diff --git a/packages/opencode/src/bus/bus-event.ts b/packages/opencode/src/bus/bus-event.ts
index efaed944066c..90a0668dba52 100644
--- a/packages/opencode/src/bus/bus-event.ts
+++ b/packages/opencode/src/bus/bus-event.ts
@@ -1,16 +1,31 @@
 import z from "zod"
 import type { ZodType } from "zod"
+import { Schema, Types } from "effect"
+import { zod } from "@/util/effect-zod"
 
 export type Definition = ReturnType<typeof define>
 
 const registry = new Map<string, Definition>()
 
-export function define<Type extends string, Properties extends ZodType>(type: Type, properties: Properties) {
-  const result = {
-    type,
-    properties,
-  }
-  registry.set(type, result)
+/**
+ * Define a bus event type with a payload schema.
+ *
+ * Accepts either a Zod schema or an Effect Schema. Effect Schemas are
+ * converted to Zod internally via the effect-zod walker so that the bus
+ * continues to use Zod as the lingua franca for serialization/validation.
+ */
+export function define<Type extends string, P extends Schema.Top>(
+  type: Type,
+  properties: P,
+): { type: Type; properties: z.ZodType<Types.DeepMutable<Schema.Schema.Type<P>>> }
+export function define<Type extends string, P extends ZodType>(
+  type: Type,
+  properties: P,
+): { type: Type; properties: P }
+export function define(type: string, properties: unknown) {
+  const zodProperties = Schema.isSchema(properties) ? zod(properties) : (properties as ZodType)
+  const result = { type, properties: zodProperties }
+  registry.set(type, result as Definition)
   return result
 }
 
diff --git a/packages/opencode/src/server/projectors.ts b/packages/opencode/src/server/projectors.ts
index cfecce526599..b5cc446b870e 100644
--- a/packages/opencode/src/server/projectors.ts
+++ b/packages/opencode/src/server/projectors.ts
@@ -1,16 +1,18 @@
-import z from "zod"
+import { Schema } from "effect"
 import sessionProjectors from "../session/projectors"
 import { SyncEvent } from "@/sync"
 import { Session } from "@/session"
 import { SessionTable } from "@/session/session.sql"
 import { Database, eq } from "@/storage"
 
+const isSessionUpdated = Schema.is(Session.Event.Updated.schema)
+
 export function initProjectors() {
   SyncEvent.init({
     projectors: sessionProjectors,
     convertEvent: (type, data) => {
-      if (type === "session.updated") {
-        const id = (data as z.infer<typeof Session.Event.Updated.schema>).sessionID
+      if (type === Session.Event.Updated.type && isSessionUpdated(data)) {
+        const id = data.sessionID
         const row = Database.use((db) => db.select().from(SessionTable).where(eq(SessionTable.id, id)).get())
 
         if (!row) return data
diff --git a/packages/opencode/src/session/message-v2.ts b/packages/opencode/src/session/message-v2.ts
index 980dd4da844f..7664b09adaa5 100644
--- a/packages/opencode/src/session/message-v2.ts
+++ b/packages/opencode/src/session/message-v2.ts
@@ -581,48 +581,48 @@ export const Event = {
     type: "message.updated",
     version: 1,
     aggregate: "sessionID",
-    schema: z.object({
-      sessionID: SessionID.zod,
-      info: Info.zod,
+    schema: Schema.Struct({
+      sessionID: SessionID,
+      info: _Info,
     }),
   }),
   Removed: SyncEvent.define({
     type: "message.removed",
     version: 1,
     aggregate: "sessionID",
-    schema: z.object({
-      sessionID: SessionID.zod,
-      messageID: MessageID.zod,
+    schema: Schema.Struct({
+      sessionID: SessionID,
+      messageID: MessageID,
     }),
   }),
   PartUpdated: SyncEvent.define({
     type: "message.part.updated",
     version: 1,
     aggregate: "sessionID",
-    schema: z.object({
-      sessionID: SessionID.zod,
-      part: Part.zod,
-      time: z.number(),
+    schema: Schema.Struct({
+      sessionID: SessionID,
+      part: _Part,
+      time: Schema.Number,
     }),
   }),
   PartDelta: BusEvent.define(
     "message.part.delta",
-    z.object({
-      sessionID: SessionID.zod,
-      messageID: MessageID.zod,
-      partID: PartID.zod,
-      field: z.string(),
-      delta: z.string(),
+    Schema.Struct({
+      sessionID: SessionID,
+      messageID: MessageID,
+      partID: PartID,
+      field: Schema.String,
+      delta: Schema.String,
     }),
   ),
   PartRemoved: SyncEvent.define({
     type: "message.part.removed",
     version: 1,
     aggregate: "sessionID",
-    schema: z.object({
-      sessionID: SessionID.zod,
-      messageID: MessageID.zod,
-      partID: PartID.zod,
+    schema: Schema.Struct({
+      sessionID: SessionID,
+      messageID: MessageID,
+      partID: PartID,
     }),
   }),
 }
diff --git a/packages/opencode/src/session/session.ts b/packages/opencode/src/session/session.ts
index d2bdbccb7dbc..3b9e2ebb4be2 100644
--- a/packages/opencode/src/session/session.ts
+++ b/packages/opencode/src/session/session.ts
@@ -28,7 +28,7 @@ import type { Provider } from "@/provider"
 import { Permission } from "@/permission"
 import { Global } from "@/global"
 import { Effect, Layer, Option, Context, Schema, Types } from "effect"
-import { zod, zodObject } from "@/util/effect-zod"
+import { fromZod, zod, zodObject } from "@/util/effect-zod"
 import { withStatics } from "@/util/schema"
 
