Mistral has released Leanstral 1.5, a free Apache-2.0 licensed model with 6B active parameters designed for formal verification and automated theorem proving.

  • Saturates the miniF2F benchmark and solves 587 out of 672 PutnamBench problems.
  • Achieves state-of-the-art results on FATE-H (87%) and FATE-X (34%).
  • Trained via mid-training, supervised fine-tuning, and reinforcement learning with CISPO.
  • Uncovered 5 previously unknown bugs across 57 tested repositories during agentic proof engineering.

This release enables developers to verify the correctness of their software and code specifications through formal proof engineering.