Karpathy had Opus 5 build a rough Middle-earth for $10

Andrej Karpathy handed Opus 5 the opening of The Lord of the Rings, along with a budget of 1 million tokens — roughly $10 — and asked it to build a 3D world using Three.js.

The model ran for about two hours and wrote 5,500 lines of code. It had to position polygon assets in 3D coordinates, then write the code to make the scene move. The final product is rough, but it does turn text into a playable 3D world.

This kind of content was rarely worth making before; the cost and time didn't add up. But an LLM can keep at it indefinitely, turning highly customized worlds into throwaway generated content. Karpathy envisions users stepping into these temporary worlds as characters, bystanders, or NPCs — like an on-demand GTA.

The experiment also exposes the limits of today's LLMs. Opus 5 can't efficiently watch video or jump into a game to test it; it can only slowly capture screenshots at various points. It also makes plenty of mistakes, leaving behind rough and glitchy content.

It can build a world, but it can't yet inspect or inhabit the world it made.