Mistral releases Leanstral 1.5 for formal code verification with 587/672 Putnam problems solved

Mistral announced Leanstral 1.5, an Apache-2.0 licensed open-source model with 119B total and 6B active parameters designed for Lean 4 proof engineering. According to Mistral, the model solves 587 of 672 PutnamBench problems, achieves 87 percent on FATE-H and 34 percent on FATE-X benchmarks, and uncovered five previously unknown bugs across 57 open-source repositories. The model is available free via Hugging Face and Mistral's API.