Co-op Session

Build the drop-in co-op layer (2–4 players) of an open-world action RPG — the Genshin lobby model: guests visit the host's world, content rules make griefing structurally impossible, and the session ends gracefully when the host leaves. Architecture level (authority, prediction, relevance) — no low-level netcode. The hub also maps the wider netcode landscape (lockstep, rollback, FPS lag-comp, MMO scale, matchmaking) and the co-op design craft so you can pick the model that fits your genre, not just copy Genshin. Primary evidence: the Grasscutter server reimplementation + the verified co-op rules already sourced across this skill family.

The architecture rule

The host's world is a server instance, authority is a per-category dial, and the content matrix is data.