fix: drain ACP test launcher streams

packages/support/acp-snapshot/tests/fixtures/fake-acp-agent.ts

@@ -18,6 +18,7 @@
 
 import { mkdirSync, readFileSync, rmSync, writeFileSync } from 'node:fs'
 import { readdirSync } from 'node:fs'
+import { spawn } from 'node:child_process'
 import { dirname, join } from 'node:path'
 import { randomUUID } from 'node:crypto'
 import { createInterface } from 'node:readline'
@@ -50,6 +51,8 @@ interface Behavior {
   echoWorkspace?: boolean
   /** Write a line to stderr on boot (spec-side stderr-capture assertions). */
   stderrNote?: string
+  /** Let a short-lived descendant retain stdio and emit one final ACP update plus stderr line after this parent exits. */
+  lateInheritedOutput?: boolean
   /** Session logs to persist on stdin EOF. */
   logs?: ScriptedLog[]
   /** Leave a stray FILE directly under the sessions root (harvest must skip it). */
@@ -256,6 +259,24 @@ function flushLogsAndExit(): void {
     writeFileSync(join(sessionsRoot, 'bucket-noise', 'notes.txt'), 'not a session log\n')
   }
   if (behavior.deleteSessionsRoot === true) rmSync(sessionsRoot, { recursive: true, force: true })
+  if (behavior.lateInheritedOutput === true) {
+    const frame = JSON.stringify({
+      jsonrpc: '2.0',
+      method: 'session/update',
+      params: {
+        sessionId,
+        update: {
+          sessionUpdate: 'agent_message_chunk',
+          content: { type: 'text', text: 'late inherited stdout' },
+        },
+      },
+    })
+    const code = [
+      `setTimeout(() => process.stdout.write(${JSON.stringify(`${frame}\n`)}), 50)`,
+      `setTimeout(() => process.stderr.write(${JSON.stringify('late inherited stderr\n')}), 75)`,
+    ].join(';')
+    spawn(process.execPath, ['-e', code], { stdio: ['ignore', 1, 2] }).unref()
+  }
   process.exit(0)
 }
 

packages/support/acp-snapshot/tests/harness.spec.ts

@@ -91,6 +91,27 @@ describe('runScenario', () => {
     expect(exited).toBe(true)
   })
 
+  it('waits for inherited stdio and buffered ACP parsing after the parent exits', { timeout: 20_000 }, async () => {
+    const { dir, fixtureFile } = await scenario({ lateInheritedOutput: true })
+    const launched = launchAcpTestAgent({
+      agent: AGENT,
+      cwd: dir,
+      env: { DSH_SNAPSHOT_FILE: fixtureFile },
+    })
+    await launched.client.initialize({ protocolVersion: PROTOCOL_VERSION, clientCapabilities: {} })
+    await launched.client.newSession({ cwd: dir, mcpServers: [] })
+    const lateUpdate = launched.waitForUpdate(update =>
+      update.sessionUpdate === 'agent_message_chunk'
+      && update.content.type === 'text'
+      && update.content.text === 'late inherited stdout')
+
+    await launched.close()
+
+    await expect(lateUpdate).resolves.toMatchObject({ sessionUpdate: 'agent_message_chunk' })
+    expect(launched.rawStdout()).toContain('late inherited stdout')
+    expect(launched.stderr()).toContain('late inherited stderr')
+  })
+
   it('drives a full turn: initialize (terminal caps), session, prompt, permission stub, harvest', { timeout: 20_000 }, async () => {
     const { fixtureFile } = await scenario({
       permissionProbe: true,