/Tech5h ago

Mistral AI releases Leanstral 1.5, a 119B MoE model for Lean 4 formal proof engineering

Story Overview

Mistral just dropped an open 119B-parameter MoE model tuned specifically for Lean 4 formal proof work, where only about 6B parameters stay active at once. It saturates the miniF2F benchmark at 100 percent while posting 587 solved problems on PutnamBench and fresh highs on the FATE-H and FATE-X sets.

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Original post

kache@yacineMTB#403inTech

Awesome stuff The ability to produce narrow models is the product

Albert Jiang@AlbertQJiang

https://mistral.ai/news/leanstral-1-5/ Leanstral 1.5 is here. SoTA on FATE-H/X, 587 on PutnamBench, saturating miniF2F, all with an Apache-2 6B active params model. We are having fun verifying code properties and catching bugs in Rust repos!

Tech report covering training environment and evaluations: https://github.com/mistralai/LeanstralSafeVerify/blob/main/LeanstralReport.pdf We also open-source LeanstralSafeVerify and FLTEval.

11:41 AM · Jul 3, 2026 · 7K Views

Developer Impact

It ships ready for agentic proof loops

The release includes support for multi-turn workflows, filesystem navigation, and Lean language server hooks, so developers can drop it straight into code-verification agents without extra scaffolding.

Access Now

Weights and a free API endpoint are live today

Everything is Apache-2.0 on Hugging Face plus a labs-tier endpoint, though longer-term pricing or enterprise hosting details remain unspecified in the announcement.

Sentiment

Many users praised Mistral's Leanstral 1.5 open 119B model for Lean 4 proof engineering due to its real-world bug verification potential and open weights, while some called the benchmarks saturated and the claims overhyped.

Pos

86.7%

Neg

13.3%

23 comments with sentiment.

Cluster Engagement

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