Unraveling the Enigma: A 128-Line Python Codebase for a Coding Agent

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 —