# Claude Formalized Fermat in Lean in 11 Days — Not a New Proof

Times of AI Desk · 2026-09-04 · Research

[https://timesof.ai/2026/09/anthropic-claude-fermats-last-theorem-lean](https://timesof.ai/2026/09/anthropic-claude-fermats-last-theorem-lean)

> 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.

## Sources

- [Anthropic: Formalizing Fermat’s Last Theorem (September 4, 2026)](https://www.anthropic.com/research/formalizing-fermats-last-theorem)
- [GitHub: anthropics/fermats-last-theorem](https://github.com/anthropics/fermats-last-theorem)