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

[jlowin]

That is weird - I'm not sure offhand and will have to try to replicate

[bitnom]

That is weird - I'm not sure offhand and will have to try to replicate

This will greatly alleviate the cumbersome setup.

Dockerfile:

# Install uv
FROM python:3.12-slim AS builder
COPY --from=ghcr.io/astral-sh/uv:latest /uv /uvx /bin/

# Change the working directory to the `app` directory
WORKDIR /app
ENV UV_COMPILE_BYTECODE=1
# Install dependencies
RUN --mount=type=cache,target=/root/.cache/uv \
    --mount=type=bind,source=uv.lock,target=uv.lock \
    --mount=type=bind,source=pyproject.toml,target=pyproject.toml \
    uv sync --locked --no-install-project --no-editable

# Copy the project into the intermediate image
ADD . /app

# Sync the project
RUN --mount=type=cache,target=/root/.cache/uv \
    uv sync --locked --no-editable

FROM python:3.12-slim

# Copy the environment, but not the source code
COPY --from=builder --chown=app:app /app/.venv /app/.venv

# Expose the port your app runs on
EXPOSE 8000

# Run the application with optimizations
CMD ["/app/.venv/bin/mymcp"]

fly.toml:

# fly.toml app configuration file generated for avdl on 2025-05-12T14:49:12-04:00
#
# See https://fly.io/docs/reference/configuration/ for information about how to use this file.
#

app = 'mymcp'
primary_region = 'atl'

[http_service]
  internal_port = 8000
  force_https = true
  auto_stop_machines = false
  auto_start_machines = true
  min_machines_running = 1
  processes = ['app']

[[vm]]
  memory = '1gb'
  cpu_kind = 'shared'
  cpus = 1

pyproject.toml:

[project]
name = "mymcp"
version = "0.1.0"
description = "Add your description here"
readme = "README.md"
requires-python = ">=3.12"
dependencies = [
    "fastmcp>=2.3.3",
    "uvicorn>=0.34.2",
]

[project.scripts]
mymcp = "mymcp:main"

[dependency-groups]
dev = [
]

[build-system]
requires = ["setuptools"]
build-backend = "setuptools.build_meta"

src/mymcp/init.py:

from fastmcp import FastMCP
from typing import List, Dict, Any, Optional, Union
import uvicorn

mcp = FastMCP("mymcp")

@mcp.tool()
def some_tool():
  """
  Do the thing
  """
  return "did the thing"

def main():
    http_app = mcp.http_app(transport="streamable-http", path="/mcp")
    uvicorn.run(http_app, host="0.0.0.0", port=8000)

if __name__ == "__main__":
    main()

then just issue fly launch and fly logs

[bitnom]

I'll contribute a cookiecutter template or something if I can get it working

[Squidward1012]

I trid http://127.0.0.1:8000/mcp on vscode is not work with 307.
But I trid http://127.0.0.1:8000/mcp/ is work.

[bitnom]

I trid http://127.0.0.1:8000/mcp on vscode is not work with 307. But I trid http://127.0.0.1:8000/mcp is work.

Thanks. I tried that before too and it didn't help with flyio. I think it's a separate issue. Here are more debug headers:

