Leanstral 1.5 Briefing: The Model Is Free, the Verification System Is Not
Mistral’s Leanstral 1.5 is an Apache-2.0 open-weight code agent for Lean 4 proof engineering. The model is free to access; the verification system around it is not. A document-first briefing on what it does, what it really costs to operate, where its benchmarks hold, and which teams should adopt it or skip it.