Mistral AI Releases Leanstral 1.5 for Lean 4 Proof Assistance
Leanstral 1.5 aims to reduce the labor of converting human-readable math and specifications into Lean 4 notation, potentially broadening formal proof use beyond expert mathematicians to practical software verification.
Reporting from 1 sources: GIGAZINE.
Mistral AI released Leanstral 1.5 on June 30, 2026, an AI model for automated theorem proving and formalization with Lean 4. The 119-billion-parameter model uses a Mixture of Experts architecture with 6.5 billion active parameters and a 256k-token context length. It is available for free under Mistral AI's Labs framework, succeeding the March 2026 version.
Mistral AI released Leanstral 1.5 on June 30, 2026, a model designed to assist with automated theorem proving and formalization for the Lean 4 proof assistant. The model has 119 billion total parameters and uses a Mixture of Experts architecture, with 6.5 billion parameters active during inference. Its 256k-token context window can handle long proof files and related code together. Leanstral 1.5 replaces the original Leanstral, which launched in March 2026 and has been discontinued. The model is available for free on Mistral AI's playground and supports API features including Chat Completions, Function Calling, and Agents & Conversations. Mistral AI has not yet published benchmarks comparing Leanstral 1.5's performance to the earlier version, and weight release policies remain unconfirmed.
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