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err:frontline:upstream:service_unavailable
The gateway sent the request to a running instance of the and couldn’t connect. The instance refused the connection or couldn’t be reached. You get 503 Service Unavailable. This isn’t err:unkey:application:service_unavailable, which the Unkey API returns when it fails on our side.
Example response
Browsers get the same error as an HTML page.

If you’re calling an API hosted on Unkey

Retry after a few seconds. A refused connection during a restart clears on its own. If it keeps happening, only the API owner can fix it. Send them the request ID.

If you own this API

The gateway tries your other instances before it gives up, so none of them accepted the connection. Common causes:
  • The process hasn’t finished starting yet.
  • The process exited.
  • The process listens on a different port than your runtime settings say.
  • The process listens only on localhost instead of all interfaces.

How to fix

  1. Check the runtime logs to see whether the process starts and stays up.
  2. Make sure the port in the runtime settings matches the port your process listens on, and that it listens on all interfaces.
  3. If the process is slow to start, check the health check settings so we can tell when it’s ready.
If instances keep being unreachable and none of these apply, contact Unkey support with the request ID.
Last modified on October 6, 2026