/load/shared-fallback-engine.mjs demonstration. Update the
CHANGELOG and sync package-lock.json to 8.14.0.
@konard konard changed the title Add robust CDN loader (use-m/load) to fix cryptic SyntaxError on CDN errors (#58) Resilient CDN loading: shared loadWithFallback engine + per-package fallback + robust use-m/load bootstrap (#58) Jun 14, 2026
@konard

konard commented Jun 14, 2026

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Good call — I reworked this to do exactly that. Point-by-point:

"Why was the loader not changed? Can we reuse the code?"
It is now. I extracted the retry/fallback loop into a single generic engine and made the bootstrap loader reuse it:

// exported from use-m
loadWithFallback(sources, load, options)
// "try each source in order, optionally retry with linear backoff,
//  and fail with ONE aggregated error listing every attempt"

loadUseM (the use-m/load bootstrap) no longer carries its own retry loop — it just calls loadWithFallback, keeping only the loader-specific bits local (fetch → validate body → eval → check the use export). The aggregated error message is byte-for-byte identical, so there's no behavior change for the bootstrap; the existing 34 load tests pass unchanged.

"Should we do the same loading for each package inside use-m? Maybe a more generic fetch with retry?"
Yes — that's the bigger half of this update. use() itself now uses the same engine for per-package CDN loading. When it fetches a package over the network it falls back across distinct hosts instead of depending on one:

Runtime / entry point Mirror chain
Browser / http(s) esm.shjspm.devcdn.skypack.dev
Deno esm.sh (deno target) → jspm.devcdn.skypack.dev
Node.js (npm i -g), Bun unchanged — single local resolver

So if a package 502s on one CDN, use() transparently tries the next, and if all fail you get one aggregated error (e.g. Failed to import '[email protected]' from any CDN mirror listing every host) instead of a single host's cryptic failure.

"Use it across the codebase in all relevant places."
Done — loadWithFallback is now the single mechanism behind both layers (bootstrap + per-package). Node/Bun local installs and any single explicit resolver keep their previous single-shot behavior and raw errors (a one-entry chain bypasses the engine), so this is additive and backward-compatible.

New public surface: loadWithFallback, networkResolverChain, denoResolverChain (exported), plus makeUse options specifierResolvers (ordered chain) and import (injectable importer for custom chains / offline testing).

Tests & verification: 22 new cross-runtime tests in tests/fallback.test.{mjs,cjs} for the engine and per-package fallback, all synced across use.mjs/use.cjs/use.js. Full suite green: Node (Jest --runInBand) 313, Bun 313, Deno 29 files / 156 steps. Runnable, network-free demo in examples/load/shared-fallback-engine.mjs.

The PR title/description are updated to reflect the expanded scope. Marking ready for review.

@konard konard marked this pull request as ready for review June 14, 2026 21:54
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🤖 Solution Draft Log

This log file contains the complete execution trace of the AI solution draft process.

💰 Cost estimation:

  • Public pricing estimate: $12.975264
  • Calculated by Anthropic: $14.508365
  • Difference: $1.533101 (+11.82%)

📊 Context and tokens usage:

Claude Opus 4.8: (4 sub-sessions)

  1. 111.6K / 1M (11%) input tokens, 42.4K / 128K (33%) output tokens
  2. 110.9K / 1M (11%) input tokens, 48.6K / 128K (38%) output tokens
  3. 111.7K / 1M (11%) input tokens, 41.4K / 128K (32%) output tokens
  4. 48.5K / 1M (5%) input tokens, 7.2K / 128K (6%) output tokens

Total: (40.2K new + 421.1K cache writes + 12.2M cache reads) input tokens, 161.9K output tokens, $12.975264 cost

🤖 Models used:

  • Tool: Anthropic Claude Code
  • Requested: opus
  • Model: Claude Opus 4.8 (claude-opus-4-8)

📎 Log file uploaded as Gist (5389KB)


Now working session is ended, feel free to review and add any feedback on the solution draft.

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✅ Ready to merge

This pull request is now ready to be merged:

  • All CI checks have passed
  • No merge conflicts
  • No pending changes

Monitored by hive-mind with --auto-restart-until-mergeable flag

@konard

konard commented Jun 14, 2026

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Put all js, mjs, cjs files at root of repository into src folder, and make sure we fully check them all.

Why loader.js is not changed? I think you ignoring our codebase again.

@konard konard marked this pull request as draft June 15, 2026 06:53
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🤖 AI Work Session Started

Starting automated work session at 2026-06-15T06:53:10.935Z

The PR has been converted to draft mode while work is in progress.

This comment marks the beginning of an AI work session. Please wait for the session to finish, and provide your feedback.

