Lean adds independent checks for its proof checker

Lean 4.35 adds lake check to export and recheck proofs inside a sandbox. Its paranoid mode runs several kernels, including a Lean implementation with a consistency proof and a Rust translation verified against that algorithm. Different implementations reduce exposure to shared compiler or runtime bugs. The consistency proof still assumes an underlying set theory and is checked by Lean itself.

Leonardo de Moura · Oct 15, 2026

Why it matters

Proof checking benefits from independent implementations as well as proofs about the implementation.

More context

Try the new checking command on a Lean project. The report also identifies scaling limits: exporting and rechecking enormous libraries still needs better binary formats and incremental checking.

Keep in mind

Formal verification narrows the trusted surface; it does not eliminate compiler, runtime or foundational assumptions.

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