Mistral AI releases Leanstral 1.5 for formal verification
Mistral AI has launched Leanstral 1.5, an open AI model under the Apache 2.0 licence. The model focuses on advanced theorem verification and automated code review.

What happened?
Mistral AI has made Leanstral 1.5 public as an open-source model under the Apache 2.0 licence. This version of the AI model is designed for advanced, formal verification of mathematical proofs and software correctness. The model is built using a Mixture-of-Experts architecture and has 119 billion total parameters, of which 6 billion are active during inference.
Key facts
| Modell | Leanstral 1.5 |
|---|---|
| Licens | Apache 2.0 |
| Parametrar (totalt) | 119 miljarder |
| Parametrar (aktiva) | 6 miljarder |
| Benchmarkresultat | 100% på miniF2F |
”Mistral has open-sourced Leanstral 1.5 under the Apache 2.0 licence, bringing advanced AI-powered theorem proving and automated code verification to developers and enterprises with self-hosting support.”
”Mistral AI has released Leanstral 1.5 as an open-source AI model under the Apache 2.0 licence, expanding access to advanced formal verification for mathematical proofs and software correctness.”
Why it matters
By making Leanstral 1.5 open source with a permissive licence, Mistral AI enables commercial use, modification, and self-hosting. This is particularly important for organisations with strict compliance and data governance requirements. The model operates in conjunction with the Lean 4 interactive theorem prover.
Who is affected?
Developers and companies working in academic mathematics and software verification are directly affected, as they gain access to a powerful tool for self-hosting. Mistral AI is a French AI company positioning itself as a leading player in open and responsible AI development.
What else you should know
Leanstral 1.5 is available via a free API endpoint and for self-hosting via Hugging Face. The model achieved 100% on both the validation and test sets in the miniF2F benchmark, according to Mistral AI.
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