Claude Formalized Fermat in Lean in 11 Days — Not a New Proof
Anthropic says Claude agents produced the first complete computer-checked Fermat’s Last Theorem formalization in 11 days, using about 13 million Lean lines and six billion output tokens. It follows an existing proof, rather than providing a new mathematical argument.
Correction / update, October 14, 2026: Removed an unverified quotation and later-event comparison. The result is attributed to Anthropic and explicitly acknowledges the existing proof and formalization projects it builds on.
Anthropic reports an 11-day formalization of an existing proof, building on Lean, Mathlib and earlier human projects. It is not a new mathematical proof of Fermat.
Anthropic (September 4) published “Formalizing Fermat’s Last Theorem.” Claim: first complete computer-checked FLT. Dozens of agents via Prove2Me (Tianyi Peng / Columbia). 11 days, ~6 billion output tokens, internal research model “roughly comparable to Fable 5.1.” **13 million** lines of Lean; ~29,500 intermediate theorems used (~30,300 proved); >5× Mathlib; Lean 4.33.1 + independent comparator. Follows Darmon–Diamond–Taylor exposition of Wiles — not a new human proof. GitHub: anthropics/fermats-last-theorem. The company’s publication is dated September 4.
What this is (and is not)#
| Item | anthropic.com/research Sep 4 |
|---|---|
| Result | Machine-checked formalization of known FLT |
| Not | A new number-theory proof or a Millennium Prize claim |
| Stack | Lean 4, Prove2Me, Fable-class agents |
The repository labels the release a research artifact and says it is not maintained or accepting contributions. The project’s scale and speed remain Anthropic-reported measurements.