FastMCP HTTP Transport: Calling MCP Servers Over HTTP

FastMCP provides an HTTP transport layer for Model Context Protocol (MCP) servers, but requires a proper MCP client to communicate. This post demonstrates the correct way to call an MCP server over HTTP using FastMCP.

What is FastMCP HTTP Transport?

FastMCP’s HTTP transport exposes MCP servers over HTTP, but maintains the MCP protocol requirements. Unlike REST APIs, you cannot use regular HTTP requests - you need an MCP-compatible client to handle the protocol properly.

Basic HTTP Server Setup

from fastmcp import FastMCP

mcp = FastMCP("Demo Server")

@mcp.tool()
def get_weather(city: str) -> str:
    """Get weather information for a city."""
    return f"The weather in {city} is sunny and 72°F"

if __name__ == "__main__":
    mcp.run(transport="http", port=8000)

Call the MCP Server

import asyncio
from fastmcp import Client

client = Client("http://localhost:8000/mcp") # mcp is needed here

async def call_tool(city: str):
    async with client:
        result = await client.call_tool("get_weather", {"city": city})
        print(result)

asyncio.run(call_tool("New York"))

Note

We need a FastMCP client or any LLM client that supports the MCP protocol to call the MCP server

https://gofastmcp.com/getting-started/quickstart#call-your-server

The regular POST request won’t work

import requests

# Call the tool via regular HTTP
response = requests.post(
    "http://localhost:8000/mcp/",
    json={"city": "New York"}
)

this will get something like `INFO:     127.0.0.1:59128 - "POST /mcp/ HTTP/1.1" 406 Not Acceptable`