Build a Claude MCP Server in Python

Create a Model Context Protocol (MCP) server in Python that Claude can use as a tool. Expose custom functions, databases, and APIs to Claude Code and Claude Desktop.

An MCP server exposes tools Claude can call during any session — without you modifying API calls. This example builds a minimal MCP server in Python using the official mcp SDK.

Installation

pip install mcp

Minimal MCP server with two tools

from mcp.server.fastmcp import FastMCP

mcp = FastMCP("my-tools")

@mcp.tool()
def get_word_count(text: str) -> int:
    """Count the number of words in a text string."""
    return len(text.split())

@mcp.tool()
def calculate(expression: str) -> float:
    """Safely evaluate a mathematical expression like '2 + 3 * 4'."""
    import ast, operator
    ops = {ast.Add: operator.add, ast.Sub: operator.sub,
           ast.Mult: operator.mul, ast.Div: operator.truediv}

    def eval_node(node):
        if isinstance(node, ast.Constant):
            return node.value
        if isinstance(node, ast.BinOp):
            return ops[type(node.op)](eval_node(node.left), eval_node(node.right))
        raise ValueError(f"Unsupported: {node}")

    tree = ast.parse(expression, mode="eval")
    return eval_node(tree.body)

if __name__ == "__main__":
    mcp.run()

MCP server with a resource (read-only data)

from mcp.server.fastmcp import FastMCP

mcp = FastMCP("company-data")

@mcp.resource("company://team-roster")
def get_team_roster() -> str:
    """Current team roster in CSV format."""
    return "name,role,team
Alice,Engineer,Platform
Bob,Designer,Product"

@mcp.tool()
def lookup_employee(name: str) -> dict:
    """Look up an employee by name. Returns role and team."""
    roster = {
        "Alice": {"role": "Engineer", "team": "Platform"},
        "Bob": {"role": "Designer", "team": "Product"}
    }
    return roster.get(name, {"error": f"{name} not found"})

if __name__ == "__main__":
    mcp.run(transport="stdio")

Register in Claude Code settings

# ~/.claude/settings.json
{
  "mcpServers": {
    "my-tools": {
      "command": "python",
      "args": ["/path/to/your/server.py"],
      "env": {}
    }
  }
}

Test the server with the MCP inspector

mcp dev /path/to/server.py

Browse available MCP servers and skills at the Claude Skills Browser. For API-level tool use (without MCP), see the tool use Python example.

Frequently asked questions

Free tools

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