Today, Mistral AI released Leanstral 1.5. It is a code agent model built for Lean 4. The release targets automated theorem proving and proof engineering. Weights are open under Apache 2.0. A free API endpoint, leanstral-1-5, is now live.
Leanstral 1.5 updates the earlier Leanstral-2603 model. It belongs to the Mistral Small 4 family.
What is Leanstral 1.5
Leanstral 1.5 is a code agent model for Lean 4, a proof assistant. A proof assistant checks every logical step mechanically. Lean 4 can express objects like perfectoid spaces and properties of Rust fragments.
The architecture is a mixture-of-experts, or MoE. An MoE routes each token to a few specialized sub-networks. This keeps compute low while total capacity stays large. Leanstral uses 128 experts, with 4 active per token.
