My app built but doesn't start

“routed port 5000; listening: 5000 (localhost only)”, or “the proxy answered HTTP 502 (cannot reach the application)”.

Your app listens on 127.0.0.1 or localhost, which only accepts connections from inside the container. Flask's app.run(debug=True), Spring's server.address and some dev servers do this.

Listen on 0.0.0.0. We set HOST=0.0.0.0 for apps that read it.

Flask: use gunicorn app:app --bind 0.0.0.0:$PORT as the start command. Spring: set SERVER_ADDRESS=0.0.0.0 in Variables. Node: app.listen(process.env.PORT, "0.0.0.0").

“routed port 8080; listening: 42073”, or a 502 with no error in the output.

The port is fixed in your code (for example app.listen(3000)) and isn't the one we route to.

Read the PORT variable in your code. Or set Port in the project's Settings to the port your app uses. When we see your app on another port, we usually re-route and redeploy by ourselves.

“the container exited or is restarting”, with an error at the end of the output.

Your app crashes during start-up: a missing variable, a failed database connection, a wrong start command, or an exception in your code.

Read the last lines of the output on the deploy page: they name the problem. The usual ones:

“X is required / must be set” → add the variable. “ECONNREFUSED 127.0.0.1” → database connection. “Cannot find module” or “No such file” → check the Start command and the App folder in Settings.

The deploy fails after waiting about 5 minutes, and the output shows the app still booting.

Very heavy start-ups (large Java apps, big models loading) on a small amount of memory.

Give the app more memory (Hobby and up, or the memory add-on), and avoid heavy work at start-up, such as downloading models: do it lazily on the first request.