Codespace port forwarding returns 404 after restart

[Ogimarinski]

🏷️ Discussion Type

Question

Body

When my Codespace is being timed out and started again my forwarded port just returns 404. Despite working before the time out and no changes made to the branch in the codespace.

[saintpepsicola]

I've run into this before. Usually the issue is that when the codespace stops, the dev server shuts down, and when it restarts the port forwarding doesn't reconnect properly.

Try these in order:

1. In the terminal inside your codespace, just restart your dev server (Ctrl+C then npm run dev or whatever you're using). Sometimes the server doesn't auto-start on resume.

2. Check if the port is actually listening: run lsof -i :PORT_NUMBER to see if anything is actually bound to that port.

3. Failing that, stop the codespace completely (not just let it timeout) from the Codespaces tab, wait 30 secs, and start it fresh.

4. You can also try gh codespace ports visibility CODESPACE_NAME --port PORT --public to reset the port visibility.

Most of the time the simple restart of the dev server does it.

[initial-d]

One extra distinction that helps here: a 404 usually means the forwarded URL is reaching something, but the application behind the forwarded port is not serving that route/path. If port forwarding itself were completely broken, I would expect connection refused, timeout, or an auth/visibility error rather than your app returning a 404.

After a Codespace resumes, I would check it in this order:

# 1. Is the dev server process actually running?
ps aux | grep -E "vite|next|node|python|rails|django" | grep -v grep

# 2. Is anything listening on the expected port?
ss -ltnp | grep :PORT
# or
lsof -i :PORT

# 3. Does the app answer from inside the codespace?
curl -i http://localhost:PORT/

If curl http://localhost:PORT/ returns 404 inside the Codespace too, the issue is the app/server route, not Codespaces forwarding.

If localhost works but the forwarded URL returns 404, then I would remove and re-add the forwarded port from the Ports tab, or set it explicitly in .devcontainer/devcontainer.json so Codespaces forwards it consistently after rebuild/resume:

{
  "forwardPorts": [3000],
  "portsAttributes": {
    "3000": {
      "label": "web app",
      "onAutoForward": "openBrowser"
    }
  }
}

GitHub documents automatic port forwarding here: https://docs.github.com/en/codespaces/developing-in-a-codespace/forwarding-ports-in-your-codespace#automatically-forwarding-a-port

One more common cause after resume: some dev servers bind only to 127.0.0.1 or restart on a different port. For Codespaces, I usually run frontend dev servers with an explicit host/port, for example:

npm run dev -- --host 0.0.0.0 --port 3000

So the quick split is:

• localhost returns 404 -> app route/config problem
• localhost works, forwarded URL fails -> port forwarding/visibility problem
• no process/listener -> dev server did not restart after the timeout

[tobycheese]

I just had the same problem for the first time. I never saw this during the last two years or so with exactly the same setup.
While I was looking for a solution, the problem disappeared by itself though. So, maybe waiting a few minutes will solve it for you too.

Let's see if it happens again.