cd /home/bitnom/Code/myproject/apps/mymcp && uv run test_client.py --url "https://mymcp.fly.dev/mcp/" --debug
INFO:test_client:Testing connection to: https://mymcp.fly.dev/mcp/
INFO:test_client:Video URL: https://www.youtube.com/watch?v=dQw4w9WgXcQ
INFO:mcp.client.streamable_http:Connecting to StreamableHTTP endpoint: https://mymcp.fly.dev/mcp/
DEBUG:mcp.client.streamable_http:Sending client message: root=JSONRPCRequest(method='initialize', params={'protocolVersion': '2024-11-05', 'capabilities': {'sampling': {}, 'roots': {'listChanged': True}}, 'clientInfo': {'name': 'mcp', 'version': '0.1.0'}}, jsonrpc='2.0', id=0)
DEBUG:httpcore.connection:connect_tcp.started host='mymcp.fly.dev' port=443 local_address=None timeout=30 socket_options=None
DEBUG:httpcore.connection:connect_tcp.complete return_value=<httpcore._backends.anyio.AnyIOStream object at 0x7f408904bf80>
DEBUG:httpcore.connection:start_tls.started ssl_context=<ssl.SSLContext object at 0x7f408902a8d0> server_hostname='mymcp.fly.dev' timeout=30
DEBUG:httpcore.connection:start_tls.complete return_value=<httpcore._backends.anyio.AnyIOStream object at 0x7f408904bdd0>
DEBUG:httpcore.http11:send_request_headers.started request=<Request [b'POST']>
DEBUG:httpcore.http11:send_request_headers.complete
DEBUG:httpcore.http11:send_request_body.started request=<Request [b'POST']>
DEBUG:httpcore.http11:send_request_body.complete
DEBUG:httpcore.http11:receive_response_headers.started request=<Request [b'POST']>
DEBUG:httpcore.http11:receive_response_headers.complete return_value=(b'HTTP/1.1', 406, b'Not Acceptable', [(b'date', b'Wed, 14 May 2025 15:56:37 GMT'), (b'server', b'Fly/bbaf6ebad (2025-05-06)'), (b'content-type', b'application/json'), (b'mcp-session-id', b'79ca8aca17de482bbf50b14c6ecfbf15'), (b'content-encoding', b'br'), (b'via', b'1.1 fly.io, 1.1 fly.io'), (b'fly-request-id', b'01JV7R67AYMT56VMAS4B32HHS0-iad'), (b'transfer-encoding', b'chunked')])
INFO:httpx:HTTP Request: POST https://mymcp.fly.dev/mcp/ "HTTP/1.1 406 Not Acceptable"
DEBUG:httpcore.http11:response_closed.started
DEBUG:httpcore.http11:response_closed.complete
ERROR:mcp.client.streamable_http:Error in post_writer: Client error '406 Not Acceptable' for url 'https://mymcp.fly.dev/mcp/'
For more information check: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/406

[bitnom]

I have made an example repo: https://github.com/bitnom/fastmcp-flyio

[ramrengaswamy]

Wonder if it has to do with proxies. I ran into a similar issue when I deployed FastMCP behind a GKE API Gateway. In my case, the proxy is a Google External Application Loadbalancer.

[maltekuehl]

Experience something similar with an nginx reverse proxy, in that case the followin solved it:

proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-Proto $scheme;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

# Change redirects to the URL
proxy_redirect http://127.0.0.1:8000/ https://mcp.example.com/

Of course, if the proxy is not under your control, you might not be able to change this. But perhaps putting another reverse proxy (like nginx) in front of it can solve the problem?

[tloula]

I ran into this issue as well when hosted in Azure Container Apps. I was able to fix it by wrapping the ASGI app in uvicorn's ProxyHeadersMiddleware.

from fastmcp.server import FastMCP
from uvicorn.middleware.proxy_headers import ProxyHeadersMiddleware

app = ProxyHeadersMiddleware(FastMCP().http_app(), trusted_hosts=["*"])

[jlowin]

@tloula if that's the case, maybe we should document your solution since I'm not sure it has a universal solution we can implement for everyone?

[tloula]

@jlowin yeah, that would likely be the simplest path forward.

[ameringe]

@bitnom were you able to fix with @tloula's solution? Having the same issue and hoping to resolve.

[monarchwadia]

We're having this issue too. It would be easier to debug such issues if there was an explicit RFC that explained how this technology worked. Please consider providing one so that people can debug these types of "advanced" problems more easily: modelcontextprotocol/modelcontextprotocol#541

[monarchwadia]

I ran into this issue as well when hosted in Azure Container Apps. I was able to fix it by wrapping the ASGI app in uvicorn's ProxyHeadersMiddleware.

from fastmcp.server import FastMCP
from uvicorn.middleware.proxy_headers import ProxyHeadersMiddleware

app = ProxyHeadersMiddleware(FastMCP().http_app(), trusted_hosts=["*"])

Could you please explain your reasoning? How does this work?