An NES emulator is hidden inside a small AI model — you can play Super Mario Bros in your browser

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An entire NES CPU hidden in an AI

The project, called nesformer, lets you run classic NES games like Super Mario Bros by feeding a game cartridge file into a stock Qwen3.5-4B model. The model doesn't just guess what comes next — every single CPU instruction that the old 6502 chip would have run is computed inside the model's weights by a compiler ¹².

Nobody trained the model for this. Every weight was worked out by a piece of software ². The trick comes from a earlier project called qwenputer by Divyansh Agrawal, which did something similar for simpler computer chips. Nesformer reuses the same compiler and the same idea: the model's internal state acts as the console's memory .

When you start a game, the model predicts one step of the CPU at a time — one token at a time — while a small helper program draws the screen, reads your buttons, and handles sound ².

How it was built

Drew Breunig used Opus 5.5 to plan the system ¹. He pasted an article about qwenputer into the model and asked it to figure out whether it could do the same thing for an NES chip . Claude Opus 5.5 agreed, and Breunig told it to go ahead — noting that speed didn't matter .

Claude also found an old set of test files made 15 years ago by Christopher Pow that check whether a 6502 CPU behaves correctly, and used those to verify the emulator worked ¹. The result? Every frame from the bundled test games matches what a regular NES emulator produces exactly ².

What it can do — and how fast

Super Mario Bros World 1-1 runs inside the model from start to finish, with every CPU token coming straight from the AI ². But don't expect smooth gameplay. Breunig himself warns it will be slow . A typical run of 600 frames (10 seconds of NES time) takes around 25 minutes, since a frame takes 1–6 seconds .

To use it, you need Python 3.11 or newer and a few standard libraries. You download the project and its weights, place them side by side, and run a command like python -m nesformer run roms/thwaite.nes --frames 600 --show ². The terminal draws each frame as the model generates it, and saves an animated GIF when finished .

The project lives on GitHub under the name dbreunig/nesformer and is released under an open license ².

This story reports on a trend for information only. It is not advice or encouragement to do any of it.

How good is it

For the bundled test games, every frame matches exactly what a conventional NES emulator produces ². The project was verified using an old collection of 6502 conformance tests originally released by Christopher Pow 15 years ago ¹. There is no benchmark comparing it to rival approaches or measuring speed in quantifiable terms .

Corrections and updates

• Corrected on 31 October 2026. An earlier version of this story included details its sources do not support. They have been corrected or removed.

Key facts

• An NES CPU emulator is compiled into the weights of a stock Qwen3.5-4B model — no training was done
• Built by Drew Breunig using Claude Opus 5.5 to plan the system
• Super Mario Bros runs inside the model but very slowly
• Uses Gated DeltaNet state as the machine's memory, from the earlier qwenputer project
• Every test frame matches a conventional 6502 emulator output exactly

Sources

1. dbreunig/nesformer — simonwillison.net
2. GitHub - dbreunig/nesformer: An NES CPU compiled into the weights of a stock Qwen3.5 model