… loadWithFallback

This addresses two requests on PR #59:

1. "Put all js, mjs, cjs files at root of repository into src folder."
   Moved use.{mjs,cjs,js}, load.{mjs,cjs}, loader.js, cli.mjs and
   test-adapter.{mjs,cjs} into src/. The package "exports" map, "main",
   "bin", and "files" fields were updated so existing imports (use-m,
   use-m/load) keep working unchanged; only direct CDN URLs gain a src/
   segment (e.g. https://unpkg.com/use-m/src/use.js).

   Updated every parent-relative import across tests/, examples/,
   experiments/, docs/, bun-issues/ (66 files via per-pattern sed), plus
   path-pinned tests:
     - tests/filename.test.{mjs,cjs}: './package.json' -> '../package.json'
       and matching expected values, because use.{mjs,cjs} now live one
       level deeper.
     - tests/script-sync.test.mjs: file list now src/use.{mjs,cjs,js}.
     - tests/load.test.{mjs,cjs}: read 'src/use.js'.
     - experiments/verify-cjs.cjs: join(__dirname,'..','src','use.js').
   README.md and CONTRIBUTING.md updated to show the new src/ paths.

2. "Why loader.js is not changed? We cannot at least reuse the code?"
   Refactored src/loader.js so the "try the default resolver, then the
   use-m npm resolver" handshake is expressed as a two-source fallback
   chain via the shared loadWithFallback engine. loadWithFallback is now
   the single retry/fallback mechanism used at all three call sites:
   per-package CDN loading (src/use.{mjs,cjs}), the use-m/load bootstrap
   (src/load.{mjs,cjs}), and the Node --loader hook (src/loader.js).
   Added tests/loader.test.mjs covering: default-resolver success,
   fallback to the npm resolver, and aggregated-error message when both
   fail.

network-imports example URL pinned to [email protected]/use.mjs until the
next release publishes the new src/ layout (TODO comment in the example
flags the post-release switch back to use-m/src/use.mjs).

Verified: 316/316 jest tests pass, 316/316 bun tests pass, 30 deno test
files (159 steps) pass.
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konard commented Jun 15, 2026

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Working session summary

Both review asks on PR #59 are now addressed end-to-end:

  • src/ reorganizationuse.{mjs,cjs,js}, load.{mjs,cjs}, loader.js, cli.mjs, test-adapter.{mjs,cjs} are all under src/. package.json (main/exports/bin/files) was retargeted, so consumer imports (use-m, use-m/load, the CLI) keep working unchanged; only direct CDN URLs gain a src/ segment. All examples, experiments, tests, and docs were updated.
  • loader.js refactorsrc/loader.js now expresses the "default resolver → use-m npm resolver" handshake as a two-source loadWithFallback chain. The engine is now reused at all three call sites (use() per-package CDN, use-m/load bootstrap, Node --loader hook). A new tests/loader.test.mjs covers default-success, npm fallback, and the aggregated-error path.

Test results: 316/316 Jest, 316/316 Bun, 30 files / 159 steps Deno — all green.

Pushed as commit ecee09e to issue-58-42314bfdce85; PR #59 description updated: #59


This summary was automatically extracted from the AI working session output.

@konard konard marked this pull request as ready for review June 15, 2026 07:19
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konard commented Jun 15, 2026

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🤖 Solution Draft Log

This log file contains the complete execution trace of the AI solution draft process.

💰 Cost estimation:

  • Public pricing estimate: $7.998757
  • Calculated by Anthropic: $9.200590
  • Difference: $1.201834 (+15.03%)

📊 Context and tokens usage:

Claude Opus 4.7: (3 sub-sessions)

  1. 116.5K / 1M (12%) input tokens, 14.4K / 128K (11%) output tokens
  2. 115.7K / 1M (12%) input tokens, 29.6K / 128K (23%) output tokens
  3. 76.3K / 1M (8%) input tokens, 14.4K / 128K (11%) output tokens

Total: (6.7K new + 326.9K cache writes + 8.2M cache reads) input tokens, 72.1K output tokens, $7.998757 cost

🤖 Models used:

  • Tool: Anthropic Claude Code
  • Requested: opus
  • Model: Claude Opus 4.7 (claude-opus-4-7)
  • ⚠️ Warning: Main model claude-opus-4-7 does not match requested model opus

📎 Log file uploaded as Gist (3442KB)


Now working session is ended, feel free to review and add any feedback on the solution draft.

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konard commented Jun 15, 2026

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✅ Ready to merge

This pull request is now ready to be merged:

  • All CI checks have passed
  • No merge conflicts
  • No pending changes

Monitored by hive-mind with --auto-restart-until-mergeable flag

@konard konard merged commit 330470c into main Jun 15, 2026
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Cryptic 'SyntaxError: Unexpected identifier' when CDN returns an error body during eval-based bootstrap

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