Fermat famously claimed to have a “truly marvelous proof” of his Last Theorem, but he never wrote it down, insisting the margin of his page was too narrow to contain it. A few centuries later, Anthropic announced a complete formalization of Fermat’s Last Theorem using 13 million lines of Lean code (clearly not what Fermat intended). Luckily, we found a wonderfully cursed Lean bug, shown below, that suggests the proof may have fit the margin after all. The issue affects all stable versions of Lean up to 4.33.1, and the patch is incorporated in v4.34.0-rc1. A “checked” proof of Fermat’s Last Theorem using Lean 4.33.1 The blue checkmarks in the screenshot above would suggest that Lean considers this proof correct. This seems odd given the amount of work Sir Andrew Wiles put into this problem and the vast size of Claude’s proof. So what is going on? The “proof” clearly doesn’t make any sense and exploits an issue in Lean. We found the issue while using GPT-5.6 to experiment with a new skill for code review. We want to clarify up front that the issue is not a kernel soundness issue, but it happens to nicely fit any discussion of strings, lengths, and substrings. The issue affects String.Pos.Raw.extract, Lean’s low-level string-slicing function. When asked to extract a one-byte slice at an astronomically large position, Lean’s logical definition returns the empty string. But the compiled native code returns the entire original string. That disagreement is enough to manufacture a contradiction. Lean’s ordinary evaluator “proves” that the tiny slice was empty, while native evaluation “proves” that the very same slice contained “a truly marvelous proof.” Put those together, and Lean concludes that the empty string equals a non-empty string. And once you have a contradiction, you can prove anything, including Fermat’s Last Theorem. On the bright side, the Lean team was considerably faster than mathematical history. About 90 minutes after we reported the issue, har

Read Full Article at Trail of Bits Blog →