Mistral Open-Sources Leanstral 1.5: AI for Math & Code Proofs

Mistral AI has open-sourced Leanstral 1.5, an AI model designed to verify mathematical proofs and check software behavior. This model is available under the Apache 2.0 license, allowing for commercial use and modification. Leanstral 1.5 works with Lean 4, an interactive theorem prover. It targets both academic mathematics and practical software verification. The model uses a Mixture-of-Experts architecture with 119 billion total parameters, but only about 6 billion are active during inference. Mistral states Leanstral 1.5 achieved a perfect score on the miniF2F benchmark for formal mathematical reasoning. It also solved 587 of 672 problems in PutnamBench, and 87% of graduate-level abstract algebra problems in FATE-H. The company demonstrated its use in verifying Rust software, where it detected 47 violated properties and 11 genuine software bugs across 57 repositories. This matters because it offers a new tool for ensuring the correctness and safety of complex software and mathematical systems.