S2S is implemented. HTTP Signatures, WebFinger, inbox/outbox, Accept, Reject, Move, Flag, NodeInfo 2.1. The publicly reachable inbox lives on the relay because that's where the stable URL is. Signing and identity come from the device. Tested with Mastodon, Pleroma, Misskey, Pixelfed, GoToSocial.

Your phone can be a server. It runs a complete ActivityPub server: HTTP Signatures, WebFinger, inbox, outbox, NodeInfo 2.1. The relay handles what a phone can't: stable URL, transport. A Bloom filter built on device filters the shared inbox to followings, local, and followed tags. Private keys and identity stay on the device. The relay never holds them.

Private keys stay on the device. With a custom domain pointing to the relay via CNAME, switching relays is done by repointing the CNAME, no ActivityPub Move needed. Without a custom domain, changing relays uses a standard Move, like anywhere else on the Fediverse.

Reception works. Profile is cached on the relay. Incoming activities go into a queue and are delivered when the phone reconnects. The relay pushes notifications to the phone via UnifiedPush or Expo, even when the app is killed. Recent posts are cached on the relay for offline visibility to other servers. Replying still requires the phone to be online.