Add regression test and changeset

- Add test verifying stdout listeners are detached on close() so
  late non-JSON output from the child process no longer fires onerror
- Add patch-level changeset

Author: felixweinberger

.changeset/stdio-cleanup-listeners-on-close.md

@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/client': patch
+---
+
+Detach stdio transport stdout/stdin listeners on `close()`. Previously, if the child process wrote to stdout during shutdown (after `close()` was called), the still-attached data listener would attempt to parse it as JSON-RPC and emit a spurious parse error via `onerror`. Fixes #780.

packages/client/test/client/crossSpawn.test.ts

@@ -173,4 +173,55 @@ describe('StdioClientTransport using cross-spawn', () => {
         // verify message is sent correctly
         expect(mockProcess.stdin.write).toHaveBeenCalled();
     });
+
+    // Regression test for #780: late stdout data after close() should not trigger onerror
+    test('should detach stdout listeners on close so late output does not fire onerror', async () => {
+        let capturedDataHandler: ((chunk: Buffer) => void) | undefined;
+        let closeCallback: (() => void) | undefined;
+
+        const mockProcess = {
+            on: vi.fn().mockReturnThis(),
+            once: vi.fn((event: string, callback: () => void) => {
+                if (event === 'spawn') callback();
+                if (event === 'close') closeCallback = callback;
+                return mockProcess;
+            }),
+            stdin: {
+                on: vi.fn(),
+                off: vi.fn(),
+                write: vi.fn().mockReturnValue(true),
+                end: vi.fn(() => closeCallback?.())
+            },
+            stdout: {
+                on: vi.fn((event: string, handler: (chunk: Buffer) => void) => {
+                    if (event === 'data') capturedDataHandler = handler;
+                }),
+                off: vi.fn()
+            },
+            stderr: null,
+            exitCode: 0,
+            kill: vi.fn()
+        };
+
+        mockSpawn.mockReturnValue(mockProcess as unknown as ChildProcess);
+
+        const transport = new StdioClientTransport({ command: 'test-command' });
+        const onerror = vi.fn();
+        transport.onerror = onerror;
+
+        await transport.start();
+        expect(capturedDataHandler).toBeDefined();
+
+        await transport.close();
+
+        // close() must detach the listener it registered
+        expect(mockProcess.stdout.off).toHaveBeenCalledWith('data', capturedDataHandler);
+        expect(mockProcess.stdin.off).toHaveBeenCalled();
+
+        // Simulate the child writing non-JSON during shutdown — the #780 scenario.
+        // Before the fix this would parse-fail and call onerror; now the listener
+        // is detached so the transport never sees it.
+        capturedDataHandler!(Buffer.from('Shutting down...\n'));
+        expect(onerror).not.toHaveBeenCalled();
+    });
 });