con-leche: an external Lean checker proven consistent (in Lean) goes open source

burny_tech · x · 2026-09-11

leanprover introduced con-leche, an external checker for the Lean theorem prover that is itself proven (in Lean) to be consistent—it will not accept a proof of False. Conceived by Joachim Breitner, its core idea is to let the checker do extra non-essential work (annotations, checks) when that makes the soundness proof easier. It's practically usable: it processes a mathlib export in about 20 minutes on a single worker thread. Source: github.com/leanprover/con-leche.

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