Leanstral 1.5 is a free Apache-2.0 licensed model with 6B active parameters that delivers significant performance upgrades in formal verification and code analysis. It achieves state-of-the-art results on multiple benchmarks and uncovers previously unknown bugs in open-source repositories.

  • Saturates miniF2F completely (100% on validation and test sets).
  • Solves 587/672 PutnamBench problems, outperforming Seed-Prover 1.5 at a fraction of the cost.
  • Achieves new state-of-the-art scores of 87% on FATE-H and 34% on FATE-X.
  • Uncovers 5 previously unreported bugs across 57 tested repositories using an automated pipeline.
  • Demonstrates strong test-time scaling, solving 587 problems with a 4M token budget.

The model enables practical formal verification by making rigorous proof engineering more accessible and cost-effective for real-world codebases.