MCP Simple Server

A minimal, reference implementation of a Model Context Protocol server with streamable HTTP transport. Built with FastMCP following the official Anthropic MCP specification 2025-06-18. Perfect starting point for building remote MCP servers.

🎯 Purpose

This project serves as a simple, well-documented reference for developers who want to:

• Build their first MCP server
• Deploy MCP servers to cloud platforms (Railway, Heroku, Render)
• Understand the MCP protocol implementation
• Create a foundation for more sophisticated MCP solutions

Features

• ✅ Two Math Tools: `add` and `multiply` functions
• ✅ Streamable HTTP Transport: Modern MCP protocol with SSE support
• ✅ Session Management: Proper MCP initialization flow
• ✅ Remote Deployment: Railway, Heroku, Render deployment configs
• ✅ Automated Testing: Complete protocol validation and debugging tools
• ✅ Claude Desktop Integration: Ready for AI assistant integration
• ✅ Reference Implementation: Well-documented code for learning

Quick Start

Local Development

git clone https://github.com/oleksandrsirenko/mcp-simple-server.git
cd mcp-simple-server
uv sync
source .venv/bin/activate
python main.py

Server starts at: `http://localhost:8000/mcp/`

Test the Server

python test_server.py

Expected output:

🧪 Starting MCP Server Tests
✅ Initialize successful - Server: Simple Server
✅ Initialized notification sent
✅ Found 2 tools: add, multiply  
✅ Add tool returned correct result
✅ Multiply tool returned correct result
🎉 All tests passed!

Available Tools

`add(a, b)`

Adds two numbers together.

Example:

{"name": "add", "arguments": {"a": 25, "b": 17}}
→ Returns: 42

`multiply(a, b)`

Multiplies two numbers together.

Example:

{"name": "multiply", "arguments": {"a": 8, "b": 6}}
→ Returns: 48

Manual Testing with curl

Local Testing (Development)

For testing your local development server running on `localhost:8000`:

1. Initialize Session

curl -X POST http://localhost:8000/mcp/ \
  -H "Content-Type: application/json" \
  -H "MCP-Protocol-Version: 2025-06-18" \
  -H "Accept: application/json, text/event-stream" \
  -d '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{"tools":{}},"clientInfo":{"name":"test-client","version":"1.0.0"}}}'

2. Send Initialized Notification

curl -X POST http://localhost:8000/mcp/ \
  -H "Content-Type: application/json" \
  -H "MCP-Protocol-Version: 2025-06-18" \
  -H "Accept: application/json, text/event-stream" \
  -H "Mcp-Session-Id: YOUR_SESSION_ID" \
  -d '{"jsonrpc":"2.0","method":"notifications/initialized"}'

3. List Tools

curl -X POST http://localhost:8000/mcp/ \
  -H "Content-Type: application/json" \
  -H "MCP-Protocol-Version: 2025-06-18" \
  -H "Accept: application/json, text/event-stream" \
  -H "Mcp-Session-Id: YOUR_SESSION_ID" \
  -d '{"jsonrpc":"2.0","id":2,"method":"tools/list"}'

4. Call Add Tool

curl -X POST http://localhost:8000/mcp/ \
  -H "Content-Type: application/json" \
  -H "MCP-Protocol-Version: 2025-06-18" \
  -H "Accept: application/json, text/event-stream" \
  -H "Mcp-Session-Id: YOUR_SESSION_ID" \
  -d '{"jsonrpc":"2.0","id":3,"method":"tools/call","params":{"name":"add","arguments":{"a":25,"b":17}}}'

Remote Testing (Production)

For testing your deployed server, replace `localhost:8000` with your deployment URL:

# Example with Railway deployment
curl -X POST https://your-app.railway.app/mcp/ \
  -H "Content-Type: application/json" \
  -H "MCP-Protocol-Version: 2025-06-18" \
  -H "Accept: application/json, text/event-stream" \
  -d '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{"tools":{}},"clientInfo":{"name":"test-client","version":"1.0.0"}}}'

Note: For comprehensive remote testing, use the automated test script:

python test_deployment.py your-app.railway.app

Deployment

Railway (Recommended)

1. Push to GitHub:

git add .
   git commit -m "ready for deployment"
   git push origin main

2. Deploy to Railway:

3. Test your deployment:

python test_deployment.py your-app-name.up.railway.app

4. Your MCP URL: `https://your-app.railway.app/mcp/`

Heroku

heroku create your-mcp-server
git push heroku main

Your MCP URL: `https://your-mcp-server.herokuapp.com/mcp/`

Render

1. Connect GitHub repository to Render

2. Render auto-detects `render.yaml` and Dockerfile

3. Deploys automatically

Your MCP URL: `https://your-service.onrender.com/mcp/`

Docker

docker build -t mcp-simple-server .
docker run -p 8000:8000 mcp-simple-server

Claude Desktop Integration

Local Server Configuration

{
  "mcpServers": {
    "simple-server": {
      "command": "python",
      "args": ["main.py"],
      "cwd": "/path/to/mcp-simple-server"
    }
  }
}

Remote Server Configuration (Recommended)

For remote servers deployed to Railway, Heroku, or Render, use the `mcp-remote` package:

{
  "mcpServers": {
    "simple-server-remote": {
      "command": "npx",
      "args": [
        "-y",
        "mcp-remote",
        "https://your-app.railway.app/mcp/",
        "--allow-http",
        "--header",
        "Accept: application/json, text/event-stream"
      ]
    }
  }
}

Key Configuration Notes:

• Use `npx` with the `-y` flag to auto-install `mcp-remote`
• Include the trailing slash in the URL: `/mcp/`
• Add the `--allow-http` flag for HTTP connections
• Include the Accept header for proper SSE support

Alternative: Direct Python Proxy (Advanced)

For advanced users or debugging purposes, you can create a custom Python proxy:

{
  "mcpServers": {
    "simple-server-proxy": {
      "command": "python",
      "args": ["claude_mcp_proxy.py"],
      "cwd": "/path/to/mcp-simple-server"
    }
  }
}

Note: This requires the `claude_mcp_proxy.py` script from the repository and is mainly for debugging purposes. Use `mcp-remote` for production.

