diff --git a/packages/opencode/src/actor/spawn.ts b/packages/opencode/src/actor/spawn.ts
index 734672466..3aee91b5e 100644
--- a/packages/opencode/src/actor/spawn.ts
+++ b/packages/opencode/src/actor/spawn.ts
@@ -195,7 +195,7 @@ export interface SpawnResult {
 export interface Interface {
   readonly spawn: (input: SpawnInput) => Effect.Effect<SpawnResult>
   readonly cancel: (sessionID: SessionID, actorID: string, mode: "graceful" | "forced") => Effect.Effect<void>
-  readonly getForkContext: (actorID: string) => Effect.Effect<ForkContext | undefined>
+  readonly getForkContext: (sessionID: SessionID, actorID: string) => Effect.Effect<ForkContext | undefined>
   /**
    * Run ONE stall-watchdog scan pass synchronously (the same body the background
    * fiber repeats every WATCHDOG_SCAN_INTERVAL_MS). Exposed for deterministic
@@ -226,7 +226,14 @@ export const layer = Layer.effect(
     // ForkContext snapshot per actor, captured at spawn for fork agents
     // (contextMode = "full"). Read by fork's runLoop (see prompt.ts) and
     // cleared on terminal status. Fiber tracking moved to SessionRunState.
+    //
+    // Keyed by (sessionID, actorID) rather than actorID alone: actorID is only
+    // unique within a (sessionID, agentType) scope (see
+    // ActorRegistry.allocateActorID), so two concurrent sessions spawning the
+    // same agentType can allocate the same actorID. A bare actorID key would
+    // let one session's forkContext clobber or be deleted by another's.
     const forkContexts = new Map<string, ForkContext>()
+    const forkContextKey = (sessionID: SessionID, actorID: string) => `${sessionID}:${actorID}`
 
     // Actors whose cancel() has begun, keyed "sessionID:actorID". Populated
     // BEFORE the fiber is interrupted so forkWork's onSuccess/onFailure notify
@@ -636,7 +643,7 @@ export const layer = Layer.effect(
                   lastFinalText = newTurn.finalText
                 }
 
-                yield* Effect.sync(() => forkContexts.delete(input.actorID))
+                yield* Effect.sync(() => forkContexts.delete(forkContextKey(input.sessionID, input.actorID)))
               }),
             onFailure: (cause) =>
               Effect.gen(function* () {
@@ -647,7 +654,7 @@ export const layer = Layer.effect(
                   outcome,
                   cancelled ? { status: "cancelled" as const } : { status: "failure" as const, error },
                 )
-                yield* Effect.sync(() => forkContexts.delete(input.actorID))
+                yield* Effect.sync(() => forkContexts.delete(forkContextKey(input.sessionID, input.actorID)))
               }),
           }),
         )
@@ -701,7 +708,7 @@ export const layer = Layer.effect(
         tools: input.tools,
       })
       if (input.forkContext) {
-        forkContexts.set(child.id, input.forkContext) // peer's actorID === child.id
+        forkContexts.set(forkContextKey(child.id, child.id), input.forkContext) // peer's actorID === child.id === sessionID
       }
       const { fiber, outcome } = yield* forkWork({
         sessionID: child.id,
@@ -747,7 +754,7 @@ export const layer = Layer.effect(
       if (input.onActorID) yield* Effect.sync(() => input.onActorID!(actorID)).pipe(Effect.ignore)
 
       if (input.forkContext) {
-        forkContexts.set(actorID, input.forkContext)
+        forkContexts.set(forkContextKey(input.sessionID, actorID), input.forkContext)
       }
 
       // Auto-inject return-format instruction for non-specialized subagents.
@@ -853,11 +860,11 @@ export const layer = Layer.effect(
           .updateStatus(sessionID, actorID, { status: "idle", lastOutcome: "cancelled" })
           .pipe(Effect.ignore)
         yield* notifyTerminal(sessionID, actorID, actor, "cancelled")
-        yield* Effect.sync(() => forkContexts.delete(actorID))
+        yield* Effect.sync(() => forkContexts.delete(forkContextKey(sessionID, actorID)))
       })
 
-    const getForkContext = Effect.fn("Actor.getForkContext")(function* (actorID: string) {
-      return forkContexts.get(actorID)
+    const getForkContext = Effect.fn("Actor.getForkContext")(function* (sessionID: SessionID, actorID: string) {
+      return forkContexts.get(forkContextKey(sessionID, actorID))
     })
 
