Audience
Formal methods engineers and Lean 4 researchers who need an open model for theorem proving, proof debugging, and real-world code verification
About Leanstral 1.5
Leanstral 1.5 is an Apache-2.0 licensed model for practical proof engineering in Lean 4, built to make formal verification more powerful and accessible. With 119B total parameters and only 6B active parameters, it delivers a major performance upgrade for theorem proving, agentic proof engineering, and real-world code verification. Leanstral 1.5 was trained through a three-stage process: mid-training, supervised fine-tuning, and reinforcement learning with CISPO. In the multiturn environment, the model receives a theorem statement, submits a proof, gets Lean compiler feedback, and refines its approach until the proof compiles or the budget is exhausted. In the code agent environment, Leanstral works like a developer in a raw filesystem: it edits files, runs bash commands, and uses the Lean language server to inspect goals, errors, and type information in real time.
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