MCP Debug

A debugging and development tool for Model Context Protocol (MCP) servers.

MCP Debug enables rapid development and testing of MCP servers with hot-swapping, session recording, and automated playback testing.

Features

Hot-Swap Development

• Replace server binaries without disconnecting MCP clients
• Add/remove servers dynamically during development
• Tool name preservation - same interface, new implementation
• Graceful disconnect/reconnect workflow for binary replacement

Session Recording & Playback

• Record JSON-RPC traffic for debugging and documentation
• Playback client mode - replay requests to test servers
• Playback server mode - replay responses to test clients
• Regression testing with recorded sessions

Development Proxy

• Multi-server aggregation with tool prefixing
• Real-time connection monitoring
• Management API for server lifecycle control
• Comprehensive logging

Installation

# Using uvx (Python - recommended)
uvx mcp-debug --help

# Using npx (Node.js)
npx @standardbeagle/mcp-debug --help

# Or install globally
pip install mcp-debug              # Python
npm install -g @standardbeagle/mcp-debug  # Node.js

# Or build from source
go install github.com/standardbeagle/mcp-debug@latest

Quick Start

# Start proxy with a config file
uvx mcp-debug --proxy --config config.yaml

# Or with mcp-tui for interactive testing
mcp-tui uvx mcp-debug --proxy --config config.yaml

Usage

Proxy Mode

# Basic proxy
uvx mcp-debug --proxy --config config.yaml

# With recording
uvx mcp-debug --proxy --config config.yaml --record session.jsonl

# With custom log file
uvx mcp-debug --proxy --config config.yaml --log /tmp/debug.log

Management Tools:

• server_add - Add a server: {name: "fs", command: "npx -y @mcp/filesystem /path"}
• server_remove - Remove server completely
• server_disconnect - Disconnect server (tools return errors)
• server_reconnect - Reconnect with new command
• server_list - Show all servers and status

Playback Modes

# Replay recorded requests to test a server
uvx mcp-debug --playback-client session.jsonl | ./your-mcp-server

# Replay recorded responses to test a client
mcp-tui uvx mcp-debug --playback-server session.jsonl

Configuration

# config.yaml
servers:
  - name: "filesystem"
    prefix: "fs"
    transport: "stdio"
    command: "npx"
    args: ["-y", "@modelcontextprotocol/filesystem", "/home/user"]
    timeout: "30s"

proxy:
  healthCheckInterval: "30s"
  connectionTimeout: "10s"
  maxRetries: 3

Environment Variables

MCP_LOG_FILE="/tmp/mcp-debug.log"  # Log location
MCP_DEBUG=1                         # Enable debug logging
MCP_RECORD_FILE="session.jsonl"     # Auto-record sessions
MCP_CONFIG_PATH="./config.yaml"     # Default config

Development Workflow

# 1. Start with empty config
mcp-tui uvx mcp-debug --proxy --config empty-config.yaml

# 2. Add your server dynamically
server_add: {name: myserver, command: ./my-server-v1}

# 3. Test tools: myserver_read_file, myserver_process, etc.

# 4. Make changes and rebuild
go build -o my-server-v2

# 5. Hot-swap the server
server_disconnect: {name: myserver}
server_reconnect: {name: myserver, command: ./my-server-v2}

# 6. Same tools work immediately with new implementation!

CLI Commands

uvx mcp-debug --help              # Show help
uvx mcp-debug --version           # Show version
uvx mcp-debug config init         # Create default config
uvx mcp-debug config show         # Show current config
uvx mcp-debug config validate     # Validate config file
uvx mcp-debug env list            # List environment variables
uvx mcp-debug env check           # Check required env vars
uvx mcp-debug tools list          # List tools with details

Project Structure

mcp-debug/
├── main.go              # CLI entry point
├── config/              # Configuration loading
├── client/              # MCP client implementation
├── integration/         # Proxy server and wrapper
├── discovery/           # Tool discovery
├── proxy/               # Request forwarding
├── playback/            # Recording and playback
└── test-servers/        # Example MCP servers

Building

# Development build
go build -o mcp-debug .

# Production build with version info
go build -ldflags "-X main.BuildTime=$(date -u +%Y-%m-%dT%H:%M:%SZ) -X main.GitCommit=$(git rev-parse HEAD)" -o mcp-debug .

# Run tests
go test ./...

Contributing

See CONTRIBUTING.md for guidelines.

License

MIT License - see LICENSE for details.