     // === T40 stall watchdog ===
diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index dc5c1608a..177e4a4be 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -3339,7 +3339,7 @@ NOTE: At any point in time through this workflow you should feel free to ask the
             // If forkCtx is missing (race / cleanup bug / spawn skipped), fail the
             // actor so the next prune turn can spawn a fresh fork.
             if (isForkAgent) {
-              const forkCtxEffect = spawnRef.current?.getForkContext(lastUser.agentID!)
+              const forkCtxEffect = spawnRef.current?.getForkContext(sessionID, lastUser.agentID!)
               const forkCtx = forkCtxEffect ? yield* forkCtxEffect : undefined
               if (!forkCtx) {
                 yield* slog.warn("fork agent runLoop: missing forkContext, failing actor", {
diff --git a/packages/opencode/test/actor/spawn.test.ts b/packages/opencode/test/actor/spawn.test.ts
index 388da8ce0..b119d3b11 100644
--- a/packages/opencode/test/actor/spawn.test.ts
+++ b/packages/opencode/test/actor/spawn.test.ts
@@ -700,7 +700,7 @@ describe("Actor forkContext lifecycle", () => {
         })
 
         // Before cancel: forkContext must be present.
-        const before = yield* actor.getForkContext(result.actorID)
+        const before = yield* actor.getForkContext(result.sessionID, result.actorID)
         expect(before).toBeDefined()
         expect(before?.system).toEqual(["test-system"])
 
@@ -708,7 +708,7 @@ describe("Actor forkContext lifecycle", () => {
         yield* actor.cancel(result.sessionID, result.actorID, "forced")
 
         // After cancel: forkContext must be gone.
-        const after = yield* actor.getForkContext(result.actorID)
+        const after = yield* actor.getForkContext(result.sessionID, result.actorID)
         expect(after).toBeUndefined()
       }),
       { git: true, config: providerCfg },
@@ -751,7 +751,7 @@ describe("mode × contextMode matrix", () => {
           forkContext: fakeForkCtx,
         })
 
-        const ctx = yield* actor.getForkContext(result.actorID)
+        const ctx = yield* actor.getForkContext(result.sessionID, result.actorID)
         expect(ctx).toBeDefined()
         expect(ctx?.system).toEqual(["test-system"])
 
@@ -761,6 +761,90 @@ describe("mode × contextMode matrix", () => {
     ),
   )
 
+  it.live("forkContext does not leak across sessions that allocate the same actorID", () =>
+    provideTmpdirServer(
+      Effect.fnUntraced(function* ({ llm }) {
+        const actor = yield* Actor.Service
+        const session = yield* Session.Service
+
+        // Two independent sessions. allocateActorID scopes its incrementing
+        // suffix by (sessionID, agentType), so the first "explore" subagent
+        // spawned in each session below is expected to collide on the same
+        // actorID (e.g. "explore-1") even though the sessions are unrelated.
+        const sessionA = yield* session.create({
+          title: "forkCtx cross-session A",
+          permission: [{ permission: "*", pattern: "*", action: "allow" }],
+        })
+        const sessionB = yield* session.create({
+          title: "forkCtx cross-session B",
+          permission: [{ permission: "*", pattern: "*", action: "allow" }],
+        })
+
+        // Hang the LLM so both fibers stay alive while we verify state.
+        yield* llm.hang
+
+        const forkCtxA = {
+          system: ["session-a"],
+          tools: {},
+          inheritedMessages: [],
+          parentPermission: [],
+          watermarkMsgID: MessageID.ascending(),
+          model: ref,
+        }
+        const forkCtxB = {
+          system: ["session-b"],
+          tools: {},
+          inheritedMessages: [],
+          parentPermission: [],
+          watermarkMsgID: MessageID.ascending(),
+          model: ref,
+        }
+
+        const resultA = yield* actor.spawn({
+          mode: "subagent",
+          sessionID: sessionA.id,
+          agentType: "explore",
+          task: "noop",
+          context: "none",
+          tools: [],
+          background: true,
+          model: ref,
+          forkContext: forkCtxA,
+        })
+        const resultB = yield* actor.spawn({
+          mode: "subagent",
+          sessionID: sessionB.id,
+          agentType: "explore",
+          task: "noop",
+          context: "none",
+          tools: [],
+          background: true,
+          model: ref,
+          forkContext: forkCtxB,
+        })
+
+        // Confirms the collision precondition: both sessions allocated the
+        // same locally-scoped actorID.
+        expect(resultA.actorID).toBe(resultB.actorID)
+
+        // Each session must read back its own forkContext, not the other's.
+        const ctxA = yield* actor.getForkContext(sessionA.id, resultA.actorID)
+        const ctxB = yield* actor.getForkContext(sessionB.id, resultB.actorID)
+        expect(ctxA?.system).toEqual(["session-a"])
+        expect(ctxB?.system).toEqual(["session-b"])
+
+        yield* actor.cancel(sessionA.id, resultA.actorID, "forced")
+
+        // Cancelling session A's actor must not clear session B's forkContext.
+        const ctxBAfterCancelA = yield* actor.getForkContext(sessionB.id, resultB.actorID)
+        expect(ctxBAfterCancelA?.system).toEqual(["session-b"])
+
+        yield* actor.cancel(sessionB.id, resultB.actorID, "forced")
+      }),
+      { git: true, config: providerCfg },
+    ),
+  )
+
   it.live("subagent + none: no forkContext stored", () =>
     provideTmpdirServer(
       Effect.fnUntraced(function* ({ llm }) {
@@ -786,7 +870,7 @@ describe("mode × contextMode matrix", () => {
           // no forkContext
         })
 
