diff --git a/examples/acp-agent/tests/snapshots/cancel-tool-calls/input.json b/examples/acp-agent/tests/snapshots/cancel-tool-calls/input.json
index 3610e1f436..0f40e9d8b6 100644
--- a/examples/acp-agent/tests/snapshots/cancel-tool-calls/input.json
+++ b/examples/acp-agent/tests/snapshots/cancel-tool-calls/input.json
@@ -5,7 +5,8 @@
     {
       "op": "promptAndCancel",
       "text": "Run two shell commands: wait for cancellation, then write skipped.txt.",
-      "afterUpdate": "tool_call"
+      "afterUpdate": "tool_call",
+      "waitForToolCallUpdate": "call_skipped"
     }
   ]
 }
diff --git a/packages/support/acp-snapshot/src/harness.ts b/packages/support/acp-snapshot/src/harness.ts
index ca00056d93..542b6d6df3 100644
--- a/packages/support/acp-snapshot/src/harness.ts
+++ b/packages/support/acp-snapshot/src/harness.ts
@@ -39,8 +39,8 @@ export type { AgentUnderTest } from './launcher.ts'
  *
  * `promptAndCancel` starts a prompt without awaiting completion, waits until
  * the client observes the selected update (`agent_message_chunk` by default),
- * then cancels and awaits completion. This keeps update/cancel order
- * deterministic for fixtures that a plain `prompt` cannot drive.
+ * then cancels and awaits completion. A named `waitForToolCallUpdate` keeps the
+ * step open for a terminal tool update that may follow the prompt response.
  */
 export type InputStep =
   | { op: 'initialize'; terminalOutput?: boolean }
@@ -48,7 +48,12 @@ export type InputStep =
   | { op: 'newSessionExpectError'; additionalDirectories?: string[] }
   | { op: 'prompt'; text: string }
   | { op: 'promptExpectError'; text: string }
-  | { op: 'promptAndCancel'; text: string; afterUpdate?: 'agent_message_chunk' | 'tool_call' }
+  | {
+    op: 'promptAndCancel'
+    text: string
+    afterUpdate?: 'agent_message_chunk' | 'tool_call'
+    waitForToolCallUpdate?: string
+  }
   | { op: 'cancel' }
   | { op: 'setConfigOption'; configId: string; value: string }
   | { op: 'setConfigOptionExpectError'; configId: string; value: string }
@@ -347,8 +352,13 @@ async function runStep(
       const promptDone = client.prompt({ sessionId, prompt: [{ type: 'text', text: step.text }] })
       const afterUpdate = step.afterUpdate ?? 'agent_message_chunk'
       await waitForUpdate(u => u.sessionUpdate === afterUpdate)
+      // Arm this before cancellation so a fast tool drain cannot outrun the waiter.
+      const toolCallUpdateDone = step.waitForToolCallUpdate === undefined
+        ? undefined
+        : waitForUpdate(u => u.sessionUpdate === 'tool_call_update' && u.toolCallId === step.waitForToolCallUpdate)
       await client.cancel({ sessionId })
       await promptDone
+      if (toolCallUpdateDone !== undefined) await toolCallUpdateDone
       return
     }
     case 'cancel': {
diff --git a/packages/support/acp-snapshot/tests/fixtures/fake-acp-agent.ts b/packages/support/acp-snapshot/tests/fixtures/fake-acp-agent.ts
index dd8ca9ce92..43b5ee3fb3 100644
--- a/packages/support/acp-snapshot/tests/fixtures/fake-acp-agent.ts
+++ b/packages/support/acp-snapshot/tests/fixtures/fake-acp-agent.ts
@@ -35,6 +35,8 @@ interface Behavior {
   prompt?: 'respond' | 'error' | 'hang-until-cancel'
   /** Emit a tool call instead of a message chunk before parking a cancellable prompt. */
   cancelAtToolCall?: boolean
+  /** Emit the parked tool call's terminal update after answering cancellation. */
+  cancelToolCallUpdate?: boolean
   /** Before responding to a prompt, send a `session/request_permission` request and echo its outcome as a chunk. */
   permissionProbe?: boolean
   /** Echo the `DSH_SNAPSHOT_*` env the harness set as a chunk (spec-side env-plumbing assertions). */
@@ -247,6 +249,19 @@ function handleFrame(frame: Record<string, unknown>): void {
         const parked = parkedPromptId
         parkedPromptId = null
         respond(parked, { stopReason: 'cancelled' })
+        if (behavior.cancelToolCallUpdate === true) {
+          send({
+            method: 'session/update',
+            params: {
+              sessionId,
+              update: {
+                sessionUpdate: 'tool_call_update',
+                toolCallId: 'call_fake_1',
+                status: 'failed',
+              },
+            },
+          })
+        }
       }
       return
     default:
diff --git a/packages/support/acp-snapshot/tests/harness.spec.ts b/packages/support/acp-snapshot/tests/harness.spec.ts
index be03b27c78..3d174b3c3e 100644
--- a/packages/support/acp-snapshot/tests/harness.spec.ts
+++ b/packages/support/acp-snapshot/tests/harness.spec.ts
@@ -334,14 +334,26 @@ describe('runScenario', () => {
     expect(result.rawStdout.indexOf('thinking about it')).toBeLessThan(result.rawStdout.indexOf('cancelled'))
   })
 
-  it('promptAndCancel can wait for a tool call before cancelling', { timeout: 20_000 }, async () => {
-    const { fixtureFile } = await scenario({ prompt: 'hang-until-cancel', cancelAtToolCall: true })
+  it('promptAndCancel can bracket cancellation with tool-call updates', { timeout: 20_000 }, async () => {
+    const { fixtureFile } = await scenario({
+      prompt: 'hang-until-cancel',
+      cancelAtToolCall: true,
+      cancelToolCallUpdate: true,
+    })
     const result = await runScenario(
-      { steps: [...boot, { op: 'promptAndCancel', text: 'hang', afterUpdate: 'tool_call' }] },
+      {
+        steps: [...boot, {
+          op: 'promptAndCancel',
+          text: 'hang',
+          afterUpdate: 'tool_call',
+          waitForToolCallUpdate: 'call_fake_1',
+        }],
+      },
       { agent: AGENT, mode: 'replay', fixtureFile },
     )
     expect(result.rawStdout).toContain('"sessionUpdate":"tool_call"')
     expect(result.rawStdout.indexOf('"sessionUpdate":"tool_call"')).toBeLessThan(result.rawStdout.indexOf('cancelled'))
+    expect(result.rawStdout.indexOf('cancelled')).toBeLessThan(result.rawStdout.indexOf('"sessionUpdate":"tool_call_update"'))
   })
 
   it('promptExpectError swallows a model-error response as the expected outcome', { timeout: 20_000 }, async () => {