Iustitia Anti-Cheat
A free, open-source Fabric anti-cheat that watches other players from your client. A basic, advanced moderation tool.
Screenshots
About this Mod
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.
Available Versions
How to Install Iustitia Anti-Cheat on Your Server
Order Server
Order a Minecraft Java server with at least 3 GB RAM (4 GB recommended).
Set fabric Loader
In the panel under "Egg", select the fabric loader and matching Minecraft version (1.21.11).
Install Mod
Open the mod browser in the dashboard and search for "Iustitia Anti-Cheat". Click "Install" – done! Alternatively, upload the .jar via SFTP to the /mods folder.
Compatibility
Mod Loaders
Minecraft Versions
1.21.11
Server-side
✗ UnsupportedRecommended RAM
4 GB(min. 3 GB)Frequently Asked Questions
Iustitia Anti-Cheat server crashes on startup – what to do?
Most common cause: wrong fabric version or insufficient RAM. Check the server log (latest.log) for "OutOfMemoryError" or "Mixin" errors. With Mado Hosting: ensure at least 3 GB RAM is allocated and the loader matches the mod version (1.21.11). You can switch loaders with one click in the panel.
Is Iustitia Anti-Cheat compatible with fabric?
Iustitia Anti-Cheat officially supports fabric for Minecraft 1.21.11. The Mado dashboard automatically detects incompatible loader combinations.
Server lagging with Iustitia Anti-Cheat – how to optimize performance?
Recommended RAM: 4 GB (per 8 players). Use /spark profiler to check if Iustitia Anti-Cheat consumes the most tick time. Common fixes: reduce server view-distance to 8-10, install "performant" or "starlight" as supplementary mods on Forge. With Mado Hosting, your server runs on NVMe SSDs with dedicated CPU cores for minimal latency.
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Rent Modded Server
Install Iustitia Anti-Cheat with just one click on your server.