coop-sessionlisted

# 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.** ``` SESSION (the host's-world model) the "host's world" = a SERVER-SIDE world instance the host owns — never the host's machine (no host advantage, no NAT, no exposed IPs; host disconnect = the instance closes) guests' own worlds DON'T EXIST while visiting (instance destroyed/ suspended; their state untouched) join = the same EnterScene handshake as solo with a TeamJoin reason; the host's world flips to MP mode; guest spawns at host leave = graceful re-creation of the guest's own instance host leaves = warn -> complete -> close: everyone ejected to their own worlds; claimed rewards survive (server-side) NO host migration — acceptable because returning home is