What is Fastify Mcp

fastify-mcp is a plugin that allows you to integrate Model Context Protocol (MCP) servers with your Fastify application using Server-Sent Events (SSE) connections.

Use cases

Use cases for fastify-mcp include building interactive web applications that need to push updates to clients in real-time, such as dashboards, collaborative tools, and live data feeds.

How to use

To use fastify-mcp, first define your MCP server by creating an instance of McpServer. Then, create a Fastify app and register the fastify-mcp plugin, passing the MCP server to it. Finally, start the Fastify server on your desired port.

Key features

Key features of fastify-mcp include easy integration with Fastify, support for managing multiple SSE sessions, and session event handling for connected and terminated sessions.

Where to use

fastify-mcp can be used in web applications that require real-time data streaming, such as chat applications, live notifications, or any scenario that benefits from server-sent events.

fastify-mcp

Integrate Model Context Protocol servers with your Fastify app.

Supports the Streamable HTTP transport as well as the legacy HTTP+SSE transport.

Usage

First, define your MCP server.

function createServer() {
  const mcpServer = new McpServer({
    name: "...",
    version: "...",
  });

  mcpServer.tool("...");
  mcpServer.resource("...");

  return mcpServer.server;
}

Create a Fastify app and register the plugin.

import { fastify } from "fastify";
import { streamableHttp } from "fastify-mcp";
import { StreamableHTTPServerTransport } from "@modelcontextprotocol/sdk/server/streamableHttp.js";

const app = fastify();

app.register(streamableHttp, {
  // Set to `true` if you want a stateful server
  stateful: false,
  mcpEndpoint: "/mcp",
  sessions: new Sessions<StreamableHTTPServerTransport>()
  createServer,
});

app.listen({ port: 8080 });

See the examples directory for more detailed examples.

Installation

# npm
npm install fastify-mcp

# yarn
yarn add fastify-mcp

Session Management

The official MCP TypeScript SDK does not support managing multiple sessions out of the box, and therefore it’s the host server’s responsibility to do so.

This package uses an in-memory mapping of each active session against its session ID to manage multiple sessions, as recommended by the MCP SDK examples.

Session Events

The Sessions class emits the following events:

• connected: Emitted when a new session is added.
• terminated: Emitted when a session is removed.
• error: Emitted when an asynchronous event handler throws an error.

const sessions = new Sessions<StreamableHTTPServerTransport>();

sessions.on("connected", (sessionId) => {
  console.log(`Session ${sessionId} connected`);
});

sessions.on("terminated", (sessionId) => {
  console.log(`Session ${sessionId} terminated`);
});

Contributing

Please file an issue if you encounter a problem when using this package. Pull requests for new features or bug fixes are also welcome.