Mistral AI introduced Leanstral, an open-source code agent designed for Lean 4 and formal proof engineering. The model is available through Apache 2.0 weights, Mistral Vibe, and a Labs API endpoint. Mistral positions it as a cost-efficient alternative for verified coding workflows, with FLTEval benchmarks comparing it against Claude family models and large open-source competitors.
This GitHub project presents a formally verified multipolygon intersection algorithm checked in Lean 4. The author argues trust comes from the Lean checker and a small human-reviewed specification, not from trusting LLM output directly. It also documents how Claude Opus versions improved on Lean proof work, with Opus 4.8 reportedly completing larger proof strategies that earlier attempts could not.