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 5dd5524ed0..647ffbb5d9 100644
--- a/packages/support/acp-snapshot/tests/fixtures/fake-acp-agent.ts
+++ b/packages/support/acp-snapshot/tests/fixtures/fake-acp-agent.ts
@@ -266,7 +266,9 @@ function flushLogsAndExit(): void {
       `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()
+    spawn(process.execPath, ['-e', code], {
+      stdio: ['ignore', process.stdout, process.stderr],
+    }).unref()
   }
   process.exit(0)
 }
diff --git a/packages/support/acp-snapshot/tests/harness.spec.ts b/packages/support/acp-snapshot/tests/harness.spec.ts
index 2c4478e158..08b069338c 100644
--- a/packages/support/acp-snapshot/tests/harness.spec.ts
+++ b/packages/support/acp-snapshot/tests/harness.spec.ts
@@ -139,6 +139,9 @@ describe('runScenario', () => {
       update.sessionUpdate === 'agent_message_chunk'
       && update.content.type === 'text'
       && update.content.text === 'late inherited stdout')
+    // Arm rejection handling before close may exhaust the stream; the later assertion still
+    // observes the original promise and turns a missing inherited frame into the test failure.
+    void lateUpdate.catch(() => undefined)
 
     await launched.close()