Debugging WebSocket-upgrade-failed error

[atacan]

I could not find why WS connection fails with a certain Api service, namely "wss://streaming.assemblyai.com/v3/ws". I could not trace what the actual error behind the 'WebSocket upgrade failed' error thrown from try await WSClient.WebSocketClient.connect.

I tried the connection with Starscream library and it works fine.

Here is the reproduction in a macOS app: https://github.com/atacan/DiscussionWSClientUpgradeFailed/

I'd appreciate it if you could take a look.

I think you don't even need API key, as I showed the logs in the Readme, with Starscream we receive the unauthorized error after the connection is established. swift-websocket client fails with or without API key.

[adam-fowler]

If you run with the logLevel set to .debug you get the following

2025-06-27T09:40:54+0100 debug Testing : [WSClient] Client connnected to streaming.assemblyai.com:443
2025-06-27T09:40:54+0100 debug Testing : [WSClient] Upgrade declined
Swift/ErrorType.swift:254: Fatal error: Error raised at top level: WebSocket upgrade failed

NIO doesn't give us anything else, which isn't helpful. I'll investigate a little further

[adam-fowler]

NIO make it very hard to capture the whole response when a websocket upgrade fails. But the response is a 502 badGateway, any idea why you might get that

[atacan]

I have no idea. With the Starscream library I sent even actual audio stream from the microphone, and it was returning me the actual transcript of the recording.

I also noticed the swift-websocket request is not captured by mitmproxy or similar, so I could not debug it from that perspective. Could it be that some default headers in the first connection request cause it? Can we see them?

For example, this is what I see from Starscream

GET /v3/ws HTTP/1.1
Host: streaming.assemblyai.com:443
Sec-WebSocket-Version: 13
Upgrade: websocket
Origin: wss://streaming.assemblyai.com
Sec-WebSocket-Key: eGhtc3ZxYmp5cXdrd3dtYQ==
Authorization: 
Connection: Upgrade

If I have such information, I could contact their customer support.

[adam-fowler]

I checked the headers and there were some slight differences. No origin header and the host header didn't include the 443 port. I rectified this and it still didn't work.

[adam-fowler]

This is the request sent by swift-web socket

GET /v3/ws HTTP/1.1
Host: streaming.assemblyai.com:443
Content-Length: 0
Origin: wss://streaming.assemblyai.com
Authorization:
Connection: upgrade
Upgrade: websocket
Sec-WebSocket-Key: UYuvryK3NW+uti+4T+/dxQ==
Sec-WebSocket-Version: 13

I send the exact same request with curl and it works fine. I'm really banging my head against a wall with this at the moment

[adam-fowler]

Just realised what it is. It is the content-length header. If I remove that it works fine. Not sure what is the expectation here. If I remove this will it break other websocket server connections? I'll need to do some investigation

[adam-fowler]

Ok. Its a GET request so it shouldn't have a content-length header. I'll post a PR to fix this

[atacan]

Interesting. Thank you! The simple URLSession web socket task has the following headers:

GET / HTTP/1.1
Host: echo.websocket.org
User-Agent: DeneWebsocketFoundation (unknown version) CFNetwork/3826.500.131 Darwin/24.5.0
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.9
Accept: */*
Connection: Upgrade
Sec-WebSocket-Extensions: permessage-deflate
Sec-WebSocket-Key: wQ+pzij1UbOFYE3p3QbC5w==
Sec-WebSocket-Version: 13
Upgrade: websocket

Swift code:

let url = URL(string: "wss://echo.websocket.org")!
let request = URLRequest(url: url)
let urlSession = URLSession(configuration: .default)
let webSocketTask = urlSession.webSocketTask(with: request)
webSocketTask.resume()
try await webSocketTask.send(.string("Working"))
webSocketTask.receive {
    print($0)
}

try await Task.sleep(for: .seconds(4))

[adam-fowler]

#25

[adam-fowler]

This should be fixed in 1.3.2

[atacan]

Thank you