Leanstral 1.5: Proof Abundance for All
Summary
Leanstral 1.5 is an open-source AI model (Apache-2.0) with 6B active parameters (119B total) that delivers major performance gains in formal verification benchmarks, including miniF2F and PutnamBench, and achieves state-of-the-art results on FATE-H and FATE-X. It demonstrates real-world code verification and bug discovery across open-source repositories, highlighting practical proof engineering in Lean 4 with a three-stage training pipeline and reinforcement learning. The release emphasizes accessibility via Hugging Face and a free API, underscoring the move toward practical formal methods in AI tooling.