betegon/express-mcp-server

Express MCP Auto-Instrumentation Examples

This repository contains three example implementations of MCP (Model Context Protocol) servers using Express.js, demonstrating different approaches to Sentry instrumentation and transport mechanisms.

Overview

The project showcases how to integrate Sentry monitoring and performance tracking with MCP servers, providing examples with full instrumentation, partial instrumentation, and SSE transport.

Installation

# Clone the repository
git clone <repository-url>
cd express-mcp-auto-instrumentation

# Install dependencies
yarn install

The Three Servers

1. Streamable HTTP Server with Full Instrumentation

File: mcp-express-streamable-http.mjs
Port: 3005

This is the most feature-rich implementation with complete Sentry instrumentation.

Features:

• Full Sentry error tracking and performance monitoring via wrapMcpServerWithSentry
• Supports both POST (JSON-RPC) and GET (SSE) endpoints
• Stateless design (fresh server instance per request)
• Advanced tools:
  • add - Basic math with progress notifications
  • add-dynamic-tool - Create new tools on the fly
  • create-resource - Create new resources dynamically
  • update-resource - Update existing resources
  • log-message - Send custom log messages
  • add-dynamic-prompt - Create prompts dynamically
• Resources and prompts support
• Comprehensive logging system

Run:

node mcp-express-streamable-http.mjs

2. SSE Server with Instrumentation

File: mcp-express-sse.mjs
Port: 3006

A simpler implementation using Server-Sent Events (SSE) transport with Sentry instrumentation.

Features:

• Full Sentry instrumentation via wrapMcpServerWithSentry
• SSE-based transport exclusively
• Session management (maintains state between requests)
• Basic add tool for mathematical operations
• Separate endpoints for SSE stream (/mcp) and messages (/messages)

Run:

node mcp-express-sse.mjs

3. Streamable HTTP Server without Wrapper Instrumentation

File: mcp-express-no-instrumentation.js
Port: 3005

A minimal implementation that imports Sentry but doesn't use the MCP-specific wrapper.

Features:

• Imports Sentry instrumentation but doesn't wrap the MCP server
• Uses older registerTool API
• Stateless design
• Basic add tool that includes a weather API call (for testing child spans)
• Demonstrates baseline MCP functionality

Run:

node mcp-express-no-instrumentation.js

API Usage

Basic Tool Call Example

POST /mcp

{
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "name": "add",
    "arguments": {
      "a": 5,
      "b": 3
    }
  },
  "id": 1
}

SSE Connection (Server 2 only)

GET /mcp - Establishes SSE stream
POST /messages?sessionId={sessionId} - Send messages to established session

Test Sentry Integration

All servers include a test endpoint:

GET /sentry-error - Triggers a test error for Sentry verification

Key Differences

Development Notes

• The instrument.mjs file must be imported using Node's --import flag for proper Sentry initialization
• All servers demonstrate different aspects of MCP server implementation
• The full instrumentation server (streamable-http) serves as the most comprehensive example
• SSE server shows how to maintain session state across requests
• The no-wrapper server demonstrates baseline functionality without MCP-specific Sentry features

License

[Add your license here]