Diamond Hitbox

Every mob in Minecraft is a box. The enderman is a tall thin one, so you hit air beside it. The spider is a flat wide one, so you hit air under it. Diamond Hitbox throws that box away and uses the mob's model instead: the actual cubes on your screen, in the pose they're in.

Hit the arm, you hit the arm. Aim at the gap between a spider's legs and your arrow goes through it.

It works on modded mobs too, and there's no list of them anywhere. The shape is taken from whatever the game draws, so a creature from another mod is handled the day you install it. Nothing to configure.

A few other things it fixes on the way: mobs can't reach you through a wall any more, your own hitbox follows your model when you crouch, swim or fly an elytra, and a boat is finally boat-shaped.

Press F3+B to see it. White is the old box, cyan is what you actually hit now.

Fabric, 1.19.4 to 1.21.11, one build per version. Needs Fabric API. Install it on the client and the server, or the two disagree.