ProxLib

ProxLib is a library mod that enables proximity-based communication between players. Close-by players can send and receive data with one another without any server-side plugin or mod relaying it. Within roughly 26 blocks of one another, players can swap packets through a basic API that ProxLib exposes, and the ProxChat mod relies on that same API to deliver proximity chat.

The data travels by encoding values as relative block positions around the sending player, which are carried in ABORT DESTROY BLOCK player-action packets. Each Minecraft packet carries 9 bits, and a message is prefixed by a 2-byte magic identifier, a 3-byte vendor packet ID and a length field. Because the packets are ordinary action packets, the server does not validate them, and the method works on vanilla servers including those with anti-cheat systems.

The library is client-side only, so only the client needs it. It runs on Fabric and is open source under the MIT License, with files for 1.20 – 26.1, and the latest Fabric file is 0.2.4+26.1 for 26.1. Mods that depend on it must ship it alongside, and mods that bundle it are not always visible in the in-game mod menu.