NotEnoughMinecraft

Inside a world, a computer block can be placed and used to play a nested copy of Minecraft. Input goes to the screen the way it would on a client, with movement, jumping, sprinting, breaking, placing and hotbar selection using the default keys, and right-click starting and finishing item use. Because the mod is server-side only, the inputs it can receive are limited to what the client sends, and the screen is rendered by the server.

Fabric API and Polymer are required. Polymer is what lets the mod work without a client install, so players join with a server resource pack that the mod requires. Files are published for Fabric and Quilt for 1.21.10, with 1.21.8 also covered in the release line. The mod is released under the MIT License and is open source, with its source code and issue tracker public.

Each fake player is a companion that behaves much like a regular player for world interactions and commands, and it appears in the player list. Each player gets one with a unique UUID derived from the real player's UUID. Rendering screens on the server costs CPU, and the uncompressed pixel data adds an expected 300-400 KiB/s of network traffic per screen under default compression, so heavy use on low-end hardware can cause problems.