Lean gets con-leche: an external checker proven consistent, never accepts a proof of False

AlexKontorovich · x · 2026-09-10

The Lean ecosystem has a new tool: con-leche (CONsistent LEan CHEcker), an external checker for Lean whose consistency is itself proven in Lean — meaning it can never accept a proof of False. The project is led by Joachim Breitner (@nomeata), strengthening the trust chain for formal verification. Mathematician Alex Kontorovich and others amplified the news.

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