We’ve now added a functionality where if you add fly-force-instance-id header to your request, with a value containing an instance id, we will forward the request to that instance only. This should also work with websockets and upgrades if the original request was made with the aforementioned header.
This is a bit of a footgun: we will not try other instances when you provide that header. If the instance is down, the connection will probably hang for a little bit and then close.