Websocket handshake fails

Python
,
1

FastAPI app Websocket handshake unsuccessful

char req[] =
    "GET /my-websocket-path/%s HTTP/1.1" "\x0D\x0A"
    "Upgrade: websocket"                "\x0D\x0A"
    "Connection: Upgrade"               "\x0D\x0A"
    "Host: %s"                          "\x0D\x0A"
    "Sec-WebSocket-Key: %s"             "\x0D\x0A"
    "Sec-WebSocket-Version: 13"         "\x0D\x0A"
    "%s%s";
===

Past SSL handshake I start the web socket handshake on the client end. See a FIN originating from the fly.io app end. Its a FastAPI/Uvicorn app on the fly.io end.

Works perfectly fine locally which leads me to believe:

  1. I am missing some header that the proxy expects
  2. Proxy sees something it does not like and rejects the connection immediately. I do not see the request being forwarded to the internal port in the logs.

I have a single app on 8080 (internal) and would at least expect the web socket accept response which I do see locally and on other deployments.

deployment toml is along the lines WebSockets and Fly · The Fly Blog

Any help getting a basic web socket working would be appreciated.
The service also sends streaming/non streaming responses on the non web socket paths defined in the app.

2

Hey @yobewakutan

Looking at the logs, I see the following messages emitted by the app:

2025-08-08 00:49:53,096 - WARNING - httptools_impl.py:184 - _should_upgrade_to_ws() ] No supported WebSocket library detected. Please use "pip install 'uvicorn[standard]'", or install 'websockets' or 'wsproto' manually.

2025-08-08 00:49:53,095 - WARNING - httptools_impl.py:182 - _should_upgrade_to_ws() ] Unsupported upgrade request.

So it looks like the request made it to the app, but some of the required Python dependencies are missing in the Docker image.

5

Connecting this remark to Pavel’s observation, I wonder if you’re running this in Docker with an on-host volume, so your dev environment is not matching what you’re building and running remotely.

6

Thanks @pavel @halfer . So a missing dependency/ import would cause the service not to start at all. Health check and checking non a non streaming endpoint in revision 3 was yielding expected results. But I will rebuild the image and push as you suggest.

7

I wonder also whether you’re not using Docker locally at all. In which case you’ve got it running on your laptop, but your Dockerfile would not build locally and/or your websockets would not work locally in Docker. If you’re not using Docker locally, for dev and for testing, that is what I would change first.

8

Ah locally docker works fine and the image goes through and works on gcp. The first two revisions failed only because of a dependency which was resolved in revision 3.

9

yeah its definitely proxy terminating the connection at the gateway and the websocket request never makes it to the internal service.

O - server.py:219 - _log_started_message() ] Uvicorn running on ``http://0.0.0.0:8080`` (Press CTRL+C to quit)
2025-08-09T01:15:48Z app[XXXXXXXX] iad [info]returning env
2025-08-09T01:15:48Z app[XXXXXXXX] iad [info]2025-08-09 01:15:48,538 - INFO - 172.19.19.89:64148 - “GET /health HTTP/1.1” 200
2025-08-09T01:15:48Z health[XXXXXXXX] iad [info]Health check on port 8080 is now passing.

And only a series of successful health checks after that.

10

@yobewakutan

Do you have the Docker image that you are using pushed to a public registry, so I can launch the same image and play with it?

Alternatively, could you keep your app running so I can check it live?

I don’t see anything in the proxy logs, so it’s a bit hard to debug.

You can also try to add flyio-debug: doit header to the request, this should give us more info. But ideally, I’d like to be able to debug it while the app is running.

11

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