Leanstral: Mistral's Free AI Code Agent That Proves Its Code Works

What if AI could not just write your code—but mathematically prove it's correct?

Mistral just dropped Leanstral, an open-source AI code agent designed for Lean 4 (a proof assistant). Unlike regular coding AI that might hallucinate a solution, Leanstral formally verifies its implementations against strict specifications.

Why This Matters for Builders

Here's the problem with current AI coding tools: they generate code that "looks right" but fails in edge cases. You still need human review, and that becomes the bottleneck when you're moving fast.

Leanstral changes the equation. It uses Lean 4 as a perfect verifier—if Leanstral says the code works, it's mathematically proven. No "should probably work" or "looks good to me."

The Numbers Are Insane