Configuration File Locations:

• macOS: `~/Library/Application Support/Claude/claude_desktop_config.json`
• Windows: `%APPDATA%\Claude\claude_desktop_config.json`

Test with Claude

After integration, ask Claude:

• "Can you add 42 and 18 for me?"
• "What's 7 times 9?"
• "What tools do you have available?"

Claude will use your MCP server to perform calculations! 🎉

Development

Adding New Tools

@mcp.tool()
def subtract(a: float, b: float) -> float:
    """Subtract two numbers"""
    return a - b

@mcp.tool()
def divide(a: float, b: float) -> float:
    """Divide two numbers"""
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

Environment Variables

• `HOST`: Server host (default: 127.0.0.1, use 0.0.0.0 for deployment)
• `PORT`: Server port (default: 8000, Railway sets this automatically)

HOST=0.0.0.0 PORT=3000 python main.py

Note: For Railway deployment, FastMCP will automatically bind to `0.0.0.0:$PORT`.

Project Structure

mcp-simple-server/
├── main.py                    # MCP server (~25 lines)
├── test_server.py             # Local server tests (~300 lines)
├── test_deployment.py         # Remote deployment tests
├── test_host_binding.py       # Host binding tests
├── test_proxy_script.py       # Proxy testing script
├── test_streamable_app.py     # Streamable HTTP tests
├── test_tool_verification.py  # Tool verification tests
├── debug_railway_server.py    # Railway debugging utilities
├── debug_fastmcp.py           # FastMCP debugging utilities
├── claude_mcp_proxy.py        # Claude Desktop proxy (optional)
├── start.sh                   # Shell startup script
├── pyproject.toml             # Project configuration
├── README.md                  # This documentation
├── uv.lock                    # Dependency lock file
├── .gitignore                 # Git ignore patterns
├── .python-version            # Python version specification
├── Dockerfile                 # Docker deployment
├── railway.toml               # Railway configuration
├── Procfile                   # Heroku configuration
└── render.yaml                # Render configuration

Architecture

• FastMCP: High-level MCP implementation from Anthropic
• Streamable HTTP: Modern transport with SSE streaming support
• Session Management: Stateful connections with session IDs
• JSON-RPC 2.0: Standard protocol for message exchange
• Protocol 2025-06-18: Latest MCP specification
• Port 8000: Default FastMCP server port (configurable via PORT env var)

Technical Details

Server Implementation

• Framework: FastMCP (official Anthropic library)
• Transport: Streamable HTTP with Server-Sent Events
• Protocol: MCP 2025-06-18 specification
• Dependencies: `httpx>=0.28.1`, `mcp>=1.9.4`

MCP Protocol Flow

1. Client sends `initialize` request

2. Server responds with capabilities and session ID

3. Client sends `initialized` notification

4. Normal operations begin (tools/list, tools/call, etc.)

Tool Response Format

Tools return simple Python values (float, int, str) which FastMCP automatically wraps in the proper MCP response format.

Troubleshooting

Server Won't Start

# Check if port is in use
lsof -i :8000

# Try different port
PORT=3000 python main.py

MCP Protocol Errors

# Run automated test
python test_server.py

# Check server logs for detailed errors

Claude Desktop Not Connecting

1. Verify JSON configuration syntax - Use a JSON validator

2. Check server URL accessibility - Test with `curl` or browser

3. Restart Claude Desktop after config changes

4. Ensure proper MCP endpoint path - Use `/mcp/` with trailing slash

5. Use `mcp-remote` for remote servers - Don't use `curl` for remote connections

Test Remote Deployment

Test your deployed server with the provided script:

# Test your deployed server (replace with your URL)
python test_deployment.py your-app.railway.app

# Or with full URL
python test_deployment.py https://your-app.railway.app

This will run the complete MCP protocol test suite against your remote server.

Common Issues

• Wrong endpoint: Use `/mcp/` (with trailing slash)
• Missing headers: Include all required MCP headers
• Session management: Must send `initialized` notification after `initialize`
• Remote connections: Use `mcp-remote`, not `curl` for Claude Desktop
• Port binding: Use `0.0.0.0:$PORT` for deployment, not `127.0.0.1`

Dependencies

dependencies = [
    "httpx>=0.28.1",   # HTTP client for testing
    "mcp>=1.9.4",      # Official Anthropic MCP library
]

The project uses:

• mcp: Official Anthropic MCP Python SDK
• httpx: Modern HTTP client for automated testing
• Python: Requires Python >=3.10

Contributing

1. Fork the repository

2. Make your changes

3. Run tests: `python test_server.py`

4. Test deployment: `python test_deployment.py your-test-url`

5. Ensure all tests pass

6. Submit a pull request

License

MIT License

Resources

• MCP Specification 2025-06-18
• FastMCP Documentation
• Claude Desktop
• Railway Deployment
• Render Deployment
• mcp-remote Package