Fastify Mcp Server

A robust Fastify plugin that provides seamless integration with the Model Context Protocol (MCP) through streamable HTTP transport. This plugin enables your Fastify applications to act as MCP servers, allowing AI assistants and other clients to interact with your services using the standardized MCP protocol.

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Table of Contents

• Fastify MCP Server Plugin
  • Table of Contents
  • Overview
  • Features
    • Core Functionality
    • Advanced Features
  • Installation
  • Quick Demo
  • Quick Start
  • API Reference
    • Plugin Options
    • MCP Decorator
    • Session Events
  • HTTP Protocol
    • POST /mcp
    • GET /mcp
    • DELETE /mcp
    • Session Management
  • Advanced Usage
    • Custom Error Handling
    • Health Monitoring
    • Graceful Shutdown
  • Session Storage
    • Built-in Session Stores
      • In-Memory Session Store (Default)
      • Redis Session Store
    • Custom Session Store
    • How It Works
    • Comparison
    • Docker Compose Example
  • Authentication: Bearer Token Support
    • Enabling Bearer Token Authentication
    • How It Works
      • Example Tool with authentication information
      • Example Error Response
      • Example using PAT in Visual Studio Code
  • Well-Known OAuth Metadata Routes
    • Registering Well-Known Routes
    • Endpoints
  • Custom Transport Options
    • Available Options
    • Use Cases
    • Example: Custom Session ID with Prefix
  • Development
    • Setup
    • Scripts
    • Testing
  • Contributing
  • License
  • Related Projects

Overview

The Model Context Protocol (MCP) is an open standard that enables AI assistants to securely connect to external data sources and tools. This plugin provides a streamable HTTP transport implementation for MCP servers built with Fastify, offering:

• High Performance: Built on top of Fastify's high-performance HTTP server
• Session Management: Automatic handling of MCP sessions with proper lifecycle management
• Event-Driven Architecture: Real-time session monitoring and error handling
• Type Safety: Full TypeScript support with comprehensive type definitions
• Production Ready: Robust error handling, graceful shutdown, and monitoring capabilities

Features

Core Functionality

• ✅ MCP Server Integration: Seamless integration with @modelcontextprotocol/sdk
• ✅ Streamable HTTP Transport: Full support for MCP's streamable HTTP protocol
• ✅ Session Management: Automatic session creation, tracking, and cleanup
• ✅ Session Storage: Support for in-memory and custom session stores (Redis included)
• ✅ Request Routing: Intelligent routing for different MCP request types
• ✅ Authentication: Optional Bearer token support for secure access
• ✅ Error Handling: Comprehensive error handling with proper MCP error responses

Advanced Features

• ✅ Event System: Listen to session lifecycle events (creation, destruction, errors)
• ✅ Session Statistics: Real-time monitoring of active sessions
• ✅ Graceful Shutdown: Proper cleanup of all sessions during server shutdown
• ✅ Configurable Endpoints: Customizable MCP endpoint paths
• ✅ Custom Session Stores: Implement your own session storage backend
• ✅ Custom Transport Options: Configure transport behavior with custom session ID generation and callbacks
• ✅ TypeScript Support: Full type safety and IntelliSense support

Installation

npm install fastify-mcp-server @modelcontextprotocol/sdk

Quick Demo

To quickly see the plugin in action, you can run the demo server:

# Run with in-memory session storage
npm run dev

# Run with Redis session storage
npm run dev:redis

# Start MCP inspector to interact with the server
npm run inspector

This will start a Fastify server with the MCP plugin enabled, allowing you to interact with it via the MCP inspector or any MCP-compatible client.

Quick Start

import Fastify from 'fastify';
import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';
import FastifyMcpServer, { getMcpDecorator } from 'fastify-mcp-server';

const app = Fastify({ logger: true });

// Create MCP server factory function
function createMcpServer () {
  const mcp = new McpServer({
    name: 'my-mcp-server',
    version: '1.0.0'
  });

  // Define MCP tools
  mcp.tool('hello-world', () => ({
    content: [{ type: 'text', text: 'Hello from MCP!' }]
  }));

  return mcp;
}

// Register the plugin
await app.register(FastifyMcpServer, {
  createMcpServer,
  endpoint: '/mcp' // optional, defaults to '/mcp'
});

// Get MCP decorator for advanced features
const mcpServer = getMcpDecorator(app);

