Go MCP HTTP Bridge - Master Documentation

Overview

A Go application that wraps MCP (Model Context Protocol) servers with HTTP streaming (SSE), fully compatible with llama.cpp's StreamableHTTP transport. This bridge allows web-based MCP clients to communicate with subprocess-based MCP servers.

Architecture

┌─────────────────┐
│   Client        │
│   (LLM/App)     │
└────────┬────────┘
         │
         │ HTTP POST + SSE Stream
         ▼
┌──────────────────────────────┐
│  Go MCP HTTP Bridge          │
│  ┌────────────────────────┐  │
│  │ HTTP Server            │  │
│  │ - POST /mcp/{ns}/msg   │  │
│  │ - GET  /mcp/{ns} (SSE) │  │
│  └────────────────────────┘  │
│  ┌────────────────────────┐  │
│  │ Protocol Handler       │  │
│  │ - initialize           │  │
│  │ - tools/list           │  │
│  │ - tools/call           │  │
│  └────────────────────────┘  │
│  ┌────────────────────────┐  │
│  │ Subprocess Manager     │  │
│  │ - test-server          │  │
│  └────────────────────────┘  │
└──────────────────────────────┘

Features

✅ Implemented (All Phases 1-6 Complete)

1. HTTP Streaming Server

   • SSE streaming (GET /mcp/{namespace})
   • HTTP POST endpoint (POST /mcp/{namespace}/message)
   • Proper CORS support
   • Connection lifecycle management

2. Subprocess Management

   • On-demand spawning
   • Connection reuse
   • Exponential backoff restarts (1s, 2s, 4s... up to 60s)
   • Graceful shutdown (SIGTERM/SIGINT handling)
   • Process state tracking

3. JSON-RPC 2.0

   • Request/response parsing
   • Message validation
   • Error handling
   • Notification support

4. MCP Protocol Support

   • initialize - Handshake with server info
   • tools/list - List available tools
   • tools/call - Execute tools
   • ping - Health check

5. Security

   • Input validation (JSON-RPC, arguments)
   • Command injection prevention
   • CORS and origin validation
   • Message size limits (1MB request, 10MB response)
   • Connection limits (configurable, default: 5)

6. Logging

   • Structured JSON logging with slog
   • Context-aware logs (namespace, session, request ID)
   • Connection event tracking
   • Subprocess event logging

7. Debug Tools

   • /debug - Dashboard
   • /debug/stream - Real-time message log

8. Health Checks

   • /health - Bridge status
   • /health/{namespace} - Per-server status

9. Prometheus Metrics

   • /metrics endpoint
   • HTTP request metrics (mcp_http_requests_total, mcp_http_request_duration_seconds)
   • Tool call metrics (mcp_tool_calls_total, mcp_tool_calls_duration_seconds, mcp_tool_errors_total)
   • Active sessions gauge (mcp_active_sessions)
   • Subprocess state metrics (mcp_subprocess_state)

10. Enhanced Debug Streaming

    • /debug/stream - Real-time DEBUG and INFO logs
    • JSON-RPC 2.0 compliant SSE format
    • No verbose flag needed

🔧 Configuration

Full configuration example:

bridge:
  port: 8080
  allowed_origins:
    - http://localhost:3000
    - http://127.0.0.1:3000
  request_timeout: 30s
  idle_timeout: 5m
  connection_limit: 5
  request_size_limit: 1048576
  response_size_limit: 10485760

servers:
  filesystem:
    name: "filesystem"
    binary: "/usr/bin/node"
    args:
      - "/path/to/mcp-filesystem-server/index.js"
    env:
      HOME: "/home/user"
    timeout: 30s
    max_restarts: 3
    auto_start: true

  git:
    name: "git"
    binary: "npx"
    args:
      - "-y"
      - "@modelcontextprotocol/server-git"
    timeout: 60s
    max_restarts: 5
    auto_start: false

For complete configuration options, see docs/CONFIG.md.

