Port Forwarding for HTTP POST not working

I’m trying to test a .NET Core app in Codespaces which has endpoints that accept an HTTP POST request for form submissions. However when trying to use an action in the app that does a POST the codespaces hostname for the app returns an HTTP 405 status code.

Does the port-forwarding implementation only support HTTP GET requests, or is there some configuration I’m missing to enable it?

I have the following in my devcontainer.json:

  "forwardPorts": [ 5000 ],
    "5000": {
      "label": "Application",
      "onAutoForward": "openBrowserOnce",
      "protocol": "http"

I’ve set the logging level to Debug for all categories and I see no messages when submitting the POST, so I’m fairly sure the response is coming from the nginx proxy in Codespaces and not my app.

So it turns out that this is specific to the path /signin, which was the login URL for my app. If I change it to /sign-in, then it works as expected.

I guess /signin is reserved by the proxy or something.