Mistral Releases Leanstral: First Open-Source Formal Proof Agent for Lean 4 — 746pts HN

Mistral AI·high signal

Mistral shipped Leanstral, a 119B-parameter MoE model (6.5B active) purpose-built for formal proof engineering in Lean 4 repositories. It achieves 26.3 on FLTEval at pass@2, outperforming Claude Sonnet 4.6 (23.7) at 93% lower cost — $36 per task vs. $549. Released Apache 2.0 with free API endpoint (labs-leanstral-2603), integrated into Mistral Vibe, and designed as a bridge between vibe-coded output and mathematically verified correctness.