-        const ctx = yield* actor.getForkContext(result.actorID)
+        const ctx = yield* actor.getForkContext(result.sessionID, result.actorID)
         expect(ctx).toBeUndefined()
 
         yield* actor.cancel(result.sessionID, result.actorID, "forced")
@@ -822,7 +906,7 @@ describe("mode × contextMode matrix", () => {
 
         // For peer, result.actorID === child.id (the new session id)
         expect(result.actorID).not.toBe(parent.id)
-        const ctx = yield* actor.getForkContext(result.actorID)
+        const ctx = yield* actor.getForkContext(result.sessionID, result.actorID)
         expect(ctx).toBeDefined()
         expect(ctx?.system).toEqual(["test-system"])
 
@@ -857,7 +941,7 @@ describe("mode × contextMode matrix", () => {
           // no forkContext
         })
 
-        const ctx = yield* actor.getForkContext(result.actorID)
+        const ctx = yield* actor.getForkContext(result.sessionID, result.actorID)
         expect(ctx).toBeUndefined()
 
         yield* actor.cancel(result.sessionID, result.actorID, "forced")
diff --git a/packages/opencode/test/inbox/fork-agent-compat.test.ts b/packages/opencode/test/inbox/fork-agent-compat.test.ts
index 2d7d16528..61b4fa129 100644
--- a/packages/opencode/test/inbox/fork-agent-compat.test.ts
+++ b/packages/opencode/test/inbox/fork-agent-compat.test.ts
@@ -315,7 +315,7 @@ describe("Fork-agent inbox compat (Plan 4 / Task 5)", () => {
         const forkActorID = result.actorID
 
         // Tier 2 — structural: capture forkContext BEFORE inbox send.
-        const forkCtxBefore = yield* actor.getForkContext(forkActorID)
+        const forkCtxBefore = yield* actor.getForkContext(result.sessionID, forkActorID)
         expect(forkCtxBefore).toBeDefined()
         expect(forkCtxBefore?.system).toEqual(["inherited-system-prompt"])
         expect(forkCtxBefore?.inheritedMessages).toHaveLength(2)
@@ -356,7 +356,7 @@ describe("Fork-agent inbox compat (Plan 4 / Task 5)", () => {
 
         // Tier 2 — structural: forkContext must still have the exact same
         // inheritedMessages snapshot. Drain must not touch forkContexts.
-        const forkCtxAfter = yield* actor.getForkContext(forkActorID)
+        const forkCtxAfter = yield* actor.getForkContext(result.sessionID, forkActorID)
         expect(forkCtxAfter).toBeDefined()
         expect(forkCtxAfter?.inheritedMessages).toStrictEqual(forkCtxBefore?.inheritedMessages)
         expect(forkCtxAfter?.system).toEqual(["inherited-system-prompt"])
diff --git a/packages/opencode/test/session/bootstrap-skip-system.test.ts b/packages/opencode/test/session/bootstrap-skip-system.test.ts
index 88cfc989c..380411687 100644
--- a/packages/opencode/test/session/bootstrap-skip-system.test.ts
+++ b/packages/opencode/test/session/bootstrap-skip-system.test.ts
@@ -53,7 +53,7 @@ const stubActor = Layer.succeed(
   Actor.Service.of({
     spawn: () => Effect.die("Actor.spawn unexpectedly called in system-spawn skip path"),
     cancel: () => Effect.die("Actor.cancel unexpectedly called in system-spawn skip path"),
-    getForkContext: () => Effect.succeed(undefined),
+    getForkContext: (_sessionID: string, _actorID: string) => Effect.succeed(undefined),
   }),
 )
 
