dbreunig/nesformer

What it claims

nesformer, by Drew Breunig, is a Nintendo Entertainment System 6502 CPU emulator baked into the weights of Qwen3.5-4B. It plays NES ROMs such as Super Mario Bros inside the model, very slowly. Simon Willison's link-blog post of 30 October 2026 adds the two Opus 5.5 prompts Breunig used to build it, which Breunig posted in a Hacker News comment.

How it works

The post gives the build process as prompts, not as an architecture. The first prompt asked Opus 5.5 to read Divyansh Agrawal's qwenputer article (https://divagr.com/blog/qwenputer) and any relevant source code. It then asked whether the same technique could emulate the NES chip and RAM, as MiSTer FPGA projects do in hardware. It also asked what model size would be needed, and whether smaller, faster models could reach the NES's standard clock speed. The page does not record Opus's answers to those questions.

The second prompt set the build goal: "I think we should build it! Just for fun. It's okay if it's not fast. We want to be able to have the weights, and a cli to run them that we can point at a ROM."

For verification, Opus found and used a set of NES CPU conformance tests that Christopher Pow released about 15 years earlier.

How to use it

The intended deliverable is the model weights plus a command-line tool that takes a ROM path. The post gives no install command, CLI syntax, weight download location or licence. Those details are in the dbreunig/nesformer repository.

Limits

Performance is described only as "very slowly". The post gives no frame rate, emulated clock speed or hardware figure. It does not say whether the emulation covers more than the 6502 CPU, such as the NES picture or audio chips. It also does not say how many of Pow's conformance tests pass. The source is a short link post that relays a Hacker News comment, and it contains no measurements.