Leanstral 1.5: Proof Abundance for All

Leanstral 1.5 is a free, open-source AI model designed to automatically generate mathematical proofs and verify computer code using the Lean 4 formal verification system. The model was trained through three stages: mid-training, supervised fine-tuning, and reinforcement learning, allowing it to learn from compiler feedback when proofs fail and to work across multiple files like a developer would. It matters because formal verification can catch bugs and prove properties are correct, and this model makes that process more practical and affordable than existing alternatives, as demonstrated by discovering previously unknown bugs in real open-source repositories and solving complex mathematical problems at a fraction of the cost of competing systems.