diff --git a/packages/opencode/test/session/checkpoint-child-session.test.ts b/packages/opencode/test/session/checkpoint-child-session.test.ts
index 171391c35..d869c22db 100644
--- a/packages/opencode/test/session/checkpoint-child-session.test.ts
+++ b/packages/opencode/test/session/checkpoint-child-session.test.ts
@@ -76,7 +76,7 @@ const recordingActor = Layer.effect(
           }
         }),
       cancel: () => Effect.void,
-      getForkContext: () => Effect.succeed(undefined),
+      getForkContext: (_sessionID: string, _actorID: string) => Effect.succeed(undefined),
     })
     spawnRef.current = impl
     yield* Effect.addFinalizer(() =>
diff --git a/packages/opencode/test/session/checkpoint-drain.test.ts b/packages/opencode/test/session/checkpoint-drain.test.ts
index 2eece2c84..ea43294a5 100644
--- a/packages/opencode/test/session/checkpoint-drain.test.ts
+++ b/packages/opencode/test/session/checkpoint-drain.test.ts
@@ -48,7 +48,7 @@ const hangingActor = Layer.effect(
           }
         }),
       cancel: () => Effect.void,
-      getForkContext: () => Effect.succeed(undefined),
+      getForkContext: (_sessionID: string, _actorID: string) => Effect.succeed(undefined),
     })
     spawnRef.current = impl
     yield* Effect.addFinalizer(() =>
diff --git a/packages/opencode/test/session/checkpoint-fork-mode.test.ts b/packages/opencode/test/session/checkpoint-fork-mode.test.ts
index ced893b8e..a47acd986 100644
--- a/packages/opencode/test/session/checkpoint-fork-mode.test.ts
+++ b/packages/opencode/test/session/checkpoint-fork-mode.test.ts
@@ -59,7 +59,7 @@ const recordingActor = Layer.effect(
           }
         }),
       cancel: () => Effect.void,
-      getForkContext: () => Effect.succeed(undefined),
+      getForkContext: (_sessionID: string, _actorID: string) => Effect.succeed(undefined),
     })
     spawnRef.current = impl
     yield* Effect.addFinalizer(() =>
diff --git a/packages/opencode/test/session/checkpoint-main-slice.test.ts b/packages/opencode/test/session/checkpoint-main-slice.test.ts
index b5beb492b..00b4e7ec3 100644
--- a/packages/opencode/test/session/checkpoint-main-slice.test.ts
+++ b/packages/opencode/test/session/checkpoint-main-slice.test.ts
@@ -58,7 +58,7 @@ const recordingActor = Layer.effect(
           }
         }),
       cancel: () => Effect.void,
-      getForkContext: () => Effect.succeed(undefined),
+      getForkContext: (_sessionID: string, _actorID: string) => Effect.succeed(undefined),
     })
     spawnRef.current = impl
 
