128 lines of Python.
That's the entire core of a coding agent — the loop that powers tools like Claude Code and Cursor. I didn't believe it either, so I built one from scratch. Then I pointed it at a failing test, and it read the file, ran the test, saw the traceback, fixed the code, and re-ran it — choosing every step itself. No one hard-coded that.
It's open source (MIT), with a phased roadmap you can follow:
Why build one instead of reading one
I use coding agents every day. As an AI engineer, I think they're the breakout use case for LLMs right now. But using something and understanding it are different things.
