Serve over stdio

A host that launches your server as a local child process talks to it over stdio: JSON-RPC requests arrive on stdin, responses leave on stdout. To host one endpoint that many clients connect to, serve the same factory over HTTP instead.

Serve a factory over stdio

serveStdio takes a factory; it owns the transport and calls the factory to build the instance that serves the connection.

import { McpServer } from '@modelcontextprotocol/server';
import { serveStdio } from '@modelcontextprotocol/server/stdio';

const handle = serveStdio(() => {
    const server = new McpServer({ name: 'notes', version: '1.0.0' });
    // server.registerTool(...) — one factory builds the instance that serves the connection
    return server;
});

The process is now an MCP server. A host that spawns it lists and calls whatever the factory registered; until one does, the process waits on stdin.

Coming from v1?

serveStdio replaces the new StdioServerTransport() + server.connect(transport) wiring — run the codemod, then see the upgrade guide.

INFO

serveStdio serves older clients from the same factory by default; the legacy option and the full story are on Legacy clients. The entry also owns which protocol revision each connection negotiates — see Protocol versions.

Log to stderr, never stdout

Announce readiness with console.error, which writes to stderr.

console.error('notes server is listening on stdio');

stdout is the JSON-RPC channel: the host parses every line of it as a protocol message. Add one console.log('debug: starting the notes server') to the program above and send it an initialize request. Its two output streams now carry:

[stdout] debug: starting the notes server
[stdout] {"result":{"protocolVersion":"2025-06-18","capabilities":{},"serverInfo":{"name":"notes","version":"1.0.0"}},"jsonrpc":"2.0","id":1}
[stderr] notes server is listening on stdio

The protocol channel opens with a line no JSON-RPC parser accepts, ahead of the initialize response. The console.error banner went to stderr, which the host keeps out of the channel and shows in its server log.

Test it with the Inspector

The MCP Inspector launches your server command itself and connects to it over stdio.

npx @modelcontextprotocol/inspector node ./build/server.js

In the browser tab it opens, click Connect; the Tools tab lists and calls everything the factory registered, without configuring the server in a host.

Shut down cleanly

serveStdio returns a StdioServerHandle; its close() tears down the pinned server instance and the transport.

process.on('SIGINT', () => {
    void handle.close();
});

close() resolves once the instance the factory built and the underlying transport are both shut down.

Recap

• serveStdio(factory) is the stdio entry point: it owns the transport and calls your factory to build the instance that serves the connection.
• stdout is the protocol channel; log with console.error.
• One console.log puts a line no JSON-RPC parser accepts into the stream the host parses.
• npx @modelcontextprotocol/inspector <command> exercises a stdio server without configuring it in a host.
• The returned StdioServerHandle's close() tears down the pinned instance and the transport.