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.

Anthropic's new Model Context Protocol is powerful way to give broadcast new functionality and context to LLMs. However, developing MCP servers can be 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:

# Basic usage
fastmcp dev your_server.py

# Install package in editable mode from current directory
fastmcp dev your_server.py --with-editable .

# Install additional packages
fastmcp dev your_server.py --with pandas --with numpy

# Combine both
fastmcp dev your_server.py --with-editable . --with pandas --with numpy

The --with flag automatically includes fastmcp and any additional packages you specify. The --with-editable flag installs the package from the specified directory in editable mode, which is useful during development.

Installing in Claude

To use your server with Claude Desktop:

# Basic usage
fastmcp install your_server.py --name "My Server"

# Install package in editable mode
fastmcp install your_server.py --with-editable .

# Install additional packages
fastmcp install your_server.py --with pandas --with numpy

# Combine options
fastmcp install your_server.py --with-editable . --with pandas --with numpy