Labs Leanstral 1.5.1

The point release of Mistral's experimental proof engineering line, Labs Leanstral 1.5.1 supersedes the initial Leanstral 1.5 endpoint while keeping its recipe intact, a mixture-of-experts design with 119 billion total parameters, 6.5 billion active per token, and a 256K context window. Like 1.5 it writes and checks Lean 4 proofs, translating informal mathematics into machine-checkable statements and searching for proofs autonomously. As the current revision it carries the refinements accumulated since the June 2026 debut of 1.5, making it the endpoint to reach for when staying on the freshest build of the line matters. The workload profile is unchanged, automated theorem proving, autoformalization, and formal verification of program correctness.