Leanstral 1.5: Proof Abundance for All

Leanstral 1.5, a free Apache-2.0 licensed model with 6B active parameters (119B total), delivers significant performance upgrades in formal verification. It saturates miniF2F, solves 587/672 PutnamBench problems, and achieves state-of-the-art results on FATE-H (87%) and FATE-X (34%). The model is trained through a three-stage process: mid-training, supervised fine-tuning, and reinforcement learning with CISPO, utilizing both multiturn and code agent environments. It demonstrates practical proof engineering by uncovering 5 previously unknown bugs across 57 tested repositories. The post details evaluation benchmarks, code verification case studies (AVL Trees, bug discovery), and provides setup instructions for Mistral Vibe.