Mistral introduces Leanstral 1.5 for Lean 4 proof engineering
Leanstral 1.5 targets Lean 4 proof engineering. Its June release is now followed by a September retirement notice for the experimental hosted model.
Correction added October 1, 2026. Mistral records June 30 as Leanstral 1.5’s release date, rather than July 2. Its model page describes a Lean 4 formal proof model with 119 billion total parameters, 6.5 billion active parameters, and an Apache 2.0 license.
Formal math models are a product category now. Labs that can close contest lemmas in Lean are selling a different capability than “writes Python.”
Specs and license
The published model specification confirms the parameter counts and license. Access to model weights and access to a hosted endpoint are separate questions. Teams evaluating a formal proof model should check both the license and the deployment route they intend to use.
The model targets formal proof engineering and automated theorem proving. Its outputs still need to satisfy the proof assistant’s verification requirements.
Benchmark claims on miniF2F and PutnamBench
Earlier versions of this article repeated claims about miniF2F saturation and PutnamBench leadership from release trackers. The model documentation cited here does not establish those benchmark results, so they should not be treated as verified performance claims.
A useful evaluation needs the benchmark version, sampling budget, proof verification setup, and a reproducible result. Those details matter more than a claim that a model has finished a benchmark.
Why formal proving models matter now
Formal proof models aim to produce objects that a proof assistant can check. That changes how a proposed result can be evaluated, while leaving questions about problem selection, assumptions, and the contribution of human researchers. Those questions also shape the debate over credit for AI contributions to mathematics.
A proof checker can test a submitted proof against its formal statement. It does not decide whether the statement captures the intended research question or how credit should be shared.
Current availability of Leanstral 1.5
Availability update added October 1, 2026. Mistral now labels Leanstral 1.5 deprecated. The model page lists September 29 for deprecation and the changelog lists retirement on September 30. Readers should check current model lifecycle guidance before planning a hosted deployment.
This article records the historical model release. Its hosted availability has changed since that announcement.
