Leanstral 1.5
2026-07-02 · Publisher : Mistral AI · Domain : Artificial Intelligence
Mistral AI releases Leanstral 1.5, an Apache 2.0 open source agent with 119 billion parameters (6 billion active) for Lean 4 proofs, which saturates miniF2F and solves 587 of 672 PutnamBench problems.
What changes
- Update to Leanstral with a higher-quality supervised fine-tuning mix and extended long-context reasoning.
- Strong scaling with compute budget: from 44 problems at 50k tokens up to 587 at 4M tokens.
- Five previously unknown bugs found across 57 tested repositories.
- Free API endpoint.
### Specifications - Parameters: 119B total, 6B active. License: Apache 2.0. - API identifier: labs-leanstral-1-5 according to the changelog (the post cites leanstral-1-5). Retirement announced for September 30, 2026. - Weights: Hugging Face, Leanstral-1.5-119B-A6B. Usage via Mistral Vibe (vibe --agent lean). - Context and cutoff: not disclosed. ### Pricing | Item | Price | |---|---| | API | Free (Labs endpoint) | | Average cost per PutnamBench problem | about 4 USD | ### Benchmarks | Benchmark | Score | |---|---| | miniF2F | 100% | | PutnamBench | 587 / 672 | | FATE-H | 87% | | FATE-X | 34% | | FLTEval pass@1 | 28.9 | Scores published by the vendor. ### Key takeaways The announced retirement of the endpoint on September 30, 2026 makes switching to the open weights necessary for any lasting use.
Links
Tags : LLM, Mistral, Leanstral, Open Source, Preuve formelle