fix: handle EPIPE errors in StdioServerTransport gracefully

[MaxwellCalkin]

Note: This PR was authored by Claude (AI), operated by @MaxwellCalkin.

Fixes #1564

Problem

When an MCP client disconnects abruptly (closes stdio pipes before the server can write a response), stdout.write() throws an unhandled EPIPE error that crashes the entire Node.js process:

node:events:486
      throw er; // Unhandled 'error' event
      ^
Error: EPIPE: broken pipe, write
    at Socket._write (node:internal/net:75:18)
    ...
    at StdioServerTransport.send (...)

This happens because StdioServerTransport has no error event listener on the stdout stream, and stdout.write() in send() is not wrapped in error handling.

Fix

Three changes to StdioServerTransport:

1. Add _onstdouterror handler — an arrow function (matching the existing pattern for _ondata/_onerror) that forwards stdout errors to onerror and triggers a clean close().

2. Register/unregister the listener — start() adds this._stdout.on('error', this._onstdouterror); close() removes it with .off().

3. Wrap stdout.write() in try/catch in send() — if write() throws synchronously, the promise rejects instead of crashing the process.

This matches how the client-side StdioClientTransport already handles errors on stdin and stdout.

Tests

Added two new tests:

• "should handle stdout write errors gracefully" — verifies that async write errors (callback with error) trigger onerror and close()
• "should handle synchronous stdout write throws gracefully" — verifies that synchronous write() throws are caught and the promise rejects

Changeset

Included a patch changeset for @modelcontextprotocol/server.

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🦋 Changeset detected

Latest commit: 75c8800

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
@modelcontextprotocol/server	Patch
@modelcontextprotocol/express	Patch
@modelcontextprotocol/hono	Patch
@modelcontextprotocol/node	Patch

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commit: f81337b