Leanstral 1.5: Proof Abundance for All

Leanstral 1.5, a free Apache-2.0 licensed model with 6B active parameters, delivers a major performance upgrade in formal verification, saturating miniF2F, solving 587/672 PutnamBench problems, and achieving state-of-the-art results on FATE-H (87%) and FATE-X (34%). Trained through 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 across 57 repositories tested. Fully open-sourced and available via Hugging Face and a free API, Leanstral 1.5 is now accessible for practical proof engineering in Lean 4.

Since its launch, Leanstral has offered an open, practical approach to proof engineering in Lean 4. Today, we are releasing Leanstral 1.5, a free Apache-2.0 licensed model with 119B total and only 6B active parameters, delivering a performance upgrade that makes formal verification more powerful and accessible than ever.

Leanstral 1.5 saturates miniF2F, solves 587/672 PutnamBench problems, and achieves a new state-of-the-art of %87 on FATE-H and 34% on FATE-X. Beyond benchmarks, it verifies complex code properties and uncovers previously unknown bugs in open-source repositories—proving that rigorous formal methods can be both effective and practical for real-world use.

Leanstral 1.5 goes through a three-stage process: mid-training, supervised fine-tuning, and reinforcement learning with CISPO. Leanstral 1.5 leverages extensive training on two RL environments:

In the multiturn environment, the model is given a theorem statement and must either prove or disprove it. The model submits a proof, receives Lean compiler feedback, and refines its approach with each attempt. If the proof compiles it succeeds; otherwise the loop continues until the model either solves the problem or exhausts its budget.