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There is a simple piece of code that can check simple steps, and many people agree this checker is correct. Then there is a formalization of the theorem which many people agree defines the theorem accurately. Then there is 13 million lines of proof that nobody has read, but the proof checker validated each step. That's enough.

So, all you have to verify is the formalization of the theorem, and believe that the proof checker is free of bugs. You don't have to read the actual proof.

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You still have to trust that the AI didn't exploit a bug in the Lean kernel. There was just such an instance of a bug a little over a month ago:

https://leodemoura.github.io/blog/2026-8-1-postmortem-for-ke...

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True, .. and. In this case, the original proof is considered rigorously checked, so finding a bug in the kernel would be nice to know about, but in my opinion would not take away from the accomplishment (FLT in lean using agents) nor the many benefits of getting these mathematical objects formalized and usable in Lean in the future.
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This was my question as well. The way I understand it, it's like a compiler, it implements rules, in this case logic/math rules that tell you whether something follows from assumptions you've given it.

But how do you know you told it what you intended to tell it?

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A human definitely didn't, but one of the benefits of formal verification is that even if the work done to achieve something is slop-y or excessively verbose, solvers like Lean guarantee that the initial proposition (assuming it was written correctly and in this case was definitely reviewed by humans) is definitively True. This is true across other domains of formal verification outside of math as well
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guaranteed, up to lean itself having bugs that are exploited by the LLM :shrug:
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Do you have proof of this bug or something? Is this just envy against computers now ?
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as mentioned elsewhere, there was a bug in the lean kernel exploited by AI to prove a false statement roughly a month ago

https://leodemoura.github.io/blog/2026-8-1-postmortem-for-ke...

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Got it. Thanks. I feel people are using this single story to downplay this feat. There's definitely a chance but I don't see any indication of similar bugs in here or the openai's proofs that were created a month ago as i think these companies might've vetted it enough and the other team who's working on similar lean proof for this also seems to have acknowledged this feat
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I also doubt this is leveraging a lean4 kernel bug, but I also do not think that a 13m LoC proof that has not been human reviewed closes the book on our understanding of Fermat's Last Theorem, in part because of the decided possibility of a kernel bug being used somewhere in those 13m lines.
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How about all of these bugs from last week?

https://leodemoura.github.io/blog/2026-8-24-postmortem-for-t...

...I'm not saying this FLT result is compromised. I suppose things depend on your perspective where we are on the spectrum of "finding more bugs means there are fewer left to discover" vs. "finding more bugs probably means there are still unexplored corners out there".

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Well considering the proof is pretty much accepted by mathematicians to be correct (I'll be happy with that!), it would be sort of unnecessary to cheat. Maybe if some aspect is really tricky to formalize it could have done something there? If I had to search for it, I would go for parts of the original proof that are "outsourced" to other mathematical works. Imagine one of the agents struggling to download a paper due to a paywall or whatever and just deciding to cheat lol
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The nice thing about theorem provers is that you don't need to read the intermediate lines. You need to make sure that the goal/result actually matches what you think it says - but everything in the middle is validated by the prover.
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The point of writing Lean code is that Lean checks it accordingly. Lean is a domain specific language to encode mathematical reasoning in a way that can’t be fooled.

Note to other users: don’t downvote this kind of comment, answer it.

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  encode mathematical reasoning in a way that can’t be fooled.
I would be a bit careful asserting that in full generality, given https://github.com/James-Hanson/junk-theorems-in-lean
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This has nothing to do with Lean, e.g.

> The first coordinate of the polynomial X^2 (X^3 + X + 1 ) is equal to the prime factorization of 30 .

We defined polynomials as their coefficient functions in my algebra class, and it makes sense that you'd define a prime factorization as a function from primes to N, which naturally extends to a function N->N. So this junk theorem is part of normal math too. It just says in an obtuse way that they're both the function that's 1 at 2, 3, and 5, and 0 elsewhere.

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junk theorems aren't the concern, soundness issues in the lean kernel are the concern.

Notably, junk theorems are true. Nobody would debate that the junk theorem is true. The main thing people would say is that junk theorems, while being true, are sensitive to precisely how you encoded mathematics, so despite being true, they are perhaps not conceptually meaningful.

As an example of a junk theorem, sasy you use the definition of the natural numbers using von neumann ordinals

https://en.wikipedia.org/wiki/Set-theoretic_definition_of_na...

Then for any natural numbers n, m, they're implicitly sets. So n \intersect m = min(n,m). This is the wrong way to think about natural numbers. You should not use this ever in proofs. But this isn't because your proofs would be false, but instead because it is a fundamentally confusing way to think about the natural numbers. It is in this sense it is a "junk theorem".

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Isn’t there some theorem that any sufficiently complex mathematical languages will have statements that can’t be proven? :)
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This would be funny if it were relevant. Seems like a statement about false negatives instead of false positives.

False negative = could not find a proof of a true theorem.

False positive = erroneous proof of a theorem.

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Is Lean a DSL? I’d argue it’s a general purpose programming language that excels at proofs.
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Well, there’s actually a very small set of operations that allow all computation, so it doesn’t take much to be a DSL and a GP too; I’d be surprised if a proof language couldn’t swing it.
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it is a general-purpose programming language. for example, it's standard library allows you to do file io, networking, etc.
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No. No human checked it. But a type checker did. And that is much better.
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