Mistral Releases Leanstral 1.5 for Formal Proofs, Cuts Cost to ~$4 Per Problem

According to OneMillion_AI, Mistral AI recently released Leanstral 1.5, a formal proof model for Lean 4 with 119 billion total parameters and 65 billion active parameters. The model is released under Apache-2.0 license with free API access. On PutnamBench, Leanstral 1.5 achieves an average cost of approximately $4 per problem to solve, significantly lower than prior systems which cost tens to hundreds of dollars per problem.

The model solves 587 out of 672 PutnamBench problems and reaches 87% on abstract algebra benchmark FATE-H and 34% on FATE-X, setting new performance records for its category. Beyond mathematical proofs, Leanstral 1.5 was also applied to code verification, discovering 11 real bugs across 57 open-source Rust repositories, with 5 previously unreported.