fix(snapshot): include agent stderr in failures

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

@@ -24,6 +24,8 @@ interface ScriptedLog {
 
 /** The whole scripted behavior for one run. Every field defaults to the least surprising choice. */
 interface Behavior {
+  /** Exit during startup after writing any configured stderr note. */
+  failOnBoot?: boolean
   /** Reject every `session/new` (exercises the expect-error step without extra dirs). */
   rejectNewSession?: boolean
   /** Reject `session/new` only when `additionalDirectories` is non-empty (the real bridge's rule). */
@@ -62,6 +64,7 @@ const behavior: Behavior = fixtureFile === ''
   : JSON.parse(readFileSync(join(dirname(fixtureFile), 'behavior.json'), 'utf8')) as Behavior
 
 if (behavior.stderrNote !== undefined) process.stderr.write(`${behavior.stderrNote}\n`)
+if (behavior.failOnBoot === true) process.exit(7)
 
 let nextOutboundId = 1000
 let sessionId = ''

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

@@ -38,6 +38,14 @@ async function scenario(behavior: object): Promise<{ dir: string; fixtureFile: s
 const boot: InputStep[] = [{ op: 'initialize' }, { op: 'newSession' }]
 
 describe('runScenario', () => {
+  it('includes agent stderr when the ACP connection closes during startup', { timeout: 20_000 }, async () => {
+    const { fixtureFile } = await scenario({ failOnBoot: true, stderrNote: 'fake agent requested startup failure' })
+    await expect(runScenario(
+      { steps: [{ op: 'initialize' }] },
+      { agent: AGENT, mode: 'replay', fixtureFile },
+    )).rejects.toThrow(/agent stderr:\nfake agent requested startup failure/)
+  })
+
   it('drives a full turn: initialize (terminal caps), session, prompt, permission stub, harvest', { timeout: 20_000 }, async () => {
     const { fixtureFile } = await scenario({
       permissionProbe: true,