Entity Desync Viewer

This Fabric mod for Minecraft 1.21 enables the debug feature renderServerSideHitbox which is present as an unused function in the game's code. Enable hitboxes with F3+B and this mod will show the server-side hitboxes in green in addition to the normal client-side ones in white.

This only works for the integrated server in singleplayer mode.

Usage examples include observing desync after sprint-punching entities or standing on a client-side piston head.