FastMCP

Note

: This is experimental software. The Model Context Protocol itself is only a few days old and the specification is still evolving.

A fast, pythonic way to build Model Context Protocol (MCP) servers.

The Model Context Protocol is an extremely powerful way to give LLMs access to tools and resources. However, building MCP servers can be difficult and cumbersome. FastMCP provides a simple, intuitive interface for creating MCP servers in Python.

Installation

MCP servers require you to use uv as your dependency manager.

Install uv with brew:

brew install uv

(Editor's note: I was unable to get MCP servers working unless uv was installed with brew.)

Install FastMCP:

uv pip install fastmcp

Quick Start

Here's a simple example that exposes your desktop directory as a resource and provides a basic addition tool:

from pathlib import Path
from fastmcp import FastMCP

# Create server
mcp = FastMCP("Demo")

@mcp.resource("dir://desktop")
def desktop() -> list[str]:
    """List the files in the user's desktop"""
    desktop = Path.home() / "Desktop"
    return [str(f) for f in desktop.iterdir()]

@mcp.tool()
def add(a: int, b: int) -> int:
    """Add two numbers"""
    return a + b

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

Features

Resources

Resources are data sources that can be accessed by the LLM. They can be files, directories, or any other data source. Resources are defined using the @resource decorator:

@mcp.resource("file://config.json")
def get_config() -> str:
    """Read the config file"""
    return Path("config.json").read_text()

Tools

Tools are functions that can be called by the LLM. They are defined using the @tool decorator:

@mcp.tool()
def calculate(x: int, y: int) -> int:
    """Perform a calculation"""
    return x + y

Development

Running the Dev Inspector

FastMCP includes a development server with the MCP Inspector for testing your server:

fastmcp dev your_server.py

Installing in Claude

To use your server with Claude Desktop:

fastmcp install your_server.py --name "My Server"

Configuration

FastMCP can be configured via environment variables with the prefix FASTMCP_:

• FASTMCP_DEBUG: Enable debug mode
• FASTMCP_LOG_LEVEL: Set logging level (DEBUG, INFO, WARNING, ERROR, CRITICAL)
• FASTMCP_HOST: HTTP server host (default: 0.0.0.0)
• FASTMCP_PORT: HTTP server port (default: 8000)

License

Apache 2.0