// Start the server
await app.listen({ host: '127.0.0.1', port: 3000 });

API Reference

Plugin Options

type FastifyMcpServerOptions = {
  createMcpServer: () => McpServer; // MCP Server factory function
  endpoint?: string; // Custom endpoint path (default: '/mcp')
  authorization?: {
    // Authorization configuration
    bearerMiddlewareOptions: {
      verifier: OAuthTokenVerifier; // Custom verifier for Bearer tokens
      requiredScopes?: string[]; // Optional scopes required for access
      resourceMetadataUrl?: string; // Optional URL for resource metadata
    };
    oauth2?: {
      // OAuth2 metadata configuration
      authorizationServerOAuthMetadata: OAuthMetadata; // OAuth metadata for authorization server
      protectedResourceOAuthMetadata: OAuthProtectedResourceMetadata; // OAuth metadata for protected resource
    };
  };
  sessionStore?: SessionStore; // Optional custom session store implementation
  transportOptions?: StreamableHTTPServerTransportOptions; // Optional transport configuration options
};

MCP Decorator

The plugin decorates your Fastify instance with an MCP server that provides several useful methods:

const mcpServer = getMcpDecorator(app);

// Get session statistics
const stats = mcpServer.getStats();
console.log(`Active sessions: ${stats.activeSessions}`);

// Access session manager for event handling
const sessionManager = mcpServer.getSessionManager();

// Create a new MCP server instance (useful for per-session customization)
const newMcpInstance = mcpServer.create();

Session Events

Monitor session lifecycle with event listeners:

const sessionManager = mcpServer.getSessionManager();

// Session created
sessionManager.on('sessionCreated', (sessionId: string) => {
  console.log(`New MCP session: ${sessionId}`);
});

// Session destroyed
sessionManager.on('sessionDestroyed', (sessionId: string) => {
  console.log(`MCP session ended: ${sessionId}`);
});

// Transport errors
sessionManager.on('transportError', (sessionId: string, error: Error) => {
  console.error(`Error in session ${sessionId}:`, error);
});

HTTP Protocol

The plugin exposes three HTTP endpoints for MCP communication:

POST /mcp

• Purpose: Create new sessions or send requests to existing sessions
• Headers:
  • content-type: application/json
  • mcp-session-id: <session-id> (optional, for existing sessions)
• Body: MCP request payload

GET /mcp

• Purpose: Retrieve streaming responses
• Headers:
  • mcp-session-id: <session-id> (required)
• Response: Server-sent events stream

DELETE /mcp

• Purpose: Terminate sessions
• Headers:
  • mcp-session-id: <session-id> (required)

Session Management

Sessions are managed through a dedicated SessionManager class that:

• Creates new transport instances with unique session IDs
• Tracks active sessions in memory
• Handles session lifecycle events
• Provides graceful cleanup on shutdown
• Emits events for monitoring and logging

Advanced Usage

Custom Error Handling

sessionManager.on('transportError', (sessionId, error) => {
  console.error(`Transport error: ${error.message}`);
});

Health Monitoring

// Periodic health check
setInterval(() => {
  const stats = mcpServer.getStats();
  console.log(`Health Check - Active Sessions: ${stats.activeSessions}`);

  // Alert if too many sessions
  if (stats.activeSessions > 100) {
    console.warn('High session count detected');
  }
}, 30000);

Graceful Shutdown

import closeWithGrace from 'close-with-grace';

closeWithGrace({ delay: 500 }, async ({ signal, err }) => {
  if (err) {
    app.log.error({ err }, 'server closing with error');
  } else {
    app.log.info(`${signal} received, server closing`);
  }

  // Fastify close will handle MCP session cleanup automatically
  await app.close();
});

Session Storage

The plugin provides a flexible session storage system that allows you to choose or implement your own storage backend. By default, sessions are stored in memory, but you can use Redis or create your own custom implementation.

