FrontierThe story, in brief
Mistral AI Releases Leanstral 1.5: An Apache-2.0 Lean 4 Code Agent Model Solving 587 of 672 PutnamBench Problems
587 of 672. That's Mistral's new Leanstral 1.5 solving Putnam-level math proofs—open source, Apache 2.0, and it's already finding real bugs.
The KeyNews take
Why it matters
Mistral ships a specialized code agent model that outperforms on formal reasoning benchmarks while staying fully open source. This signals a shift toward task-specific, efficient MoE models competing with closed generalists on narrow but valuable domains.
The key facts
6 to knowLeanstral 1.5 solves 587 of 672 PutnamBench problems
119B parameter mixture-of-experts model activating 6.5B parameters per token
Apache 2.0 license (fully open source)
Saturates miniF2F benchmark
Specialized for Lean 4 formal verification and code generation
Includes bug-finding case studies and deployment code
Go to the source
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Publisher excerpt: Mistral AI released Leanstral 1.5, a free Apache-2.0 code agent model for Lean 4. It saturates miniF2F and solves 587 of 672 PutnamBench problems. The 119B mixture-of-experts activates 6.5B parameters per token. We break down its architecture, benchmarks, real bug-finding case studies, and…