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This page lists important things to keep in mind during development to avoid issues.

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Packets must be sent during Tick

Most operation packets must only be sent during player update events (such as onPlayerUpdate or onMotionUpdate), because these events are triggered when the player updates.
If you send them in other events (e.g., onPacketReceive), it may trigger anticheats (such as Post checks). If you are unsure whether your packet sending is safe, you can test on eu.loyisa.cn with anticheat set to Vulcan and Grim (use /ac vulcan grim).
If no BadPackets or Post flags are triggered, it is generally safe.

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Rotation must support movement correction

The Rotation Manager provides the following method for setting rotations: 
void applyRotation(RotationData rotationData, int speed, boolean correction);
If correction is set to true, movement correction will be applied during rotation. You must use applyRotation inside onPlayerUpdate, because if you call it in onMotionUpdate, the client’s movement has already been updated and cannot be corrected. Unless you never plan to support Hypixel or GrimAC servers, you must either provide this option or enable movement correction by default.

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Do not manually send Transaction packets

Transaction packets are also known as CPacket0FTransaction or C0F. Most anticheats use them (due to their instant acknowledgment nature) to validate player positions, implementing reach or hitbox checks via latency compensation. If you manually send these packets, it breaks the transaction order. Most anticheats will automatically ban players that send invalid C0F packets.

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Rendering methods must only be called in render events

Rendering methods (e.g., 2D/GUI drawing) must only be called inside onRender2D. Some rendering methods (e.g., collision box rendering) must instead be called inside onRender3D.