 const log = Log.create({ service: "session" })
@@ -215,40 +215,44 @@ export const MessagesInput = Schema.Struct({
   limit: Schema.optional(Schema.Number),
 }).pipe(withStatics((s) => ({ zod: zod(s) })))
 
+const SessionUpdateInfoSchema = fromZod(
+  updateSchema(zodObject(Info)).extend({
+    share: updateSchema(zodObject(Share)).optional(),
+    time: updateSchema(zodObject(Time)).optional(),
+  }),
+)
+
 export const Event = {
   Created: SyncEvent.define({
     type: "session.created",
     version: 1,
     aggregate: "sessionID",
-    schema: z.object({
-      sessionID: SessionID.zod,
-      info: Info.zod,
-    }),
+    schema: {
+      sessionID: SessionID,
+      info: Info,
+    },
   }),
   Updated: SyncEvent.define({
     type: "session.updated",
     version: 1,
     aggregate: "sessionID",
-    schema: z.object({
-      sessionID: SessionID.zod,
-      info: updateSchema(zodObject(Info)).extend({
-        share: updateSchema(zodObject(Share)).optional(),
-        time: updateSchema(zodObject(Time)).optional(),
-      }),
-    }),
-    busSchema: z.object({
-      sessionID: SessionID.zod,
-      info: Info.zod,
-    }),
+    schema: {
+      sessionID: SessionID,
+      info: SessionUpdateInfoSchema,
+    },
+    busSchema: {
+      sessionID: SessionID,
+      info: Info,
+    },
   }),
   Deleted: SyncEvent.define({
     type: "session.deleted",
     version: 1,
     aggregate: "sessionID",
-    schema: z.object({
-      sessionID: SessionID.zod,
-      info: Info.zod,
-    }),
+    schema: {
+      sessionID: SessionID,
+      info: Info,
+    },
   }),
   Diff: BusEvent.define(
     "session.diff",
@@ -394,7 +398,7 @@ export interface Interface {
 
 export class Service extends Context.Service<Service, Interface>()("@opencode/Session") {}
 
-type Patch = z.infer<typeof Event.Updated.schema>["info"]
+type Patch = Schema.Schema.Type<typeof Event.Updated.schema>["info"]
 
 const db = <T>(fn: (d: Parameters<typeof Database.use>[0] extends (trx: infer D) => any ? D : never) => T) =>
   Effect.sync(() => Database.use(fn))
diff --git a/packages/opencode/src/sync/index.ts b/packages/opencode/src/sync/index.ts
index 125d8c95506e..2248ed15d2ab 100644
--- a/packages/opencode/src/sync/index.ts
+++ b/packages/opencode/src/sync/index.ts
@@ -1,5 +1,5 @@
 import z from "zod"
-import type { ZodObject } from "zod"
+import { Schema, Types } from "effect"
 import { Database, eq } from "@/storage"
 import { GlobalBus } from "@/bus/global"
 import { Bus as ProjectBus } from "@/bus"
@@ -9,23 +9,36 @@ import { EventSequenceTable, EventTable } from "./event.sql"
 import { WorkspaceContext } from "@/control-plane/workspace-context"
 import { EventID } from "./schema"
 import { Flag } from "@/flag/flag"
+import { zod } from "@/util/effect-zod"
+
+type StructLike<Fields extends Schema.Struct.Fields> = Fields | Schema.Struct<Fields>
 
 export type Definition = {
   type: string
   version: number
   aggregate: string
-  schema: z.ZodObject
+  schema: Schema.Top
+  busSchema: Schema.Top
+  properties: z.ZodTypeAny
+}
 
