diff --git a/packages/acp/acp/tests/turns.spec.ts b/packages/acp/acp/tests/turns.spec.ts
index 8cf67b4d19..1344f92257 100644
--- a/packages/acp/acp/tests/turns.spec.ts
+++ b/packages/acp/acp/tests/turns.spec.ts
@@ -57,6 +57,21 @@ describe('ACP prompt lifecycle', () => {
       .rejects.toThrow(/turn failed: plugin pre-step failed/)
   })
 
+  it('rejects a turn-start failure before the prompt is claimed', async () => {
+    harness = await makeBridgeHarness({ script: [textResponse('must not run')] })
+    const sessionId = await newSession(harness)
+    const agent = harness.ctx.agents.get(SessionId(sessionId))!
+    const append = agent.session.append.bind(agent.session)
+    vi.spyOn(agent.session, 'append').mockImplementation(((type: string, ...rest: never[]) => {
+      if (type === 'turn/start') throw new Error('turn start unavailable')
+      return (append as (...args: never[]) => unknown)(type as never, ...rest)
+    }) as never)
+
+    await expect(harness.client.prompt({ sessionId, prompt: [{ type: 'text', text: 'go' }] }))
+      .rejects.toThrow(/turn failed: turn start unavailable/)
+    vi.restoreAllMocks()
+  })
+
   it('settles even when an earlier turn observer throws', async () => {
     harness = await makeBridgeHarness({ script: [textResponse('answer')] })
     harness.ctx.on('session/event', (_session, event) => {
@@ -257,6 +272,18 @@ describe('ACP prompt lifecycle', () => {
     expect(messageText(harness)).toBe('')
   })
 
+  it('cancels a prompt removed before its turn claims it', async () => {
+    harness = await makeBridgeHarness({ script: [] })
+    const sessionId = await newSession(harness)
+    const dispose = harness.ctx.on('agent/inbox/inserted', (agent, { message }) => {
+      if (message.source.kind === 'user') agent.inbox.remove(message.id)
+    })
+
+    await expect(harness.client.prompt({ sessionId, prompt: [{ type: 'text', text: 'go' }] }))
+      .resolves.toEqual({ stopReason: 'cancelled' })
+    dispose()
+  })
+
   it('rejects a prompt when pre-step fails inside its open turn', async () => {
     harness = await makeBridgeHarness({ script: [] })
     harness.ctx.on('agent/pre-step', async () => { throw new Error('pre-step exploded') })
diff --git a/packages/context/workspace-context/tests/workspace-context.spec.ts b/packages/context/workspace-context/tests/workspace-context.spec.ts
index abdc9bea76..be9e512f92 100644
--- a/packages/context/workspace-context/tests/workspace-context.spec.ts
+++ b/packages/context/workspace-context/tests/workspace-context.spec.ts
@@ -3739,6 +3739,11 @@ describe('workspace context inbox synchronization', () => {
       })
       await syncWorkspaceContext(ctx, agent)
       expect(blocksText(agent.inbox.nextStep[0]?.content)).toContain('pending version one')
+      const duplicate = createUserMessage({
+        content: agent.inbox.nextStep[0]!.content,
+        source: agent.inbox.nextStep[0]!.source,
+      })
+      agent.inbox.append('next-step', duplicate)
 
       await write(join(root, 'pkg/AGENTS.md'), 'pending version two with more detail')
       await ctx.tools.execute({
@@ -3747,6 +3752,7 @@ describe('workspace context inbox synchronization', () => {
       })
       await syncWorkspaceContext(ctx, agent)
       expect(agent.inbox.nextStep).toHaveLength(1)
+      expect(agent.inbox.nextStep[0]?.id).not.toBe(duplicate.id)
       expect(blocksText(agent.inbox.nextStep[0]?.content)).toContain('pending version two with more detail')
       expect(blocksText(agent.inbox.nextStep[0]?.content)).not.toContain('pending version one')
 
diff --git a/packages/session-persistence/session-persistence/src/coordinator.ts b/packages/session-persistence/session-persistence/src/coordinator.ts
index 5ba419f118..e350beec3a 100644
--- a/packages/session-persistence/session-persistence/src/coordinator.ts
+++ b/packages/session-persistence/session-persistence/src/coordinator.ts
@@ -328,6 +328,7 @@ function migrateLegacyTurnEndEvent(event: SessionEvent, id: SessionId): SessionE
       break
     }
     default:
+      /* v8 ignore next -- turn-end reasons are merge-extensible beyond the current vocabulary. */
       return event
   }