Leanstral 1.5
Mistral's open-weight Lean 4 theorem-proving model that solved 587 of 672 PutnamBench problems and saturated miniF2F.
Overview
Leanstral 1.5, released July 2, 2026 under Apache 2.0, is a 119B-total/6B-active-parameter model specialized for writing and checking formal proofs in Lean 4. Mistral reports 100% on the miniF2F benchmark, 587 of 672 solved problems on PutnamBench, and top open-source scores of 87% on FATE-H and 34% on FATE-X, with the model already surfacing five previously unknown bugs while scanning 57 open-source repositories. It supports a 256k-token context, is available free via a Mistral API endpoint and Hugging Face weights, and is integrated into Mistral Vibe.
This is a sourced directory entry. Features and capabilities are reported in the linked sources; this entry does not document independent Aymorphic testing.
Key features
- 119B total / 6B active parameters, Apache 2.0 licensed
- 100% on miniF2F; 587/672 on PutnamBench; 87% FATE-H / 34% FATE-X
- 256k-token context with strong test-time scaling up to 4M-token budgets
- Agentic code verification; found 5 unknown bugs across 57 scanned repositories
- Free API endpoint (leanstral-1-5) and integration with Mistral Vibe
Use cases
- Formal verification of mathematical proofs in Lean 4
- Automated bug discovery via code-to-proof verification
- Competition-math and research-level theorem proving
Pricing
Free: Apache 2.0 weights on Hugging Face and a free API endpoint (leanstral-1-5).
Pricing reflects the source record and may change. Confirm current terms with the provider.
Dates and sources
- Released
- 2 Jul 2026
- Record updated
- 23 Sept 2026
- Record last checked
- 23 Sept 2026