Built-in Session Stores

In-Memory Session Store (Default)

import { InMemorySessionStore } from 'fastify-mcp-server';

await app.register(FastifyMcpServer, {
  createMcpServer
  // sessionStore option is optional - InMemorySessionStore is used by default
});

Redis Session Store

For production deployments or distributed systems, use the Redis session store:

import { RedisSessionStore } from 'fastify-mcp-server';

const redisStore = new RedisSessionStore(redisClient); // Pass your Redis client instance

await app.register(FastifyMcpServer, {
  createMcpServer,
  sessionStore: redisStore
});

Custom Session Store

You can implement your own session store by implementing the SessionStore interface:

import type { SessionStore, SessionData } from 'fastify-mcp-server';

class MyCustomSessionStore implements SessionStore {
  async load (sessionId: string): Promise<SessionData | undefined> {
    // Load session from your storage backend
  }

  async save (sessionData: SessionData): Promise<void> {
    // Save session to your storage backend
  }

  async delete (sessionId: string): Promise<void> {
    // Delete session from your storage backend
  }

  async getAllSessionIds (): Promise<string[]> {
    // Return all session IDs
  }

  async deleteAll (): Promise<void> {
    // Delete all sessions
  }
}

// Use your custom store
await app.register(FastifyMcpServer, {
  createMcpServer,
  sessionStore: new MyCustomSessionStore()
});

How It Works

The session store is responsible for persisting session metadata (session ID and creation time). The plugin manages transports and MCP server connections in memory for performance, while session metadata can be stored in your chosen backend.

• Session Creation: When a client initializes, session metadata is saved to the store
• Session Retrieval: Session data is loaded from the store to validate existing sessions
• Session Cleanup: Sessions are removed from the store when destroyed
• Transport Management: Active transports are maintained in memory for fast access

Comparison

Feature	In-Memory	Redis	Custom
Persistence	Lost on restart	Persists across restarts	Depends on implementation
Scalability	Single instance	Multiple instances	Depends on implementation
Performance	Fastest	Slightly slower (network)	Depends on implementation
Use Case	Development, single instance	Production, distributed systems	Specialized requirements
Setup	No configuration needed	Requires Redis server	Custom implementation needed

Docker Compose Example

A docker-compose.yaml is provided for local development with Redis:

docker compose up -d
npm run dev:redis

Authentication: Bearer Token Support

You can secure your MCP endpoints using Bearer token authentication. The plugin provides a bearerMiddlewareOptions option, which enables validation of Bearer tokens in the Authorization header for all MCP requests.

Enabling Bearer Token Authentication

Pass the authorization.bearerMiddlewareOptions option when registering the plugin. It accepts BearerAuthMiddlewareOptions from the SDK:

import type { BearerAuthMiddlewareOptions } from '@modelcontextprotocol/sdk/server/auth/middleware/bearerAuth.js';

await app.register(FastifyMcpServer, {
  createMcpServer,
  authorization: {
    bearerMiddlewareOptions: {
      verifier: myVerifier, // implements verifyAccessToken(token)
      requiredScopes: ['mcp:read', 'mcp:write'], // optional
      resourceMetadataUrl: 'https://example.com/.well-known/oauth-resource' // optional
    }
  }
});

• verifier: An object with a verifyAccessToken(token) method that returns the decoded token info or throws on failure. It must implements the OAuthTokenVerifier interface from the SDK.
• requiredScopes: (Optional) Array of scopes required for access.
• resourceMetadataUrl: (Optional) URL included in the WWW-Authenticate header for 401 responses.

How It Works

The plugin uses a Fastify preHandler hook applied in the context of the MCP registered routes (see addBearerPreHandlerHook) to:

• Extract the Bearer token from the Authorization header (Authorization: Bearer TOKEN).
• Validate the token using your verifier.
• Check for required scopes and token expiration.
• Attach the decoded auth info to the request object (req.raw.auth).
• Respond with proper OAuth2 error codes and WWW-Authenticate headers on failure.

Example Tool with authentication information

You can access the validated authentication information in your MCP tools via the authInfo parameter:

mcp.tool('example-auth-tool', 'Demo to display the validated access token in authInfo object', ({ authInfo }) => {
  return {
    content: [
      {
        type: 'text',
        // Just a bad example, do not expose sensitive information in your LLM responses! :-)
        text: `Authenticated with token: ${authInfo.token}, scopes: ${authInfo.scopes.join(', ')}, expires at: ${new Date(authInfo.expiresAt).toISOString()}`
      }
    ]
  };
});

Example Error Response

If authentication fails, the response will include a WWW-Authenticate header:

HTTP/1.1 401 Unauthorized
WWW-Authenticate: Bearer error="invalid_token", error_description="Token has expired"
Content-Type: application/json

{"error":"invalid_token","error_description":"Token has expired"}

Example using PAT in Visual Studio Code

{
  "inputs": [
    {
      "type": "promptString",
      "id": "bearer_token",
      "description": "Enter your MCP Bearer Token",
      "password": true
    }
  ],
  "servers": {
    "my-mcp-server": {
      "url": "http://localhost:9080/mcp",
      "headers": {
        "Authorization": "Bearer ${input:bearer_token}"
      }
    }
  }
}

Well-Known OAuth Metadata Routes

The plugin can automatically register standard OAuth 2.0 metadata endpoints under the .well-known path, which are useful for interoperability with OAuth clients and resource servers. You can test metadata discovery with the MCP inspector in the Authentication tab.

