When the app runs on this machine and its port is taken, the server moves to the next free one, trying up to twenty, and PUBLIC_URL with it, since the OAuth redirect and the app's links name that address, and logs a warning saying so. It moves only when PUBLIC_URL is loopback on that same port, so a deployment, or a loopback URL that names another port, keeps its port and fails to start as before, since a reverse proxy points at it. A port counts as taken when it is at every address or at 127.0.0.1 or ::1 alone, since macOS lets a listener on every address share a port with one on 127.0.0.1, which then still gets the requests for http://127.0.0.1:3000. The tunnel picks the free port itself and points both ssh and the server at it. The foundation spec and .env.example describe it, and the tests cover the probe and when the environment moves.