On September 4, Anthropic published something that sounds like a headline from a decade in the future: the first complete, computer-checked proof of Fermat's Last Theorem, written by a team of Claude agents working largely autonomously over 11 days. Thirteen million lines of Lean. Nearly 30,000 intermediate theorems. About six billion output tokens. I want to talk about a detail that most coverage will bury, because it is the only part that matters if you build software with agents instead of...