A phone, a game concept and an agent that checked its work

Simon Willison gave Claude Fable 5 an old game concept, reference images, image-generation access and instructions to work independently while pushing progress frequently. From his phone, he asked for a playable, mobile-friendly 3D browser game.

The transcript shows the agent generating textures, building the game and inspecting desktop and mobile screenshots in Chromium. It discovered that the raccoon was invisible on mobile and fixed it.

The short human request contained a much longer development process. Asset creation, inspection and correction happened inside the agent’s run; they were not eliminated by the initial prompt. The result is one project, but the transcript makes the work behind it unusually visible.