A developer used AI agents to rewrite Minecraft in Rust

Independent developer Ludovic Marie has released Horizon, a client for Minecraft: Java Edition 26.3 rewritten from scratch in Rust. The interesting part is not the language but who wrote the code. The project site says it plainly: most of the code is written by AI agents, mainly Claude sessions that Ludovic runs, plus contributors' own AI tools. Every pull request opened by an agent says so in its first line.

Nobody reads every line by hand. Instead, each change goes through automated tests that feed the same inputs to Horizon and to the real Minecraft and compare the results. As of October 25 there were 326 of them, covering everything from world generation to mobs, redstone and commands. The authors admit that passing them doesn't prove the game behaves identically everywhere.

In the team's own benchmarks on an RTX 4080 at render distance 32, Horizon loads a world in 9 seconds versus 59 for the regular Java edition and uses 1.6 GB of memory instead of 3.3. On an old ThinkPad with integrated graphics at 12 chunks it averages 209 frames per second against 126. It isn't faster at everything, though: at render distance 32 the original game draws more frames, 849 versus 672.

You still need your own legal copy of the game, since no Mojang files ship with the project. The code is open under the MIT license, mods are written anew and run in a WebAssembly sandbox, and Forge or Fabric mods won't work. Most public servers are off limits for now because Microsoft account sign-in isn't supported yet.

There was a scandal too. On October 24 the project dropped shader support after a developer of the Iris mod showed that this code had been written from Iris's source. The rules now say code may only be written from the game itself and public documentation.

The agents rewrote a game people spent 15 years building. Checking their work is still a human job.