Mistral releases Leanstral 1.5 for Lean 4 proof engineering

MarkTechPost reports that Mistral released Leanstral 1.5, an Apache-2.0 code-agent model for Lean 4 proof engineering. The model is described as a 119B-parameter mixture-of-experts system with 6.5B active parameters and a 256k-token context window. Mistral says it solved 587 of 672 PutnamBench problems and reached 100% on miniF2F, positioning the release for formal-verification and math-agent use cases.

Key details: Leanstral 1.5 targets Lean 4 proof engineering, The model is released under Apache 2.0, Mistral reports 587 of 672 PutnamBench problems solved.

Why it matters: Formal-proof agents are becoming a competitive model niche, with open licensing and long context making the tooling more practical for researchers.