Registering Well-Known Routes

To enable these endpoints, provide the authorization.oauth2.authorizationServerOAuthMetadata and/or authorization.oauth2.protectedResourceOAuthMetadata options when registering the plugin:

import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';
import FastifyMcpServer from 'fastify-mcp-server';

function createMcpServer () {
  return new McpServer({
    name: 'my-mcp-server',
    version: '1.0.0'
  });
}

const authorizationServerMetadata = {
  issuer: 'https://your-domain.com',
  authorization_endpoint: 'https://your-domain.com/oauth/authorize',
  token_endpoint: 'https://your-domain.com/oauth/token'
  // ...other OAuth metadata fields
};

const protectedResourceMetadata = {
  resource: 'https://your-domain.com/.well-known/oauth-protected-resource'
  // ...other resource metadata fields
};

await app.register(FastifyMcpServer, {
  createMcpServer,
  authorization: {
    oauth2: {
      authorizationServerOAuthMetadata: authorizationServerMetadata, // Registers /.well-known/oauth-authorization-server
      protectedResourceOAuthMetadata: protectedResourceMetadata // Registers /.well-known/oauth-protected-resource
    }
  }
});

Endpoints

• GET /.well-known/oauth-authorization-server — Returns the OAuth authorization server metadata.
• GET /.well-known/oauth-protected-resource — Returns the OAuth protected resource metadata.

These endpoints are registered only if the corresponding metadata options are provided.

Custom Transport Options

You can customize the behavior of the underlying StreamableHTTPServerTransport by providing transportOptions when registering the plugin. This allows you to configure advanced transport features such as custom session ID generation and transport lifecycle callbacks.

Available Options

import type { StreamableHTTPServerTransportOptions } from '@modelcontextprotocol/sdk/server/streamableHttp.js';

await app.register(FastifyMcpServer, {
  createMcpServer,
  transportOptions: {
    // Custom session ID generator function
    sessionIdGenerator: () => {
      // Return your custom session ID
      return `custom-${Date.now()}-${Math.random()}`;
    },
    // Callback invoked when session is initialized
    onsessioninitialized: async (sessionId) => {
      console.log(`Session ${sessionId} initialized`);
    }
    // Additional StreamableHTTPServerTransport options...
  }
});

Use Cases

• Custom Session IDs: Generate session IDs that match your existing ID format or include specific metadata
• Session Initialization Hooks: Perform custom logic when sessions are created (e.g., logging, metrics, notifications)
• Transport Configuration: Fine-tune transport behavior for specific deployment requirements

Example: Custom Session ID with Prefix

await app.register(FastifyMcpServer, {
  createMcpServer,
  transportOptions: {
    sessionIdGenerator: () => {
      return `mcp-prod-${randomUUID()}`;
    }
  }
});

Development

Setup

# Clone the repository
git clone https://github.com/flaviodelgrosso/fastify-mcp-server.git
cd fastify-mcp-server

# Install dependencies
npm install

# Run development server with hot reload
npm run dev

Scripts

• npm run dev - Run development server with in-memory session storage
• npm run dev:redis - Run development server with Redis session storage
• npm run build - Build TypeScript to JavaScript
• npm test - Run test suite with 100% coverage
• npm run test:lcov - Generate LCOV coverage report
• npm run inspector - Launch MCP inspector for testing

Testing

The project maintains 100% test coverage. Run tests with:

npm test

Contributing

Contributions are welcome! Please read our contributing guidelines and ensure:

1. Tests pass with 100% coverage
2. Code follows the established style (enforced by Biome)
3. Commits follow conventional commit format
4. Changes are properly documented

License

ISC © Flavio Del Grosso

Related Projects

• Model Context Protocol - Official MCP specification and servers
• Fastify - Fast and low overhead web framework
• MCP SDK - TypeScript SDK for MCP