-  // This is temporary and only exists for compatibility with bus
-  // event definitions
-  properties: z.ZodObject
+type MutableType<S extends Schema.Top> = Types.DeepMutable<Schema.Schema.Type<S>>
+
+type DefinedEvent<Type extends string, Agg extends string, SchemaDef extends Schema.Top, BusDef extends Schema.Top> = Definition & {
+  type: Type
+  aggregate: Agg
+  schema: SchemaDef
+  busSchema: BusDef
+  properties: z.ZodType<MutableType<BusDef>>
 }
 
+type Data<Def extends Definition> = MutableType<Def["schema"]>
+
 export type Event<Def extends Definition = Definition> = {
   id: string
   seq: number
   aggregateID: string
-  data: z.infer<Def["schema"]>
+  data: Data<Def>
 }
 
 export type SerializedEvent<Def extends Definition = Definition> = Event<Def> & { type: string }
@@ -54,7 +67,7 @@ export function init(input: { projectors: Array<[Definition, ProjectorFunc]>; co
   for (let [type, version] of versions.entries()) {
     let def = registry.get(versionedType(type, version))!
 
-    BusEvent.define(def.type, def.properties || def.schema)
+    BusEvent.define(def.type, def.properties)
   }
 
   // Freeze the system so it clearly errors if events are defined
@@ -69,22 +82,37 @@ export function versionedType(type: string, version?: number) {
   return version ? `${type}.${version}` : type
 }
 
+function struct<Fields extends Schema.Struct.Fields>(value: StructLike<Fields>) {
+  return (Schema.isSchema(value) ? value : Schema.Struct(value as Fields)) as Schema.Struct<Fields>
+}
+
 export function define<
   Type extends string,
   Agg extends string,
-  Schema extends ZodObject<Record<Agg, z.ZodType<string>>>,
-  BusSchema extends ZodObject = Schema,
->(input: { type: Type; version: number; aggregate: Agg; schema: Schema; busSchema?: BusSchema }) {
+  Fields extends Schema.Struct.Fields & Record<Agg, Schema.Top>,
+  BusFields extends Schema.Struct.Fields = Fields,
+>(input: {
+  type: Type
+  version: number
+  aggregate: Agg
+  schema: StructLike<Fields>
+  busSchema?: StructLike<BusFields>
+}): DefinedEvent<Type, Agg, Schema.Struct<Fields>, Schema.Struct<BusFields>> {
   if (frozen) {
     throw new Error("Error defining sync event: sync system has been frozen")
   }
 
-  const def = {
+  const schema = struct(input.schema)
+  const busSchema = (input.busSchema ? struct(input.busSchema) : schema) as Schema.Struct<BusFields>
+  const properties = zod(busSchema) as unknown as z.ZodType<MutableType<typeof busSchema>>
+
+  const def: DefinedEvent<Type, Agg, typeof schema, typeof busSchema> = {
     type: input.type,
     version: input.version,
     aggregate: input.aggregate,
-    schema: input.schema,
-    properties: input.busSchema ? input.busSchema : input.schema,
+    schema,
+    busSchema,
+    properties,
   }
 
   versions.set(def.type, Math.max(def.version, versions.get(def.type) || 0))
@@ -143,10 +171,10 @@ function process<Def extends Definition>(def: Def, event: Event<Def>, options: {
         const result = convertEvent(def.type, event.data)
         if (result instanceof Promise) {
           void result.then((data) => {
-            void ProjectBus.publish({ type: def.type, properties: def.schema }, data)
+            void ProjectBus.publish({ type: def.type, properties: def.properties }, data)
           })
         } else {
-          void ProjectBus.publish({ type: def.type, properties: def.schema }, result)
+          void ProjectBus.publish({ type: def.type, properties: def.properties }, result)
         }
 
         GlobalBus.emit("event", {
@@ -266,7 +294,7 @@ export function payloads() {
           id: z.string(),
           seq: z.number(),
           aggregateID: z.literal(def.aggregate),
-          data: def.schema,
+          data: zod(def.schema),
         })
         .meta({
           ref: `SyncEvent.${def.type}`,
diff --git a/packages/opencode/src/util/effect-zod.ts b/packages/opencode/src/util/effect-zod.ts
index edbbf4d542c9..e949b255f545 100644
--- a/packages/opencode/src/util/effect-zod.ts
+++ b/packages/opencode/src/util/effect-zod.ts
@@ -49,6 +49,14 @@ function isZodType(value: unknown): value is z.ZodTypeAny {
   return typeof value === "object" && value !== null && "_zod" in value
 }
 
