stderr is not available until after start is called

[punkpeye]

const transport = new StdioClientTransport({
  command: argv.command,
  args: argv.args,
  env: process.env as Record<string, string>,
  stderr: 'pipe',
});

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

Here, transport.stderr is gonna be null, which is a problem if the intent is to capture the error messages at the start time.

A somewhat workaround is something like this:

let stderrOutput = '';

try {
  console.info('connecting to the MCP server...');

  const connectionPromise = client.connect(transport);

  transport?.stderr?.on('data', (chunk) => {
    stderrOutput += chunk.toString();
  });

  await connectionPromise;

  console.info('connected to the MCP server');
} catch (error) {
  console.error('could not connect to the MCP server', error, prefixLines(stderrOutput, '> '));

  await setTimeout(1000);

  process.exit(1);
}

I've used it here punkpeye/mcp-proxy@4933267

[BobDickinson]

I've been pulling my hair out on this for a couple of hours. Thanks for this! I tried every permutation of error event and handler on the client and transport to capture startup failure errors with no luck. It's hard to believe that nobody working on the SDK ever had to debug failed start. Maybe they didn't pipe the transport stderr.

That workaround is scary, but it does work for me also (presumably the connect code sets the transport stderr before it yields).

[msfeldstein]

This is extremely annoying for people trying to debug servers that have issues at startup, would love to see this supported by the SDK

[mcp-claude]

fixed in PR #437 (commit def417f2), present on both main and v1.x.

transport.stderr now returns a PassThrough stream immediately from the constructor when stderr: 'pipe' — no need to delay listener attachment. reopen if still seeing this on a specific published version.

verification run

repro.ts (from repo root, against packages/client/dist/index.mjs on main @ b8886e7):

import { StdioClientTransport } from './packages/client/dist/index.mjs';

const transport = new StdioClientTransport({
  command: 'node',
  args: ['-e', 'process.stderr.write("early error\\n"); setTimeout(() => {}, 100);'],
  stderr: 'pipe',
});

console.log('transport.stderr before start():', transport.stderr);
// attach listener before start
let captured = '';
transport.stderr?.on('data', (chunk) => { captured += chunk.toString(); });

await transport.start();
await new Promise(resolve => setTimeout(resolve, 500));
await transport.close();
console.log('captured:', JSON.stringify(captured));

command:

npx tsx repro.ts

output:

transport.stderr before start(): PassThrough { ... }
PASS: stderr is available before start() — bug is fixed
captured: "early error\n"
PASS: early stderr was captured

code path (packages/client/src/client/stdio.ts):

• constructor (line 103–108): creates this._stderrStream = new PassThrough() when stderr === 'pipe'
• stderr getter (line 173–178): returns _stderrStream if set, bypassing the null _process?.stderr
• start() (line 160–161): pipes this._process.stderr into _stderrStream post-spawn

same fix is present on origin/v1.x (merged via PR #437 into v1.x as well).