Mistral AI released Leanstral 1.5 as labs-leanstral-1-5 via Labs API, Mistral Vibe, and Hugging Face (Apache-2.0). Specs: 119B total, 6.5B active, 256k context, $0 in Labs. Scheduled retirement September 30, 2026 on Labs endpoint.
Benchmarks vs prior Leanstral (March 2026, retired June 30): FLTEval pass@8 improved 31.9→43.2. Long-horizon proof: 2.7M+ tokens across 22 context compactions for AVL-tree complexity proof.
Targets proof engineers, formal methods researchers, and verified software teams. OpenATP maintainer updating package to point at Leanstral 1.5.