A developer used the Claude Opus 5.5 model to build an experimental NES processor emulator
With the help of the Claude Opus 5.5 model, Drew Breunig built an experimental GitHub project called nesformer — an NES processor emulator. According to the author, the model independently found and used a 15-year-old processor conformance test suite and created a CLI for running ROM files.
Drew Breunig published a project on GitHub called nesformer — an NES (Nintendo Entertainment System) processor emulator built with the help of the Claude Opus 5.5 model. The impetus was an article about the "qwenputer" technique, which describes emulating a chip using a neural network. Breunig asked the model Opus 5.5 to study this approach and determine whether the same technique could be used to emulate the NES console's chip and RAM (similar to what FPGAs do today in projects like MiSTer), what model size would be needed, and whether smaller and faster models could achieve the standard clock signal speed.
After this analysis, Breunig asked the model to actually build the system for fun, noting that speed wasn't important — the goal was to have the model weights and a CLI tool for running ROM files available. According to the author, while working, the Opus 5.5 model independently found and used an NES processor conformance test suite released 15 years ago by Christopher Pow.
The project was described on his blog by Simon Willison after it appeared on Hacker News, and he also links to the specific prompts that Breunig posted in a comment on Hacker News. The sources do not provide details on the resulting accuracy, speed, or model size.
Why it matters
This is a hobby project without direct commercial impact, but it documents a specific case where the Claude Opus 5.5 model, given an open-ended brief, independently carried out technical research (finding a relevant test suite from 2010) and then assembled a working tool with a CLI interface. For developers, it's an example of how to formulate open-ended, research-oriented prompts for agentic coding.
Relevant practical impact
What this means
For individuals
It shows how the Claude Opus 5.5 model can be guided by an open-ended brief to conduct independent research (finding a test suite) and build a working tool from scratch, which is an inspiration for your own experimentation with agentic coding.
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