Leanstral 1.5: Proof Abundance for All
Leanstral 1.5 is a new open-source model with 6B active parameters designed for formal verification and proof engineering in Lean 4. It utilizes a three-stage training process including reinforcement learning to achieve state-of-the-art results on mathematical benchmarks.
Why it matters
Advancements in formal verification models improve the reliability of software by automating the detection of bugs and verifying complex code properties.
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.
The article reports on a technical product release and its benchmark performance metrics.
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