leanstral-1-5

Leanstral 1.5 is Mistral AI's open-source code agent model purpose-built around Lean 4, the proof assistant used to formalise advanced mathematics and to verify software specifications such as properties of Rust fragments. The Hugging Face model card frames it as an updated release of the earlier Leanstral checkpoint and groups it inside the Mistral Small 4 family, combining multimodal capabilities with an efficient expert-routed architecture that the maintainers describe as both performant and cost-effective relative to closed-source alternatives.

Under the hood Leanstral 1.5 uses a Mixture-of-Experts design with 128 experts and 4 active per token, totalling 119B parameters with 6.5B activated per token, and supports a 256k-token context window. Mistral's documentation positions the v1.5 update as an optimised Lean 4 formal proof engineering model aimed at automated theorem proving and autoformalization, accepting both text and image inputs while producing text outputs. Practically, it fits research and engineering teams who want a self-hostable, specialised model for formal verification workflows, interactive proof construction, and code-agent tasks that lean on Lean 4 as the backbone representation.