The Validating a Lean Proof page says Comparator, meant for high-risk cases such as proof marketplaces, high-reward proof competitions and unaligned AI, builds the proof in a sandbox, exports the proof term, and outside the sandbox replays the proofs and checks that the proved theorem statements match those in a trusted challenge file.

The exact words it rests on

This should only be necessary for high risk scenarios (proof marketplaces, high-reward proof competitions, unaligned AI). ... Comparator will build the proof in a sandboxed environment, to protect against malicious code in the build step. The proof term is exported to a serialized format. Outside the sandbox ... it validates the exported format, replays the proofs using both Lean's kernel and/or an external checker and also ensures that the proved theorem statements match those in the trusted challenge file.

What the source said when we opened it, on 18 Oct 2026.

The source

Lean reference: Validating a Lean Proof
Lean FRO / Lean project

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Checked

Checked by the notis newsroom on 18 Oct 2026, against the source above.

In the story

How Lean checks an AI-written proof, and where that stops 18 Oct 2026