#lobby

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.

Networking.CreateLobby is async: Networking.IsActive is still false on the same frame, so any branch on IsActive takes the wrong path. Gate on your own synchronous mode enum instead.

When a lobby query drives a connect, validate the exact lobby candidate you will connect to (not results[0]), or a peer can connect to a lobby it never validated (build-skew, wrong-host connect that no gate catches).

A published-build client join reloads the game assembly, wiping all statics. The reconstruct-not-reset fix from the scene-handoff case has nothing to reconstruct from unless join intent is persisted to disk.

Networking.QueryLobbies appends a hidden:0 filter unless you pass a truthy hidden key, so a Hidden lobby is structurally excluded from every ordinary query. The bool overload param is includeServers, not hidden-inclusion. And editor hosts force Private privacy, which no filter overrides.

A Steam lobby survives its host switching to a different s&box game with stale metadata: a joiner connects and loads the wrong game's content.