StdioClientTransport.close() doesn't clean up listeners, causing EventEmitter memory leak warning #3070

Bug: StdioClientTransport.close() doesn't clean up listeners, causing EventEmitter memory leak warning

Description

When using local (stdio) MCP servers, Node.js emits the following warning:

MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 drain listeners added to [WriteStream]. MaxListeners is 10. Use emitter.setMaxListeners() to increase limit

Root cause

In StdioClientTransport:

1. start() attaches permanent listeners to the stdio pipes:

this._process.stdin?.on("error", ...)
this._process.stdout?.on("data", ...)
this._process.stdout?.on("error", ...)
this._process.stderr.pipe(this._stderrStream)  // pipe() internally adds a "drain" listener

2. close() kills the process but doesn't remove any listeners:

async close(){
  // ... kills the process ...
  // NO removeListener / off / unpipe here
}

The pipe() call on stderr automatically adds a drain listener on the source stream to handle back-pressure. When the process is killed, this listener is orphaned.

Reproduction

1. Configure any local (stdio) MCP server that produces stderr output
2. Launch an MCP client (e.g., opencode TUI)
3. Observe the warning in stderr

Minimal MCP servers that don't produce stderr output don't trigger the warning. Real MCP servers (gitlab, google-workspace, etc.) that log to stderr do trigger it.

Proposed fix

In close(), add proper cleanup:

this._process.stderr?.unpipe()
this._process.stdin?.removeAllListeners()
this._process.stdout?.removeAllListeners()
this._process.stderr?.removeAllListeners()

Or alternatively, raise the max listeners limit in start():

this._process.stderr?.setMaxListeners(20)

Impact

• Non-blocking, but indicates a potential memory leak over long sessions
• Affects all MCP clients that use the TypeScript SDK with local (stdio) servers
• Reported in opencode issue #58173: Possible EventEmitter memory leak detected: 11 drain listeners added to [WriteStream] anomalyco/opencode#58173