# Anthropic's Claude Formalizes Fermat's Last Theorem in Lean

> Anthropic says an internal research model on par with Claude Fable 5.1 produced the first complete, computer-verified proof of Fermat's Last Theorem, writing it out in Lean, a proof assistant that mechanically checks every logical step. The system generated roughly 13 million lines of Lean code and proved about 29,500 supporting theorems, and it did so in 11 days while burning through an estimated 6 billion tokens across many agents working in parallel. The headline is not that the theorem is newly true, Andrew Wiles proved it in the 1990s, but that a machine translated that sprawling human argument into a form a computer can certify from end to end, using only Lean's three standard axioms. Mathematicians had assumed a formalization on this scale would take years of painstaking human effort. If the result holds up to outside scrutiny, it is a strong signal that AI is becoming a serious collaborator in formal mathematics, a domain where correctness is absolute and there is no room to bluff.

_Section: [Daily AI Updates](https://www.wortins.com/daily-ai) · Source: Anthropic · Published Saturday, September 5, 2026_

## Wortins' read

Anthropic says an internal research model on par with Claude Fable 5.1 produced the first complete, computer-verified proof of Fermat's Last Theorem, writing it out in Lean, a proof assistant that mechanically checks every logical step. The system generated roughly 13 million lines of Lean code and proved about 29,500 supporting theorems, and it did so in 11 days while burning through an estimated 6 billion tokens across many agents working in parallel. The headline is not that the theorem is newly true, Andrew Wiles proved it in the 1990s, but that a machine translated that sprawling human argument into a form a computer can certify from end to end, using only Lean's three standard axioms. Mathematicians had assumed a formalization on this scale would take years of painstaking human effort. If the result holds up to outside scrutiny, it is a strong signal that AI is becoming a serious collaborator in formal mathematics, a domain where correctness is absolute and there is no room to bluff.

## Source

[Read the full story at Anthropic](https://www.anthropic.com/research/formalizing-fermats-last-theorem)

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