diff --git a/packages/opencode/test/session/checkpoint-rebuild-fallback-decision.test.ts b/packages/opencode/test/session/checkpoint-rebuild-fallback-decision.test.ts
index 561e4ac0a..6b9cf4b24 100644
--- a/packages/opencode/test/session/checkpoint-rebuild-fallback-decision.test.ts
+++ b/packages/opencode/test/session/checkpoint-rebuild-fallback-decision.test.ts
@@ -43,7 +43,7 @@ const controllableActor = Layer.effect(
           return { actorID: `${input.agentType}-${counter}`, sessionID: input.sessionID, outcome }
         }),
       cancel: () => Effect.void,
-      getForkContext: () => Effect.succeed(undefined),
+      getForkContext: (_sessionID: string, _actorID: string) => Effect.succeed(undefined),
     })
     spawnRef.current = impl
     yield* Effect.addFinalizer(() =>
diff --git a/packages/opencode/test/session/checkpoint-rebuild-nonblocking.test.ts b/packages/opencode/test/session/checkpoint-rebuild-nonblocking.test.ts
index 7f74c2a94..e6c715eb4 100644
--- a/packages/opencode/test/session/checkpoint-rebuild-nonblocking.test.ts
+++ b/packages/opencode/test/session/checkpoint-rebuild-nonblocking.test.ts
@@ -48,7 +48,7 @@ const hangingActor = Layer.effect(
           }
         }),
       cancel: () => Effect.void,
-      getForkContext: () => Effect.succeed(undefined),
+      getForkContext: (_sessionID: string, _actorID: string) => Effect.succeed(undefined),
     })
     spawnRef.current = impl
     yield* Effect.addFinalizer(() =>
diff --git a/packages/opencode/test/session/checkpoint-watermark-transactional.test.ts b/packages/opencode/test/session/checkpoint-watermark-transactional.test.ts
index b5b59658a..0b0ff467c 100644
--- a/packages/opencode/test/session/checkpoint-watermark-transactional.test.ts
+++ b/packages/opencode/test/session/checkpoint-watermark-transactional.test.ts
@@ -45,7 +45,7 @@ const controllableActor = Layer.effect(
           return { actorID: `${input.agentType}-${counter}`, sessionID: input.sessionID, outcome }
         }),
       cancel: () => Effect.void,
-      getForkContext: () => Effect.succeed(undefined),
+      getForkContext: (_sessionID: string, _actorID: string) => Effect.succeed(undefined),
     })
     spawnRef.current = impl
     yield* Effect.addFinalizer(() =>
diff --git a/packages/opencode/test/session/classify-integration.test.ts b/packages/opencode/test/session/classify-integration.test.ts
index 57f349bfa..83305263e 100644
--- a/packages/opencode/test/session/classify-integration.test.ts
+++ b/packages/opencode/test/session/classify-integration.test.ts
@@ -368,7 +368,8 @@ describe("classifier routing — integration", () => {
                 model: { providerID: ProviderID.make("alibaba"), modelID: ModelID.make("qwen-plus") },
               }
               spawnRef.current = {
-                getForkContext: (id: string) => Effect.succeed(id === forkActorID ? forkCtx : undefined),
+                getForkContext: (sessionID: string, actorID: string) =>
+                  Effect.succeed(actorID === forkActorID ? forkCtx : undefined),
                 spawn: () => Effect.die("spawn not used in fork-gate test"),
                 cancel: () => Effect.die("cancel not used in fork-gate test"),
               } as unknown as NonNullable<typeof spawnRef.current>
@@ -443,7 +444,8 @@ describe("classifier routing — integration", () => {
                 model: { providerID: ProviderID.make("alibaba"), modelID: ModelID.make("qwen-plus") },
               }
               spawnRef.current = {
-                getForkContext: (id: string) => Effect.succeed(id === forkActorID ? forkCtx : undefined),
+                getForkContext: (sessionID: string, actorID: string) =>
+                  Effect.succeed(actorID === forkActorID ? forkCtx : undefined),
                 spawn: () => Effect.die("spawn not used in fork content-filter test"),
                 cancel: () => Effect.die("cancel not used in fork content-filter test"),
               } as unknown as NonNullable<typeof spawnRef.current>
diff --git a/packages/opencode/test/tool/actor-cancel.test.ts b/packages/opencode/test/tool/actor-cancel.test.ts
index 77020af4c..f512055dc 100644
--- a/packages/opencode/test/tool/actor-cancel.test.ts
+++ b/packages/opencode/test/tool/actor-cancel.test.ts
@@ -71,7 +71,7 @@ beforeAll(() => {
           yield* installedRegistry.updateStatus(sessionID, actorID, { status: "idle", lastOutcome: "cancelled" }).pipe(Effect.ignore)
         }
       }),
-    getForkContext: () => Effect.succeed(undefined),
+    getForkContext: (_sessionID: string, _actorID: string) => Effect.succeed(undefined),
   } satisfies ActorInterface
 })
 afterAll(() => {
diff --git a/packages/opencode/test/tool/actor.test.ts b/packages/opencode/test/tool/actor.test.ts
index e5308d1cd..37f5c2caf 100644
--- a/packages/opencode/test/tool/actor.test.ts
+++ b/packages/opencode/test/tool/actor.test.ts
@@ -70,7 +70,7 @@ function installMockSpawn(onSpawn?: (input: SpawnInput) => void) {
           return { actorID, sessionID: input.sessionID, outcome }
         }),
       cancel: () => Effect.void,
-      getForkContext: () => Effect.succeed(undefined),
+      getForkContext: (_sessionID: string, _actorID: string) => Effect.succeed(undefined),
     }
   })
 }