diff --git a/packages/support/acp-snapshot/src/launcher.ts b/packages/support/acp-snapshot/src/launcher.ts
index de5082eca4..e39c6552f0 100644
--- a/packages/support/acp-snapshot/src/launcher.ts
+++ b/packages/support/acp-snapshot/src/launcher.ts
@@ -231,6 +231,15 @@ export function launchAcpTestAgent(options: AcpTestLaunchOptions): LaunchedAcpTe
         return
       }
 
+      const propagateFailureAfterDrain = async (): Promise<never> => {
+        await drained
+        closeUpdateStream()
+        throw failure
+      }
+      // Windows implements the supported signal names as forced termination. The exit markers
+      // may therefore arrive after the error wins the race above but before fallback begins.
+      if (!isRunning(child)) return propagateFailureAfterDrain()
+
       // An `error` after spawn is not an exit edge: in particular, a failed
       // signal can leave the subprocess live. Force termination, await the
       // already-observed exit edge, and only then propagate the child error so
@@ -240,6 +249,10 @@ export function launchAcpTestAgent(options: AcpTestLaunchOptions): LaunchedAcpTe
       child.once('error', observeFallbackError)
       if (!child.kill('SIGKILL')) {
         child.off('error', observeFallbackError)
+        // A successful earlier signal may win between the live check and this fallback call.
+        // In that case `kill()` correctly reports no process to signal; the original child error
+        // remains the shutdown result once inherited stdio and callbacks have drained.
+        if (!isRunning(child)) return propagateFailureAfterDrain()
         closeUpdateStream()
         throw new AggregateError(
           [failure, new Error('Fallback SIGKILL was not accepted by the child process')],
@@ -258,9 +271,7 @@ export function launchAcpTestAgent(options: AcpTestLaunchOptions): LaunchedAcpTe
           'ACP test agent failed and fallback termination was refused',
         )
       }
-      await drained
-      closeUpdateStream()
-      throw failure
+      return propagateFailureAfterDrain()
     },
   }
 }
diff --git a/packages/support/acp-snapshot/tests/harness.spec.ts b/packages/support/acp-snapshot/tests/harness.spec.ts
index 5589858fe9..2c4478e158 100644
--- a/packages/support/acp-snapshot/tests/harness.spec.ts
+++ b/packages/support/acp-snapshot/tests/harness.spec.ts
@@ -176,6 +176,30 @@ describe('runScenario', () => {
     }
   })
 
+  it('preserves the child error when fallback refusal races with an exit marker', async () => {
+    const { dir } = await scenario({})
+    const launched = launchAcpTestAgent({ agent: AGENT, cwd: dir })
+    await launched.spawned
+
+    const childFailure = Object.assign(new Error('signal failed while the child exited'), { code: 'EPERM' })
+    const originalKill = launched.child.kill.bind(launched.child)
+    const kill = vi.spyOn(launched.child, 'kill').mockImplementation((signal) => {
+      if (signal === 'SIGTERM') return true
+      originalKill('SIGKILL')
+      Object.defineProperty(launched.child, 'signalCode', { configurable: true, enumerable: true, writable: true, value: 'SIGKILL' })
+      return false
+    })
+    try {
+      launched.child.emit('error', childFailure)
+      await expect(launched.close('SIGTERM')).rejects.toBe(childFailure)
+      expect(kill).toHaveBeenNthCalledWith(1, 'SIGTERM')
+      expect(kill).toHaveBeenNthCalledWith(2, 'SIGKILL')
+    } finally {
+      kill.mockRestore()
+      if (launched.child.exitCode === null && launched.child.signalCode === null) originalKill('SIGKILL')
+    }
+  })
+
   it('rejects promptly when fallback termination emits an error', async () => {
     const { dir } = await scenario({})
     const launched = launchAcpTestAgent({ agent: AGENT, cwd: dir })