Leanstral 1.5: Proof abundance for all

Leanstral 1.5, a free Apache-2.0 licensed model with 6B active parameters, significantly upgrades 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%). Trained using mid-training, supervised fine-tuning, and reinforcement learning with CISPO, it excels in agentic proof engineering and real-world code verification, uncovering 5 previously unknown bugs. Fully open-sourced and available via Hugging Face and a free API, Leanstral 1.5 makes practical proof engineering in Lean 4 accessible.