Leanstral 1.5

Leanstral 1.5 is Mistral AI's open formal-verification model for the Lean 4 proof language, released July 2, 2026 under Apache 2.0. It is a 119B-parameter mixture-of-experts with 6B active parameters per token and a 256k context window.

Mistral reports it saturates miniF2F at 100%, solves 587 of 672 PutnamBench problems, and reaches state-of-the-art 87% on FATE-H and 34% on FATE-X, all at roughly $4 per problem versus an estimated $300+ for proprietary competitors.

Beyond math, it found five previously unknown bugs across 57 open-source repositories in a real-world code verification test. Weights are on Hugging Face with a free API endpoint.