FastMCP (standalone)

Common errors

Warnings

Install

Install compatibility verified last tested: 2026-05-11

Imports

• FastMCP (v2 and v3 — same import path)
  wrong

  from mcp.server.fastmcp import FastMCP

  correct

  from fastmcp import FastMCP
  
  mcp = FastMCP('My Server')
  
  @mcp.tool
  def add(a: int, b: int) -> int:
      """Add two numbers"""
      return a + b
  
  mcp.run()

  Use 'from fastmcp import FastMCP' for standalone package. 'from mcp.server.fastmcp import FastMCP' is FastMCP 1.0 bundled in the official mcp package — a different, older version.

• Client
  wrong

  from fastmcp import FastMCPClient

  correct

  from fastmcp import Client
  
  async with Client('http://localhost:8000/mcp') as client:
      tools = await client.list_tools()
      result = await client.call_tool('add', {'a': 1, 'b': 2})

  Client class replaces any earlier transport-specific client patterns. Handles transport negotiation automatically.

Quickstart stale last tested: 2026-05-11

Decorators derive tool name, description, and JSON schema from function signature and docstring automatically. mcp.run() defaults to stdio for local use; pass transport='http' for remote deployments.

# Server
from fastmcp import FastMCP

mcp = FastMCP('Demo')

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

@mcp.resource('data://{name}')
def get_data(name: str) -> str:
    """Fetch named data"""
    return f'Data for {name}'

@mcp.prompt
def review(code: str) -> str:
    return f'Review this code:\n{code}'

if __name__ == '__main__':
    mcp.run()  # stdio by default
    # mcp.run(transport='http', host='0.0.0.0', port=8000)  # remote

# ---

# Client
import asyncio
from fastmcp import Client

async def main():
    async with Client('http://localhost:8000/mcp') as client:
        tools = await client.list_tools()
        result = await client.call_tool('add', {'a': 1, 'b': 2})
        print(result)

asyncio.run(main())

# ---

# CLI usage
# fastmcp list http://localhost:8000/mcp     # list tools on any server
# fastmcp call http://localhost:8000/mcp add a=1 b=2  # call a tool
# fastmcp discover                            # find all locally configured servers