Leanstral 1.5, released by Mistral AI on June 30, 2026, updates the Lean 4 formal proof engine for automated theorem proving and autoformalization. The model expands to 119 B total parameters, with 6.5 B active during inference. Its 256k context window lets it handle large proofs while remaining free of licensing fees. Positioning Leanstral as a core tool for research labs.

Leanstral 1.5 ships with a suite of productivity features. Chat completions, function calling, and agent‑enabled conversations integrate directly into IDEs, while built‑in tools deliver structured outputs and OCR‑based annotations. The model predicts bounding‑box coordinates and extracts document QnA, enabling developers to embed automated reasoning into everyday workflows without additional tooling for rapid prototyping and continuous integration.

Pricing remains free, with no usage fees, making Leanstral a low‑cost alternative to proprietary theorem provers. The 256k context window and 6.5 B active parameters give it competitive speed, while the built‑in moderation and transcription tools broaden its applicability to compliance and accessibility. Stakeholders can now deploy Leanstral in production pipelines without vendor lock‑in for developers seeking scalable formal methods today.