#p2p

The operational recipe for shipping player-hosted (P2P) multiplayer in s&box (lobby mechanics, invite codes, the join handshake liveness contract, replication traps, and the three-rung testing ladder), covering the layer the official docs don't document and where live multi-peer sessions actually break.

Running a second s&box process under the same Steam account (an editor or dev instance alongside the published client) correlates with P2P joins to a live remote host failing rendezvous inside the engine's fixed ~3.5s connect budget. Close every second same-account instance before drawing any conclusion from a P2P connectivity failure.

The C# layer of s&box never closes per-pair Steam P2P sessions, so a host crash or task-kill poisons the pairwise transport state between two SteamIDs until Steam-side expiry clears it, typically minutes.