Anthropic: Claude completes first formalized proof of Fermat's Last Theorem

Anthropic said that 'last month' Claude finished the first complete formal proof of Fermat's Last Theorem in Lean. Coverage and follow-up posts put it at over 13 million lines and 29,000+ supporting theorems, many in areas never formalized before, produced by many Claude agents on the Prove2Me platform in 11 days (x.com/tianyi_peng/status/2101840801009426768). Kevin Buzzard, whose multi-year grant targeted the same goal, called it extraordinary. Jared Lichtman: 'Kevin Buzzard had a 5-year grant... Claude has done it in 11 days' (x.com/jdlichtman/status/2095959872563269840). Verified via syndication: 2026-09-04T18:50:48Z.

Checking that a major mathematical proof is correct can take years. Formalization—converting the mathematical reasoning into a form computer proof assistants like Lean can verify—can help.

Last month, Claude completed the first formalized proof of Fermat’s Last Theorem, one of the most famous theorems of all time. This was a project experts thought would take many years. It is the largest Lean proof ever written.

Fermat’s Last Theorem was first proven in 1995 by Sir Andrew Wiles, more than 350 years after it was conjectured. Our proof, which totals over 13 million lines of code, provides machine verification. More importantly, it proves over 29,000 other theorems that the proof requires, across many areas of math which had never before been formalized.

We see this as a major step in the long process of firming up the core of mathematical knowledge, building on work from three centuries of mathematicians and hundreds of contributors to Lean and Mathlib. We are optimistic that AI-assisted verification of mathematical proofs will help reduce the burden of refereeing mathematics in an era where more proofs are being produced than ever before.

You can read about the process on our Science Blog: https://www.anthropic.com/research/formalizing-fermats-last-theorem

And see the complete proof on GitHub: https://github.com/anthropics/fermats-last-theorem

Media: https://video.twimg.com/amplify_video/2095946062741860352/vid/avc1/1920x1080/m-keNgFBlgmjOExS.mp4?tag=29

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