Leanstral 1.5: Proof Abundance for All
Mistral AI, Thursday, July 2nd, 2026
Mistral releases Leanstral 1.5, a free Apache-2.0 model for formal verification that sets records in Lean 4 proofs.
Mistral AI released Leanstral 1.5, a free Apache-2.0 licensed model with 6B active parameters designed for formal verification in Lean 4.
The model saturates the miniF2F benchmark, solves 587 of 672 PutnamBench problems, and reaches 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, it also demonstrates practical value by uncovering 5 previously unknown bugs across 57 tested code repositories.
Leanstral 1.5 is fully open-sourced and available through Hugging Face and a free API for practical proof engineering.