🚀 Quick Start

1. Build

   cd go-mcp-bridge
   make build

2. Configure Create config.yaml with your MCP server settings:

   bridge:
     port: 8080
     allowed_origins:
       - http://localhost:3000
   
   servers:
     git:
       name: "git"
       binary: "npx"
       args:
         - "-y"
         - "@modelcontextprotocol/server-git"

3. Run

   ./bin/bridge --config config.yaml

4. Test

   # Health check
   curl http://localhost:8080/health
   
   # Send initialize request
   curl -X POST http://localhost:8080/mcp/git \
     -H "Content-Type: application/json" \
     -d '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{...}}'
   
   # Check metrics
   curl http://localhost:8080/metrics
   
   # View debug stream
   curl http://localhost:8080/debug/stream

📡 API Endpoints

Primary MCP Endpoint

GET  /mcp/{namespace}          → Start SSE stream
POST /mcp/{namespace}/message  → Send JSON-RPC request

Example Request:

curl -X POST http://localhost:8080/mcp/test/message \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{...}}'

Health Check

GET /health              → Bridge health
GET /health/{namespace}  → Namespace health

Debug Endpoint

GET /debug               → Debug dashboard (HTML)
GET /debug/stream        → Debug message stream (SSE)

Project Structure

go-mcp-bridge/
├── cmd/
│   ├── bridge/main.go       # Main entry point
│   ├── test-server/         # Go test MCP server
│   └── test-mcp-server/     # TypeScript test MCP server
├── internal/
│   ├── config/
│   │   └── loader.go        # YAML config parser
│   ├── mcp/
│   │   ├── handler.go       # Protocol handler
│   │   ├── jsonrpc.go       # JSON-RPC 2.0
│   │   └── types.go         # MCP types
│   ├── process/
│   │   └── manager.go       # Subprocess manager
│   ├── router/
│   │   └── namespace.go     # Namespace routing
│   └── server/
│       ├── http.go          # HTTP server
│       ├── sse.go           # SSE writer
│       └── metrics.go       # Prometheus metrics
├── docs/                    # Documentation
│   ├── CONFIG.md            # Configuration reference
│   ├── API.md               # API specification
│   ├── EXAMPLES.md          # Usage examples
│   ├── DEBUG.md             # Debug endpoint guide
│   └── TESTING.md           # Testing guide
├── testdata/                # Test configurations
├── bin/                     # Built binaries
├── config.yaml              # Configuration
├── Makefile                 # Build automation
├── test-tool-metrics.sh     # Tool metrics test script
└── README.md                # This file

Testing

Unit Tests

# Run all unit tests
go test ./... -v

# Specific package
go test ./internal/mcp/... -v

End-to-End Test

# Start bridge in background
./bin/bridge --config config.yaml &
BRIDGE_PID=$!

# Run tests
./test.sh

# Stop bridge
kill $BRIDGE_PID

Integration Tests (Complete)

# Test with real subprocess
./test-tool-metrics.sh

# Or run all tests
go test ./... -v

Git Branches

• master - Current stable (All phases 1-6 complete)

Roadmap

✅ Phase 1-2: Core Architecture (COMPLETE)

• ✅ HTTP streaming server
• ✅ Subprocess management
• ✅ Basic JSON-RPC handling

✅ Phase 3: MCP Protocol Support (COMPLETE)

• ✅ JSON-RPC 2.0 handling
• ✅ Initialize method
• ✅ Tools/list and tools/call
• ✅ Request-response correlation

✅ Phase 4: Security & Error Handling (COMPLETE)

• ✅ Input validation (JSON-RPC, arguments)
• ✅ Command injection prevention
• ✅ Resource limits (size, connections)
• ✅ Exponential backoff restarts
• ✅ Graceful shutdown
• ✅ Structured JSON logging

✅ Phase 5: Testing & Documentation (COMPLETE)

