Horn
An Android client for Mastodon and Bluesky.
This subdomain exists to serve one file: client-metadata.json. AT Protocol OAuth has no app-registration step — the URL of that document is the client identifier, and your server fetches it during sign-in to check which redirect addresses Horn is allowed to use. That check is what stands in for the client secret an app installed on a phone cannot keep private.
It holds no personal data and nothing is logged here. Signing in happens on your own provider's page; Horn never sees your password.