Developer Formalizes CPython into 613K Lines of Lean Using Opus, Finds Real Bugs

A developer used Claude Opus 5.5 to formally verify CPython line by line, producing 613,318 lines of Lean code that passed the type checker. The effort also uncovered a real bug in Python.

2026-11-10 ~ 2026-11-11 · 2 related posts

• Opus formalizes CPython in 613K lines of Lean — and finds real Python bugs — pbaylies · 2026-11-10
• Opus formalizes CPython in Lean: 613K lines, finds real Python bugs — banteg · 2026-11-11