Labs Leanstral 1.5.1

Released in late June 2026, Labs Leanstral 1.5 is Mistral's experimental model for formal proof engineering and the successor to the original Leanstral from March of that year. Under the hood sits a mixture-of-experts design with 119 billion total parameters and 6.5 billion active per token, tuned to write and check Lean 4 proofs across a 256K context window, with open weights under Apache 2.0. Mistral reports perfect scores on miniF2F and 587 of 672 problems solved on PutnamBench, and the model has surfaced previously unknown bugs while scanning open-source repositories. Automated theorem proving, autoformalization of informal mathematics, and machine-checked verification of program correctness are its home turf.