• ✅ Unit tests (45 tests)
  • JSON-RPC parsing (20 tests)
  • Config loading (14 tests)
  • Namespace routing (11 tests)
• ✅ Integration tests (45+ tests)
  • HTTP server tests (20+ tests)
  • Process manager tests (15+ tests)
  • Namespace isolation tests (10 tests)
• ✅ Complete documentation (docs/CONFIG.md, docs/API.md, docs/EXAMPLES.md, docs/DEBUG.md, docs/TESTING.md)

✅ Phase 6: Build & Monitoring (COMPLETE)

• ✅ 6.1 Prometheus metrics (/metrics endpoint)
  • ✅ HTTP request metrics (mcp_http_requests_total, mcp_http_request_duration_seconds)
  • ✅ Active sessions gauge (mcp_active_sessions)
  • ✅ Subprocess state metrics (mcp_subprocess_state)
  • ✅ Tool call metrics (mcp_tool_calls_total, mcp_tool_calls_duration_seconds, mcp_tool_errors_total)
  • ✅ Metrics endpoint at /metrics
• ✅ 6.2 Structured logging with slog
  • ✅ Replace fmt.Printf with slog
  • ✅ JSON structured logging
  • ✅ Context-aware logs (namespace, session, request_id)
• ✅ 6.3 Enhanced debug streaming
  • ✅ Stream DEBUG and INFO logs to /debug/stream
  • ✅ JSON-RPC 2.0 compliant SSE format
  • ✅ No verbose flag needed
• ✅ 6.4 Configuration cleanup
  • ✅ Remove unused debug_port configuration
  • ✅ Keep debug endpoints on main port 8080
• ✅ 6.5 Test script for tool metrics (test-tool-metrics.sh)
• ✅ 6.6 End-to-end verification of tool metrics recording

All phases complete as of Phase 6 merge to master.

Compatibility

✅ llama.cpp web frontend

• Compatible with StreamableHTTP transport
• Works with latest llama.cpp build

✅ MCP SDK

• Uses standard JSON-RPC 2.0 format
• Supports MCP protocol versions:
  • 2025-06-18 (latest)
  • 2025-03-26 (default)
  • 2024-11-05 (backward compat)

Known MCP Servers

These servers can be used with the bridge:

1. Git MCP (NPM)

   servers:
     git:
       name: "git"
       binary: "npx"
       args: ["-y", "@modelcontextprotocol/server-git"]

2. Filesystem MCP (uvx)

   servers:
     filesystem:
       name: "filesystem"
       binary: "uvx"
       args: ["mcp-server-filesystem", "--allowed-directory", "/data"]

3. Memory MCP (Docker)

   servers:
     memory:
       name: "memory"
       binary: "docker"
       args: ["run", "-i", "mcp/memory-server"]

4. Custom Binary

   servers:
     custom:
       name: "custom"
       binary: "./my-mcp-server"
       args: ["--port", "8080"]

See EXAMPLES.md for more configuration examples.

Troubleshooting

Subprocess won't start

• Check binary path is correct: which npx
• Verify binary is executable: chmod +x ./bin/test-server
• Check environment variables in config
• Look at structured logs for errors

Connection fails

• Verify namespace exists in config
• Check allowed_origins includes client origin
• Look at /debug/stream for real-time errors
• Check /health/{namespace} for subprocess status

Messages not streaming

• Ensure subprocess outputs JSON-RPC to stdout
• Check for newline termination on messages (\n)
• Verify SSE headers are set correctly
• Use /debug/stream to see raw messages

High restart count

• Check subprocess logs for errors
• Increase timeout if subprocess is slow
• Verify args are correct
• Check resource limits

Security errors

• Verify argument sanitization
• Check allowed hosts/origins
• Ensure binary path is safe (no shell injection)

Documentation

Document	Description
docs/CONFIG.md	Complete configuration reference
docs/API.md	HTTP API specification
docs/EXAMPLES.md	Usage examples and configurations
docs/DEBUG.md	Debug endpoint guide
docs/TESTING.md	Testing guide and test infrastructure

Contributing

Contributions welcome! Please:

1. Create a feature branch
2. Add tests for new functionality
3. Update documentation
4. Submit a pull request

License

MIT