Build a 3D Flutter kart racing game with every asset generated in code

Jhin Lee set Gemini 4 Argon, running inside Google's Antigravity editor, a deliberately odd test: a 60 FPS kart racer in Flutter where every character, texture and sound had to come from code. Lee gave short playtester-style feedback rather than specs, 15 prompts in all. The agent built four mascot characters and their karts out of spheres, cones and boxes with flutter_scene, generated asphalt and skid-mark textures at startup, ran the macOS build and screenshotted its own window to fix camera angles and collisions, wrote a Swift script that rendered the soundtrack from the Roland instrument bank shipped with macOS, then ported the game to WebAssembly and deployed it to GitHub Pages.

The game shipped in 3 hours 26 minutes of active work (4 hours 35 minutes with playtesting), about 12,000 lines of Dart, with 96.7 percent of the code shared between macOS and the web build. The web port took under seven minutes and only the audio backend changed. When the live site hung, the agent opened it in headless Chrome, found the shader files returning 404 and traced it to a nested .gitignore copied into the build. It still needed pushback: characters looked wrong from behind until it saw a race screenshot, its first audio was thin beeps, and the karts were about 22 percent too fast for a human.

The audio trick relies on a soundbank that only exists on a Mac, so the build step is not portable. The author framed this as a stress test of the model and published the source under MIT, so the result can be checked. The flutter_scene renderer needs Flutter GPU enabled, which is still an opt-in flag.