About IConnectionLost dealing with 504 error

I am trying to override the default behavior of connection lost by implement IConnectionLost. After some digging, I realized that the IConnectionLost only deals with connection error. Is there a way to catch a http error such as 5xx?

Hi @louis-she, I can’t think of a way to do it with the IConnectionLost token, which, as you say, only responds to network errors, or 503 errors in a JupyterHub context (e.g. here).

You could try polling requests yourself using the ServiceManager and listening for other error codes. It would involve a bit of extra network traffic, but may not be a big deal for your use case.

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I come up with a dirty workaround that override the fetch methods.

const originalFetch = ServerConnection.defaultSettings.fetch

ServerConnection.defaultSettings.fetch = (input: RequestInfo, init?: RequestInit) => {
  return originalFetch(input, init).then(response => {
    if (response.status > 500) {
      // Handle 5xx error
    return response

Don’t know the side effect but yet it works fine for me.