QuantumAC

A lightweight, server-side anti-cheat plugin for PaperMC 1.21.x

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QuantumAC is a server-side anti-cheat plugin built specifically for PaperMC. It catches the most common cheats through a combination of movement analysis, packet inspection, combat geometry, and world-state occlusion — without relying on client-side modifications.

All detections happen on the server. There is nothing a cheat client can do about packets that were never sent, or a lagback it never saw coming.

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Checks

Movement

• Flight — detects sustained upward or level movement in air. Lagbacks the player to their last safe ground position.
• Speed — flags horizontal movement beyond vanilla limits, accounting for sprinting, speed potions, ice, soul sand, and post-knockback momentum. Lagbacks on confirmation.
• BoatFly — detects upward boat movement exceeding vanilla physics limits per tick.
• Timer — detects client-side timer speed hacks via sliding-window packet-rate analysis. Only position-carrying packets are counted, eliminating the false positives caused by 1.21.x rotation bursts.

Combat

• KillAura — detects unnatural attack patterns: inhuman attack rate, multi-target switching within a single swing window, and robotic aim angle consistency via packet analysis.
• Reach — detects attacks and interactions beyond legitimate reach distance. Limits are adjusted per player based on ping so high-latency players are never falsely flagged while low-latency cheaters are caught precisely. Covers combat, block interaction, and entity interaction.
• Criticals — flags critical hits landed while the player is grounded, blocking criticals cheats that fire regardless of jump state.
• NoFall — catches suppressed fall damage. Reapplies blocked damage server-side and lagbacks on confirmed damage-free landings, with mace smash and wind-charge exemptions.

World

• FastBreak — enforces minimum block break times relative to tool efficiency and block hardness. The block is never removed on flag — no client feedback, no rubberbanding.
• AntiScaffold — three sub-checks: air placement detection, impossible placement angles (catches rotation-injection scaffold clients such as Wurst BackPlace and LiquidBounce RotationMode), and ghost-hand detection for replayed place packets after item switch.
• AntiJesus — detects players walking on water or lava surfaces. Exempts Frost Walker boots, accounts for boat-exit grace periods, and scales tolerance with ping to avoid false positives on laggy connections.

Occlusion & Packet Integrity

• AntiESP — server-side entity and block entity occlusion. Uses 5 ray probes per entity (centre + 4 top-face corners) to determine visibility. Occluded entities are removed from the client's entity list entirely — no cheat client can render what was never sent.
• AntiXRay — hides fully enclosed ores and storage containers (chests, shulkers, barrels, furnaces, hoppers) via fake block packets on chunk load. Client-side bypass is impossible. Pairs cleanly with Paper's engine-mode 2 for layered protection.
• AntiVelocity — server-side knockback physics verification catches clients that suppress or nullify received velocity.
• BadPacket — blocks malformed rotation data (NaN), spoofed Y-coordinates used for MaceDMG and Freecam exploits, and impossible movement deltas that exceed vanilla limits.

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Features

• Violation level system — configurable thresholds per check for alerts, kicks, and bans
• VL decay — violation levels bleed off over time so lag spikes never stack into false punishments
• Lagback — movement violations snap the player to their last safe position cleanly, no teleport spam
• Runtime check toggle GUI — enable or disable any check in-game without a reload
• Staff alerts — broadcast flagged players to online staff above a configurable VL threshold
• Per-check configuration — enable/disable, thresholds, multipliers, VL costs, and lagback behaviour
• quantumac.bypass — permission node for staff or trusted players to skip all checks

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Requirements

• PaperMC 1.21.x
• ProtocolLib 5.4.0+
• packetevents 2.12.1+

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Commands

| Command | Description |
|---|---|
| /quantumac checks | Open the check toggle GUI |
| /quantumac reload | Reload config |
| /quantumac alerts | Toggle staff alerts for yourself |
| /quantumac violations <player> | View a player's current violation levels |

Permissions

| Permission | Default | Description |
|---|---|---|
| quantumac.admin | OP | Access to all admin commands |
| quantumac.bypass | false | Bypass all anti-cheat checks |
| quantumac.alerts | OP | Receive staff alert broadcasts |