minecraft-networkinglisted

# Minecraft Networking ## Purpose Minecraft is a client-server game even in single player, where an integrated server runs on its own thread. Any state that matters lives on the server; the client holds a copy that is only as correct as what you send it. Custom packets are how a mod moves data across that boundary, and they carry two failure modes that dwarf everything else: **the client is under the player's control, so any client-sent packet is attacker-controlled input**, and **packet handlers run on a network thread, so touching the world from one corrupts it**. This skill covers the payload model on each loader and the two rules that make it safe. ## When to use - Adding any packet: a GUI button that must act server-side, a key bind that triggers a server action, syncing mod state to clients. - A GUI button works in single player and does nothing in multiplayer. - `ConcurrentModificationException`, a corrupt chunk, or a random `NullPointerException` deep in vanilla code after adding a packet. - Reviewing a mod for exploits, or a bug report that a player duplicated items or reached blocks they should not. - Migrating channel-based packet code after a 1.20.2+ or 1.20.5+ version bump. ## When NOT to use - The loader, version and mappings are unresolved. Run `minecraft-project-conventions` first. - Entity state that the vanilla tracked-data system already syncs. Use it instead — see `minecraft-entities-mobs`. - Container/menu field syncing, which has its own