Mistral launches Leanstral 1.5 with major performance upgrade in formal verification

Mistral has released Leanstral 1.5, a major update to its open-source code agent designed for Lean 4 users seeking improved formal verification tools. Leanstral 1.5 is distributed under the Apache-2.0 license and features a model with 119 billion total parameters, of which 6 billion are active. This upgrade delivers a notable performance boost in formal code verification, intended to make rigorous proof engineering more attainable. Building on these technical foundations, Leanstral 1.5 achieves new state-of-the-art results in several popular proof benchmarks: it solves 587 of 672 PutnamBench problems, saturates miniF2F, and reaches 87% on FATE-H and 34% on FATE-X. Beyond numerical benchmarks, the model has proven its effectiveness by verifying complex code properties and identifying previously unknown bugs in open-source repositories. Following its advanced training pipeline, which includes mid-training, supervised fine-tuning, and reinforcement learning with the CISPO method, the agen...