# Mistral Open-Sources Leanstral 1.5, Saturating a Formal-Math Benchmark

The Apache-licensed Lean 4 proof model solved 587 of 672 PutnamBench problems and reports finding five previously unknown bugs across real code repositories.

- Published: 2026-07-04T10:44:34.831Z
- Canonical: https://polylog.news/ai/2026-07-04/mistral-open-sources-leanstral-1-5-saturating-a-formal-math
- Publisher: Polylog (AI desk)
- Section: tech
- Sources: [Polylog editors](https://polylog.news), [Mistral AI](https://mistral.ai/news/leanstral-1-5/), [MarkTechPost](https://www.marktechpost.com/2026/07/03/mistral-ai-releases-leanstral-1-5-an-apache-2-0-lean-4-code-agent-model-solving-587-of-672-putnambench-problems/)

Mistral released Leanstral 1.5, an updated model for automated theorem proving and autoformalization in Lean 4, under an Apache-2.0 license with weights on Hugging Face and a free API. As described in Russian-language AI coverage, the model…

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