Iustitia — client-sided Fabric anticheat for Minecraft 1.21.11

What's new in 1.1

Moderation tools, rendered in world.

v1.1.0 adds a full moderator suite — an instant-replay of the scene, eyes-free directional sonar, portable evidence clips, and player exemptions — turning raw detections into a moderation workflow. All still read-only and client-sided.

🎬

Instant replay

/ius replay <player> <sec> rewinds the last 60s and replays it in-world as translucent humanoid ghost models of every tracked player at their buffered positions. Scrub with /ius replay pause|seek|step|speed, switch cameras (free / follow / POV), or use the numpad keybinds — the live game keeps running underneath and snaps back the instant it stops.

🔊

Sonar audio

/ius sonar plays a directional note on every flushed alert — pan tells you the direction, pitch tells you the distance. Keep your eyes on the fight and just listen for cheats. Additive to chat, with its own volume, never a replacement.

📦

Evidence clips

/ius clip <sec> [name] exports the last N seconds of positions + alerts to a portable .iusclip file. Play it back later with /ius playclip or browse, play, and delete from the /ius clips manager. An evidence clip, not just a screenshot — share the file with another moderator.

🚫

Player exemptions

/ius exempt <name> skips a trusted player at the check chokepoint so they never flag — or /ius clear <name> wipes a bogus tier back to green. Exemptions persist across sessions and server hops, so a trusted regular stays exempt.

What it does

Everything is read-only.

Iustitia never sends a packet, cancels an event, or touches your own player. A single HEAD mixin observes incoming packets; every detection is inference over what the server already broadcast.

🛡

Read-only HEAD mixins

One mixin, every inject at HEAD, nothing cancelled or sent. The mod only watches — it cannot lag, kick, or interfere with vanilla.

⚔

32 checks, 4 groups

Combat, movement, rotation, and packet checks — from lag-compensated reach and a 7-sub KillAura suite to blink/timer and silent-aim tracking.

〰

Lag-exemption engine

A shared lastServerLagTick / lastLagBurstTick signal exempts every movement check during lag spikes — the #1 source of false positives.

▣

Pose-aware hitboxes

Reach, Backtrack and Triggerbot use real sneaking / prone / gliding hitbox dimensions instead of a fixed 0.6×1.8 — no FP on crouching or elytra targets.

⚙

YACL in-game config

Every check's enabled flag, setbackVL, decay and threshold are editable in a Yet Another Config Lib screen, or live via /ius config.

🏷

Nametag tier prefixes

Other players get a colored, confidence-scored prefix — [+ 0] clean, [! 12] suspicious, [X 87] definitive cheater — at a glance.

✓

False-positive reduction first

Hardened through an external audit pass and an internal 33-agent self-audit. Two findings were rejected with documented rationale rather than shipped.

⌥

Free & open-source

Kotlin, Fabric, MIT-friendly. The full source is on GitHub — every threshold, guard, and constant is auditable, no black boxes.

Philosophy

False-positive reduction first.

A client-sided anticheat that cries wolf is worse than none — you stop trusting the prefix. So Iustitia is tuned conservatively: 13 of 32 checks are primary definitive (their alert proves cheating), KillAura acts as a corroborator only, and the remaining 18 are inferential. Tiers require cross-check corroboration to reach red.

01 / OBSERVE ONLY

No outgoing packets

The mod reads incoming clientbound packets and the live client world. It never sends, cancels, or modifies anything — zero risk to your connection or vanilla behavior.

02 / LAG-AWARE

Shared lag signal

Server lag freezes every rebroadcast position. A single lag-tick signal exempts Speed, Fly, Phase, PacketGap, Timer and more — the frozen-in-wall and catch-up-burst FP classes are gone.

03 / CONSERVATIVE TIERS

13 definitive + 1 corroborator, 18 inferential

Only checks whose signature is impossible for a legit player are primary definitive [X]. KillAura is a corroborator — strong when it agrees with reach/criticals, but never marks a player alone. The rest are inferential and cannot push a tier to red.

By the numbers

Honest engineering stats.

No marketing percentages — just what the mod actually is.

32

checks

13

definitive

0

outgoing packets

100%

client-side

0

telemetry

Get started

Install in three steps.

Install Fabric
Put the Fabric Loader jar in your launcher and create a Fabric 1.21.11 profile.
Drop in the mods
Place Fabric API, Fabric Language Kotlin, and the latest iustitia.jar from the releases page in .minecraft/mods/.
Join a server
The first-launch wizard picks alert presets for General / Moderation / Ranked play. Other players' nametags show their tier; chat alerts fire on flags. Run /ius help for commands.