Modern Warfare 2 Was Rewritten in Rust, and an LLM Wrote the Code

IW4L is a new engine for Call of Duty: Modern Warfare 2 (2009), written in Rust on the Bevy game engine. It ships no game files: you point it at a copy you already own. Its README ends with "This whole project is written by an LLM", and over a hundred of its commits list Claude Opus 5.5 as a co-author. I went through the repo, the developer's posts and the clips people shared to see how it works, what already runs, why the movement code was thrown out and rewritten, and where the legal line seems to be. What's in it: - What IW4L is, and why it doesn't ship the game - How it reads MW2's fast files, translates the 2009 shaders and rebuilds networking - The timeline: July playtest to 344,056 lines in one commit - Who wrote it: the Claude co-author lines in the git history - What works today (maps, bots, killcams, master server, an iPhone port) - Changing the engine with an AI coding tool: the sprint fix and the FOV slider - The GPL question and the "clean room" movement rewrite - Activision's recent takedowns, and the developer's two rules IW4L: https://github.com/vladtrc/iw4L It's experimental: expect bugs and desyncs, and everyone in a match needs the same build. You need your own copy of MW2. Clips, posts and pages shown (all credit to their creators): - vladtrc (@vtroc7) — https://x.com/vtroc7 · YouTube "da7e": https://www.youtube.com/channel/UCdL2te7jDTKMbEY2eB6j5Og - first playtest https://youtu.be/IDY3K5WjDi8 · playtest 2 https://youtu.be/ODXO2GaDa1I · status update https://youtu.be/LOQ9rEKhbmY · https://youtu.be/-7L4AmjD_XA · https://youtu.be/3zU7eQLnLic - posts: https://x.com/vtroc7/status/2089015961370185840 · https://x.com/vtroc7/status/2094197408745824719 · https://x.com/vtroc7/status/2105664089879982548 · https://x.com/vtroc7/status/2105766083483283869 · https://x.com/vtroc7/status/2108844267779035413 · https://x.com/vtroc7/status/2109377638301421637 · https://x.com/vtroc7/status/2109581826196975808 · https://x.com/vtroc7/status/2109768250275876867 · https://x.com/vtroc7/status/2109886149795721721 · https://x.com/vtroc7/status/2107575762287055208 - chasm — "The Modern Warfare 2 Rust Rewrite is Here..." https://youtu.be/9UbADrcEW5w - Ju1cedr1nker — "IW4L (new launcher) online test" https://youtu.be/f2b0znxmN7s - IW4 Pocket (MarkusSela) — https://github.com/MarkusSela/IW4-Pocket - Luckey Faraday — https://x.com/luckeyfaraday/status/2108722723849379986 - bindnac — https://x.com/bindnac/status/2109742716397748689 - IW4L docs: README, docs/provenance/movement-iw4.md, pull requests #4 and #5 Code drawn for this video (the Ghidra window and the shader snippets) is labelled as an illustration. Not affiliated with Activision or Infinity Ward. ## Chapters 0:00 MW2, but none of the code is from 2009 0:18 What IW4L is 0:37 How it reads the old game 1:20 The timeline 1:45 Who wrote it 2:06 What already works 2:44 Changing it with AI 3:15 The movement rewrite and the GPL 3:45 What the note can't prove 4:03 Activision and the line 4:31 What AI can't do