Iustitia Anti-Cheat

IUSTITIA

Iustitia is a Fabric client mod that watches every other player on your server and flags impossible world/combat interactions — the kind of thing a reach hack, a killaura, a fly hack, or a timer cheat produces. It does this entirely on your client:

Read-only on incoming packets. A single mixin (ClientPlayNetworkHandlerMixin) observes server packets and feeds them into a tracking pipeline. It never sends anything to the server and never mutates the local player. The watch follow-cam (/ius spectate, see below) is the one deliberate exception: it overrides the camera only, while you're actively spectating, and auto-reverts the instant you stop, move, get hit, or the target leaves — vanilla re-derives the camera every frame so it can never get stuck.

Other players only. It builds a server-space model of every other client player (position, yaw/pitch, sprint/sneak, hurt ticks, vehicle, deltas) from rebroadcast state, then runs 32 detection checks against that model each tick.

Fail-open everywhere. Every check and mixin body is wrapped so a thrown exception is swallowed and skipped — a detection error never crashes your client and never produces a false positive. A chunk-unloaded player is a false negative, never a false positive.

Two streams, kept separate. Chat alerts fire only when a check's violation level crosses its setback threshold. Verbose console logging (every flag, a pipeline heartbeat) is opt-in via /ius verbose for validation/debugging and is off by default — the release build is silent in latest.log unless you turn it on.

It is a tool with detection/inspection/moderation capabilities, not an enforcement tool: it tells you who looks like a cheater. It does not kick, ban, or report anyone, and it sends nothing anywhere.

REQUIREMENTS

Minecraft 1.21.11
Fabric Loader 0.19.3+
Fabric API 0.141.3+1.21.11 (any compatible)
fabric-language-kotlin 1.13.9+kotlin.2.3.10 (any compatible)
Yet Another Config Lib (YACL) 3.8.2+1.21.11 (any compatible)
Java 21
Recommended: ViaFabricPlus for playing on 1.8-era servers via protocol translation

Iustitia Website! All three library mods (Fabric API, fabric-language-kotlin, YACL) are standard and can be installed alongside any other modpack. Iustitia is written to fail-open and not crash when other mods are present.