Incorrect 307 Temporary Redirect -> 400 Bad Request after deployment

[bitnom]

Description

Example repo for issue: https://github.com/bitnom/fastmcp-flyio

All my local testing runs without errors. After deployment via a docker image, I see in the fly logs (Upon running a test client):

2025-05-13T18:28:05Z app[d891165bed3418] atl [info][05/13/25 18:28:05] INFO     Starting server "My MCP server.py:207
2025-05-13T18:28:05Z app[d891165bed3418] atl [info]                             Metadata Server"...
2025-05-13T18:28:05Z app[d891165bed3418] atl [info]INFO:     Started server process [628]
2025-05-13T18:28:05Z app[d891165bed3418] atl [info]INFO:     Waiting for application startup.
2025-05-13T18:28:05Z app[d891165bed3418] atl [info]INFO:     Application startup complete.
2025-05-13T18:28:05Z app[d891165bed3418] atl [info]INFO:     Uvicorn running on http://0.0.0.0:5000 (Press CTRL+C to quit)
2025-05-13T18:28:26Z app[d891165bed3418] atl [info]INFO:     172.16.4.26:36830 - "POST /mcp HTTP/1.1" 307 Temporary Redirect
2025-05-13T18:28:26Z app[d891165bed3418] atl [info]INFO:     172.16.4.26:36844 - "GET /mcp/ HTTP/1.1" 400 Bad Request
2025-05-13T18:28:26Z app[d891165bed3418] atl [info]INFO:     172.16.4.26:36830 - "POST /mcp HTTP/1.1" 307 Temporary Redirect
2025-05-13T18:28:26Z app[d891165bed3418] atl [info]INFO:     172.16.4.26:36844 - "GET /mcp/ HTTP/1.1" 400 Bad Request
2025-05-13T18:29:49Z app[d891165bed3418] atl [info]INFO:     172.16.4.26:49232 - "POST /mcp HTTP/1.1" 307 Temporary Redirect
2025-05-13T18:29:49Z app[d891165bed3418] atl [info]INFO:     172.16.4.26:49250 - "GET /mcp/ HTTP/1.1" 400 Bad Request

how to fix this redirect to work with production url?

Test client:

import asyncio
from fastmcp import Client
from fastmcp.client.transports import StreamableHttpTransport

mcp_host = "https://mymcp.fly.dev/mcp"
# mcp_host = "http://127.0.0.1:5000/mcp"

async def example():

    async with Client(StreamableHttpTransport(mcp_host)) as client:
        print(await client.ping())

if __name__ == "__main__":
    asyncio.run(example())

client log:

uv run fastmcp_client.py 
Error in post_writer: Client error '400 Bad Request' for url 'https://mymcp.fly.dev/mcp'

server config:

import asyncio
import logging
from fastmcp import FastMCP
from typing import List, Dict, Any, Optional, Union

mcp = FastMCP("YouTube Transcript & Metadata Server", host="0.0.0.0", base_path="/mcp")  # have tried with and withotu base_path

@mcp.tool()
def fetch_video_data_with_concatenated_transcript() -> List[Dict[str, Any]]:
    """
    test tool

    Returns:
        some text
    """
    return "this is a test"

if __name__ == "__main__":
    # Run the MCP server when the script is executed directly
    asyncio.run(mcp.run_streamable_http_async(transport="streamable-http", port=5000, host="0.0.0.0", log_level="debug"))

EXPOSE 5000 is set in the dockerfile.

Example Code

Version Information

FastMCP version:                                                                      2.3.3
MCP version:                                                                          1.8.0
Python version:                                                                      3.12.7

Additional Context

No response