Build a pixel-art game a child designs by playtesting each build

A Redditor with 20+ years as a quantitative engineer, but no game experience, built a private unicorn game for their six-year-old daughter with Claude Sonnet 5.5 doing all of the coding. The daughter designed most maps and mini-games. Each round she played the latest build, the parent wrote down her notes (up to 25 items at once) and handed them to Claude, and she played the result the next day. Work started from a tech spec, with small throwaway spikes to test Phaser and a Cloudflare Worker backend before committing. Claude built a software 3D renderer that turns code-defined models into pixel sprites, a synthesizer for all sound effects and music, a map compiler that checks every door and spawn is reachable, and 3,000+ tests. It kept a decision log (245 rows over 11 deploys) and a handoff file for new sessions, and the parent asked it not to use subagents so one thread stayed auditable.

The game is live and free to play: eight places, five friends that follow as a herd, six kinds of magic, playground mini-games and an optional spelling mode, installable to a home screen and playable offline. It runs on two iPhones, a Mac and mostly a Kindle Fire. When spells stuttered on the Kindle, Claude added a low-end mode that turns itself on when frame rate drops. It caught a flicker bug by measuring it in the running game (20 flips in 103 seconds, none after the fix). An early pure-3D face looked blank, so faces became hand-placed pixels.

The post names the model, not the tool it ran in, so the harness is unknown. Claude could not see or hear the game, so it checked its work by reading screenshots and measuring audio with loudness meters. Phaser and one font are the only outside assets. The game is fan-made around characters the poster does not own, and the full source is not open yet.