What they did
The poster zipped the original Mac release of SimCity 2000 and gave Opus 5.5 a single prompt asking for a TypeScript WebGL low-poly replica with the full game logic. The agent decompiled the PowerPC binary with Ghidra and translated it function by function, fanning the work out across 27 subagents. A follow-up prompt asked it to make sure every aspect of the game was implemented accurately; the agent's answer was to build a verification harness in Python that runs the original under the Unicorn PowerPC emulator with faked Mac system calls and compares game state with the port. All 3D and vector assets were generated in code; bitmaps came from GPT Image 2; music and sound effects are CC-licensed because AI audio sounded bad to him. Later passes added smog, depth of field and ambient sound.
How it went
A running version in the browser after about two hours, and, per the poster, identical game state across 10,000 simulated days once the harness was in place. The game is live at dator.dev/city2026. Commenters found it slow on phones and one called it poorly optimised; the poster said depth of field is on by default and can be turned off. Another commenter reported Opus 5.5 doing the same for Red Alert 2: a launcher that skips menus, bug fixes, custom units and 32 parallel games for balance testing, kept private over copyright worries.
Worth knowing
This is a derivative of a commercial game and not a clean-room reverse engineering job: the same agents read the old code and wrote the new one, as one commenter pointed out. The harness is not named; 27 subagents and a Ghidra workflow imply an agentic coding tool rather than the chat app. The poster built it to test what the model could do, not to distribute.