Why Build Your Own MCP Server

While existing MCP server libraries are already rich, there are still many scenarios that require customization:

Connecting to internal company APIs (external tools can't access)

Handling specific business logic

Integrating with private databases

Wrapping complex workflows into simple tools

Development Environment Setup

bash
Install the MCP Python SDK
pip install mcp
Or use uv
uv add mcp

The Simplest MCP Server

python
simple_server.py
from mcp.server import Server
from mcp.server.stdio import stdio_server
from mcp.types import Tool, TextContent
import mcp.types as types
Create a server instance
app = Server("my-tools")
Register the list of tools
@app.list_tools()
async def list_tools() -> list[Tool]:
    return [
        Tool(
            name="get_weather",
            description="Get weather information for a specified city",
            inputSchema={
                "type": "object",
                "properties": {
                    "city": {"type": "string", "description": "City name"}
                },
                "required": ["city"]
            }
        ),
        Tool(
            name="calculate_discount",
            description="Calculate the price after discount",
            inputSchema={
                "type": "object",
                "properties": {
                    "original_price": {"type": "number"},
                    "discount_percent": {"type": "number"}
                },
                "required": ["original_price", "discount_percent"]
            }
        )
    ]
Implement tool logic
@app.call_tool()
async def call_tool(name: str, arguments: dict):
    if name == "get_weather":
        city = arguments["city"]
        # In a real project, call the actual weather API
        weather_data = {"city": city, "temp": "25°C", "condition": "Sunny"}
        return [TextContent(type="text", text=str(weather_data))]
    
    elif name == "calculate_discount":
        original = arguments["original_price"]
        discount = arguments["discount_percent"]
        final_price = original * (1 - discount / 100)
        return [TextContent(
            type="text",
            text=f"Original price {original} yuan, after {100-discount}% off: {final_price:.2f} yuan"
        )]
    
    raise ValueError(f"Unknown tool: {name}")
Start the server (stdio mode)
if __name__ == "__main__":
    import asyncio
    asyncio.run(stdio_server(app))

Register with Claude Desktop

json
{
  "mcpServers": {
    "my-tools": {
      "command": "python",
      "args": ["/path/to/simple_server.py"]
    }
  }
}

Hands-on: Building a Database Query Tool

python
import sqlite3
from mcp.server import Server
from mcp.server.stdio import stdio_server
from mcp.types import Tool, TextContent
app = Server("db-tools")
DB_PATH = "./mydata.db"
@app.list_tools()
async def list_tools():
    return [
        Tool(
            name="query_database",
            description="Execute a SQL query (read-only) and return results",
            inputSchema={
                "type": "object",
                "properties": {
                    "sql": {"type": "string", "description": "SELECT SQL statement"}
                },
                "required": ["sql"]
            }
        )
    ]
@app.call_tool()
async def call_tool(name: str, arguments: dict):
    if name == "query_database":
        sql = arguments["sql"]
        
        # Security check: only allow SELECT
        if not sql.strip().upper().startswith('SELECT'):
            return [TextContent(type="text", text="Error: Only SELECT queries are allowed")]
        
        conn = sqlite3.connect(DB_PATH)
        cursor = conn.cursor()
        cursor.execute(sql)
        columns = [desc[0] for desc in cursor.description]
        rows = cursor.fetchall()
        conn.close()
        
        result = f"Columns: {columns}\nResults ({len(rows)} rows):\n"
        for row in rows[:20]:  # Return at most 20 rows
            result += str(row) + "\n"
        
        return [TextContent(type="text", text=result)]
if __name__ == "__main__":
    import asyncio
    asyncio.run(stdio_server(app))

Debugging Tips

bash
Test if the server starts correctly
echo '{"jsonrpc":"2.0","id":1,"method":"tools/list"}' | python simple_server.py
Use MCP Inspector for visual testing
npx @modelcontextprotocol/inspector python simple_server.py

Publishing to the MCP Community

Package as a Python package (pyproject.toml)

Publish to PyPI: pip publish

Submit your MCP server on mcp.so or GitHub

Users can then install directly with uvx your-package-name