fastmcp

What it is

A TypeScript framework for building MCP servers capable of handling client sessions. It includes features such as authentication, headers through context, session tracking, image/audio content support, logging, error handling, HTTP streaming (with SSE compatibility), HTTPS, CORS, custom HTTP routes, edge runtime support, stateless mode, and an in-memory transport for unit testing.

How it works

The library exposes a FastMCP class to create and start an MCP server with a given configuration (name, version, authentication). It supports multiple transports (stdio, httpStream, SSE), and can connect HTTP streaming or SSE clients depending on transport. It provides a getApp() to add custom HTTP routes via an underlying Hono app and includes built-in utilities for content blocks (image, audio) and resource embedding. It can be used in edge environments via EdgeFastMCP and Cloudflare Workers integration. It includes a Quickstart demonstrating server creation, adding a tool, and starting with transportType: "stdio". It also shows usage for HTTP streaming and HTTPS with SSL options, as well as CORS customization.

Getting started

Install with:

npm install fastmcp

Quickstart example:

import { FastMCP } from "fastmcp";
import { z } from "zod";

const server = new FastMCP({
  name: "My Server",
  version: "1.0.0",
});

server.addTool({
  name: "add",
  description: "Add two numbers",
  parameters: z.object({
    a: z.number(),
    b: z.number(),
  }),
  execute: async (args) => {
    return String(args.a + args.b);
  },
});

server.start({
  transportType: "stdio",
});

Test commands:

git clone https://github.com/punkpeye/fastmcp.git
cd fastmcp

pnpm install
pnpm build

# Test the addition server example using CLI:
npx fastmcp dev src/examples/addition.ts
# Test the addition server example using MCP Inspector:
npx fastmcp inspect src/examples/addition.ts

Remote Server Options

server.start({
  transportType: "httpStream",
  httpStream: { port: 8080 }
});

For SSE:

import { SSEClientTransport } from "@modelcontextprotocol/sdk/client/sse.js";

HTTPS support:

server.start({
  transportType: "httpStream",
  httpStream: {
    port: 8443,
    sslCert: "./path/to/cert.pem",
    sslKey: "./path/to/key.pem",
    sslCa: "./path/to/ca.pem",
  },
});

CORS customization example:

server.start({
  transportType: "httpStream",
  httpStream: {
    port: 8080,
    cors: {
      origin: "http://localhost:3000",
      allowedHeaders: ["Content-Type", "Authorization", "Accept", "Mcp-Session-Id", "Mcp-Protocol-Version", "Last-Event-Id", "X-Custom-Header"],
      credentials: true
    }
  },
});

Custom HTTP routes:

const app = server.getApp();
app.get("/api/users", async (c) => {
  return c.json({ users: [] });
});

Edge Runtime:

import { EdgeFastMCP } from "fastmcp/edge";
const server = new EdgeFastMCP({ name: "My Edge Server", version: "1.0.0" });
export default server;

Recent releases

• v4.12.4 (2026-08-04): Bug Fixes — add timeout to image and audio content fetch
• v4.12.3 (2026-08-04): Bug Fixes — allow contents write
• v4.12.2 (2026-08-01): Bug Fixes — auth: use timing-safe comparison for JWT and consent signatures
• v4.12.1 (2026-07-27): Bug Fixes — export SessionError and FastMCPError from the package root
• v4.12.0 (2026-07-25): Features — add FastMCP.connect for custom transports

Traction

3246 stars, 300 forks, 22 open issues

Behind the repo

Not provided

Caveats

License: MIT Created: 2024-12-27 Last push: 2026-08-04