Lean 4 proof of Fermat's Last Theorem: how Claude did it in 11 days

Anthropic has successfully formalized Fermat's Last Theorem in Lean 4 using a multi-agent system powered by Claude. The project, which took 11 days to complete, generated 13 million lines of code and 29,500 intermediate theorems. The proof has been verified by mathematician Kevin Buzzard and an independent Rust-based kernel, confirming it relies only on Lean's standard axioms. While the result is mathematically identical to the existing proof by Andrew Wiles, the achievement marks a significant milestone in autoformalization. The project utilized a directed acyclic graph to coordinate agents, overcoming initial collaboration failures. Although the code is machine-generated and largely unreadable, it demonstrates the potential for AI to handle complex, high-stakes formal verification tasks. This development highlights a shift in how mathematical proofs can be validated, moving from human peer review to machine-checked, automated verification, effectively completing one of the most challenging formalization goals in the field.