onerror and other listener not remove after client close (stdio)

[shellRaining]

Describe the bug

This is my first time using MCP. I noticed that after closing the MCP client, there's an issue with JSON parsing failure. I'm not entirely sure whether the problem lies with the server or the SDK, and I hope to get some assistance. thanks~

To Reproduce

create a new mcp client

import { Client } from "@modelcontextprotocol/sdk/client/index.js";
import { StdioClientTransport } from "@modelcontextprotocol/sdk/client/stdio.js";

function createTransport() {
 let transport: StdioClientTransport;

 transport = new StdioClientTransport({
 command: "uvx",
 args: ["docker-mcp"],
 stderr: "pipe",
 });
 transport.onerror = async (error) => {
 console.log("error", error);
 };
 transport.onclose = async () => {
 console.log("close");
 };

 return transport;
}

const client = new Client(
 { name: "shellRaining", version: "1.0.0" },
 { capabilities: {} },
);

const transport = createTransport();
client.connect(transport);

await new Promise((resolve) => {
 setTimeout(resolve, 1000);
});

await transport.close()
await client.close()

then run this project, it will show below error message

close
error 21 | clear() {
22 | this._buffer = undefined;
23 | }
24 | }
25 | export function deserializeMessage(line) {
26 | return JSONRPCMessageSchema.parse(JSON.parse(line));
 ^
SyntaxError: JSON Parse error: Unexpected identifier "Shutting"
 at deserializeMessage (/Users/shellraining/Documents/playground/mcp/typescript-sdk/dist/esm/shared/stdio.js:26:44)
 at processReadBuffer (/Users/shellraining/Documents/playground/mcp/typescript-sdk/dist/esm/client/stdio.js:141:50)
 at <anonymous> (/Users/shellraining/Documents/playground/mcp/typescript-sdk/dist/esm/client/stdio.js:103:22)
 at emit (node:events:95:22)
 at addChunk (internal:streams/readable:265:47)
 at readableAddChunkPushByteMode (internal:streams/readable:243:18)
 at handleNumberResult (internal:streams/native-readable:69:18)
 at <anonymous> (internal:streams/native-readable:43:68)

Expected behavior

not occur error message

[shellRaining]

I tried adding the following part of the code and found that the aforementioned error no longer occurs. If this change doesn't break the protocol implementation, may I submit a PR?

[ochafik]

Hi @shellRaining, thanks for filing this!

I tried adding the following part of the code and found that the aforementioned error no longer occurs. If this change doesn't break the protocol implementation, may I submit a PR?

We'd probably want to remove only the very listeners we registered, not all of them.

[shellRaining]

Okay, I'll take another look at this part of the code.

[mcp-claude]

bug confirmed on v1.x (@modelcontextprotocol/sdk@1.29.0); fixed on main via commit 64897f78 (PR #1762) which catches SyntaxError in ReadBuffer.readMessage() and skips non-JSON lines. v1.x needs a backport.

root cause: StdioClientTransport.close() nulls this._process but never removes the stdout data listener registered in start(). When the subprocess writes non-JSON text after close, the still-live listener calls processReadBuffer() → readMessage() → deserializeMessage() → JSON.parse() throws SyntaxError → surfaces via onerror.

workaround (v1.x): assign transport.onerror after close() to a no-op, or catch and filter SyntaxError in your onerror handler.

repro script (node repro_v1.mjs, v1.x branch)

// repro_v1.mjs — run from repo root on v1.x after `npm run build`
import { StdioClientTransport } from './dist/esm/client/stdio.js';

const serverScript = `
 setTimeout(() => {
 process.stdout.write("Shutting down...\\n");
 process.exit(0);
 }, 300);
`;

const errors = [];
const transport = new StdioClientTransport({ command: 'node', args: ['-e', serverScript] });
transport.onerror = (err) => errors.push(err);

await transport.start();
await transport.close(); // listener stays on stdout
await new Promise(r => setTimeout(r, 700)); // wait for non-JSON write

if (errors.length > 0) {
 console.log('BUG REPRODUCED: onerror called after transport.close()');
 errors.forEach(e => console.log(' ', e.message));
 process.exit(1);
}
console.log('No errors.');

command: node repro_v1.mjs

output:

BUG REPRODUCED: onerror called after transport.close()
 Unexpected token 'S', "Shutting down..." is not valid JSON

code path

src/client/stdio.ts:150-153 registers the listener:

this._process.stdout?.on('data', chunk => {
 this._readBuffer.append(chunk);
 this.processReadBuffer(); // → readMessage() → deserializeMessage() → JSON.parse()
});

src/client/stdio.ts:204-243 (close()) never calls removeListener/off on stdout.

src/shared/stdio.ts:13-26 (readMessage() on v1.x) — no try/catch; throws SyntaxError directly to caller.

fix on main (packages/core/src/shared/stdio.ts, commit 64897f78):

try {
 return deserializeMessage(line);
} catch (error) {
 if (error instanceof SyntaxError) { continue; } // skip non-JSON lines
 throw error;
}

note: main's fix suppresses the visible error but doesn't remove the dangling listener. a complete fix would also call processToClose.stdout?.off('data', handler) in close(), as discussed in the issue.

suggested fix

// src/shared/stdio.ts
 readMessage(): JSONRPCMessage | null {
 if (!this._buffer) {
 return null;
 }
 
- const index = this._buffer.indexOf('\n');
- if (index === -1) {
- return null;
+ while (true) {
+ const index = this._buffer.indexOf('\n');
+ if (index === -1) {
+ return null;
+ }
+
+ const line = this._buffer.toString('utf8', 0, index).replace(/\r$/, '');
+ this._buffer = this._buffer.subarray(index + 1);
+ try {
+ return deserializeMessage(line);
+ } catch (error) {
+ if (error instanceof SyntaxError) {
+ // skip non-JSON lines (e.g. subprocess log output)
+ if (!this._buffer || this._buffer.length === 0) {
+ return null;
+ }
+ continue;
+ }
+ throw error;
+ }
 }
-
- const line = this._buffer.toString('utf8', 0, index).replace(/\r$/, '');
- this._buffer = this._buffer.subarray(index + 1);
- return deserializeMessage(line);
 }

test to verify: appending a non-JSON line followed by a valid JSON-RPC message should return null for the non-JSON line and the parsed message for the valid one, without throwing.