+// Bridge a Zod-first schema into Effect Schema while preserving the original
+// Zod for downstream JSON Schema/OpenAPI generation.
+export function fromZod<T extends z.ZodTypeAny>(value: T) {
+  return Schema.declare((input): input is z.output<T> => value.safeParse(input).success).annotate({
+    [ZodOverride]: value,
+  })
+}
+
 function walk(ast: SchemaAST.AST): z.ZodTypeAny {
   const cached = walkCache.get(ast)
   if (cached) return cached
diff --git a/packages/opencode/test/bus/bus.test.ts b/packages/opencode/test/bus/bus.test.ts
index 3df179787dd3..7cde38a01426 100644
--- a/packages/opencode/test/bus/bus.test.ts
+++ b/packages/opencode/test/bus/bus.test.ts
@@ -1,4 +1,5 @@
 import { afterEach, describe, expect, test } from "bun:test"
+import { Schema } from "effect"
 import z from "zod"
 import { Bus } from "../../src/bus"
 import { BusEvent } from "../../src/bus/bus-event"
@@ -10,6 +11,8 @@ const TestEvent = {
   Pong: BusEvent.define("test.pong", z.object({ message: z.string() })),
 }
 
+const EffectTestEvent = BusEvent.define("test.effect-schema.ping", Schema.Struct({ value: Schema.Number }))
+
 function withInstance(directory: string, fn: () => Promise<void>) {
   return Instance.provide({ directory, fn })
 }
@@ -76,6 +79,22 @@ describe("Bus", () => {
         await Bus.publish(TestEvent.Ping, { value: 1 })
       })
     })
+
+    test("accepts Effect Schema event definitions", async () => {
+      await using tmp = await tmpdir()
+      const received: number[] = []
+
+      await withInstance(tmp.path, async () => {
+        Bus.subscribe(EffectTestEvent, (evt) => {
+          received.push(evt.properties.value)
+        })
+        await Bun.sleep(10)
+        await Bus.publish(EffectTestEvent, { value: 42 })
+        await Bun.sleep(10)
+      })
+
+      expect(received).toEqual([42])
+    })
   })
 
   describe("unsubscribe", () => {
diff --git a/packages/opencode/test/sync/index.test.ts b/packages/opencode/test/sync/index.test.ts
index 866bcaa31ad0..9c9ff3081894 100644
--- a/packages/opencode/test/sync/index.test.ts
+++ b/packages/opencode/test/sync/index.test.ts
@@ -1,4 +1,5 @@
 import { describe, test, expect, beforeEach, afterEach, afterAll } from "bun:test"
+import { Schema } from "effect"
 import { tmpdir } from "../fixture/fixture"
 import z from "zod"
 import { Bus } from "../../src/bus"
@@ -43,13 +44,13 @@ describe("SyncEvent", () => {
       type: "item.created",
       version: 1,
       aggregate: "id",
-      schema: z.object({ id: z.string(), name: z.string() }),
+      schema: { id: Schema.String, name: Schema.String },
     })
     const Sent = SyncEvent.define({
       type: "item.sent",
       version: 1,
       aggregate: "item_id",
-      schema: z.object({ item_id: z.string(), to: z.string() }),
+      schema: { item_id: Schema.String, to: Schema.String },
     })
 
     SyncEvent.init({
@@ -128,6 +129,51 @@ describe("SyncEvent", () => {
         })
       }),
     )
+
+    test(
+      "accepts Effect Schema event definitions",
+      withInstance(async () => {
+        SyncEvent.reset()
+        try {
+          const Created = SyncEvent.define({
+            type: "item.effect.created",
+            version: 1,
+            aggregate: "id",
+            schema: { id: Schema.String, name: Schema.String },
+          })
+
+          SyncEvent.init({
+            projectors: [SyncEvent.project(Created, () => {})],
+          })
+
+          const events: Array<{
+            type: string
+            properties: { id: string; name: string }
+          }> = []
+          const received = new Promise<void>((resolve) => {
+            Bus.subscribeAll((event) => {
+              events.push(event)
+              resolve()
+            })
+          })
+
+          SyncEvent.run(Created, { id: "evt_1", name: "schema" })
+
+          await received
+          expect(events).toEqual([
+            {
+              type: "item.effect.created",
+              properties: {
+                id: "evt_1",
+                name: "schema",
+              },
+            },
+          ])
+        } finally {
+          setup()
+        }
+      }),
+    )
   })
 
   describe("replay", () => {