Mistral AI released Leanstral 1.5, an open-source model trained to formally verify mathematical proofs and software correctness in the Lean 4 language.
It achieves perfect scores on some math benchmarks and ranks at the top of open-source models on several formal algebra tests, and in real-world testing it identified five previously unknown bugs in actual open-source code repositories.
What happened
Mistral AI released Leanstral 1.5, a free open-source model licensed under Apache 2.0, designed for formal verification in Lean 4 (a programming language for verifying math proofs and software correctness). The model scores 100 percent on miniF2F, solves 587 of 672 problems on PutnamBench, and achieves 87 and 34 percent on the algebra benchmarks FATE-H and FATE-X. In practical testing, it scanned 57 open-source repositories and caught five previously unknown bugs, including an overflow bug in the Rust library varinteger.
Why it matters
The model ranks at the top of the open-source field on PutnamBench, FATE-H, and FATE-X—only the closed-source Aleph Prover surpasses it on PutnamBench. For software teams and mathematicians, this suggests that open-source formal verification is reaching a level where it can detect real-world defects in production code, potentially reducing costly bugs before deployment.
What to watch
Leanstral 1.5 is available now through Hugging Face and via a free API, making it immediately accessible to developers and researchers.
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