fix: drain failed ACP launches

packages/support/acp-snapshot/src/launcher.ts

@@ -178,13 +178,14 @@ export function launchAcpTestAgent(options: AcpTestLaunchOptions): LaunchedAcpTe
   // `closed` follows parser exhaustion. Capture both eagerly so a caller that
   // invokes close after process exit still joins the complete drain boundary.
   const stdioClosed = new Promise<void>(resolve => child.once('close', () => { resolve() }))
-  const drained = Promise.all([stdioClosed, client.closed]).then(async () => {
+  const drained = Promise.allSettled([stdioClosed, client.closed]).then(async ([, clientResult]) => {
     // The ACP SDK's readable loop dispatches client callbacks without awaiting
     // them. Once `closed` settles no new callbacks can start, but callbacks
     // already in flight still belong to this launch's teardown boundary.
     while (inFlightClientCallbacks.size > 0) {
       await Promise.allSettled([...inFlightClientCallbacks])
     }
+    if (clientResult.status === 'rejected') throw clientResult.reason
   })
   // A caller may await a pending update without calling close(). Make natural
   // stream exhaustion terminal for those waiters too, but only after the
@@ -206,6 +207,7 @@ export function launchAcpTestAgent(options: AcpTestLaunchOptions): LaunchedAcpTe
       try {
         await spawned
       } catch (error: unknown) {
+        await drained.catch(() => undefined)
         closeUpdateStream()
         throw error
       }

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

@@ -62,8 +62,17 @@ describe('runScenario', () => {
   it('surfaces an asynchronous child spawn failure through startup and close', async () => {
     const { dir } = await scenario({})
     const launched = launchAcpTestAgent({ agent: AGENT, cwd: join(dir, 'missing') })
+    let stdioClosed = false
+    let clientClosed = false
+    launched.child.once('close', () => { stdioClosed = true })
+    void launched.client.closed.then(
+      () => { clientClosed = true },
+      () => { clientClosed = true },
+    )
     await expect(launched.spawned).rejects.toMatchObject({ code: 'ENOENT' })
     await expect(launched.close()).rejects.toMatchObject({ code: 'ENOENT' })
+    expect(stdioClosed).toBe(true)
+    expect(clientClosed).toBe(true)
   })
 
   it('centralizes ACP boot, captures, updates, fail-closed permissions, and shutdown', { timeout: 20_000 }, async () => {