Mistral AI has released Leanstral, the first open-source code agent designed specifically for Lean 4, a proof assistant used in formal verification. Unlike existing systems that wrap generalist models, Leanstral is trained to operate efficiently in realistic formal repositories with just 6B active parameters. The model is available under Apache 2.0 license through Mistral Vibe and a free API endpoint.

As AI coding agents mature, the bottleneck has shifted from generation to verification. Manual review of machine-generated logic requires specialized expertise that slows engineering velocity, particularly in high-stakes domains like mathematics and mission-critical software. Leanstral addresses this by enabling formal proofs against strict specifications, allowing humans to dictate requirements rather than debug generated code.

Benchmarks show Leanstral outperforming both open-source competitors and the Claude family at dramatically lower costs. While Claude Opus 4.6 achieves top quality at $1,650, Leanstral reaches competitive scores for just $36. The model demonstrates particular strength in practical scenarios, successfully migrating code for breaking changes in Lean 4.29.0-rc6 and translating Rocq definitions to Lean while proving properties about programs. Available today through Mistral Vibe with zero setup using /leanstall, Leanstral represents a significant step toward verified code generation.