Mistral releases Leanstral 1.5 for automated theorem proving in Lean 4

Leanstral 1.5 targets a narrow but high-value niche: automated theorem proving and program-correctness verification using the Lean 4 formal proof assistant. Its architecture is a mixture-of-experts model with 119 billion total parameters but only 6.5 billion active during processing, and a 256k-token context window — large enough to ingest long proof files and related code, a practical requirement for formal verification work where context spans entire proof developments.

It supports Chat Completions, Function Calling, and agent APIs, and is available free via Mistral's Labs playground, lowering the barrier for mathematicians and formal-methods engineers to experiment.

The release is a reminder that not every frontier advance is a general-purpose chatbot — specialized models for math, code correctness, and scientific reasoning are a growing segment, echoing OpenAI's GeneBench-Pro biology benchmark and reports of AI solving unsolved math problems (an r/OpenAI thread on nine solved problems drew 244 upvotes this week). Competitively, Leanstral sits alongside efforts from DeepMind and others in AI-for-math, and the free Labs availability is a developer-acquisition move for Mistral. The MoE efficiency (6.5B active of 119B) is the technical highlight, keeping inference cheap despite the large parameter count. Watch whether it produces verifiable proofs that hold up under Lean's strict checker.