GrimAC is an open source Minecraft anticheat designed for 1.21 and supports 1.8-1.21. It is free while in beta. It will eventually become paid and/or will include offering additional subscription based paid checks. Geyser players are fully exempt.

This project is considered feature complete for the 2.0 (open-source) branch of this project. If you would like a bugfix or enhancement and cannot sponsor the work, pull requests are welcome.

Downloads
These are the only official download links.

• Modrinth
• Hangar
• SpigotMC
• For bleeding edge builds use Github artifacts
  

Installation notes
Java 17 is now required.

• Paper, Spigot, and Folia are all currently supported.
• If you use Geyser, place Floodgate on the backend server so grim can exempt bedrock players. Grim cannot access the Floodgate API if it is on the proxy.
• If you use ViaVersion, it should be on the backend server as movement is highly dependent on client version.

Support & wiki information

• Support & discussion: Discord
• Report issues: Issues
• Wiki & examples: Wiki

Developer information

• Source code: Github
• API source & info: GrimAPI
  

• We have a 1:1 replication of the player's possible movements
• This covers everything from basic walking, swimming, knockback, cobwebs, to bubble columns
• It even covers riding entities from boats to pigs to striders
• Built upon covering edge cases to confirm accuracy
• 1.13+ clients on 1.13+ servers, 1.12- clients on 1.13+ servers, 1.13+ clients on 1.12- servers, and 1.12- clients on 1.12- servers are all supported regardless of the large technical changes between these versions.
• The order of collisions depends on the client version and is correct
• Accounts for minor bounding box differences between versions, for example:
  
  • Single glass panes will be a + shape for 1.7-1.8 players and * for 1.9+ players 1.13+ clients on 1.8 servers see the + glass pane hitbox due to ViaVersion
  • Many other blocks have this extreme attention to detail.
  • Waterlogged blocks do not exist for 1.12 or below players
  • Blocks that do not exist in the client's version use ViaVersion's replacement block
  • Block data that cannot be translated to previous versions is replaced correctly
  • All vanilla collision boxes have been implemented

• All movement checks and the overwhelming majority of listeners run on the netty thread
• The anticheat can scale to many hundreds of players, if not more
• Thread safety is carefully thought out
• The next core allows for this design

• The anticheat keeps a replica of the world for each player
• The replica is created by listening to chunk data packets, block places, and block changes
• On all versions, chunks are compressed to 16-64 kb per chunk using palettes
• Using this cache, the anticheat can safely access the world state
• Per player, the cache allows for multithreaded design
• Sending players fake blocks with packets is safe and does not lead to falses
• The world is recreated for each player to allow lag compensation
• Client sided blocks cause no issues with packet based blocks. Block glitching does not false the anticheat.

• World changes are queued until they reach the player
• This means breaking blocks under a player does not false the anticheat
• Everything from flying status to movement speed will be latency compensated

• The player's inventory is tracked to prevent ghost blocks at high latency, and other errors