Mistral Releases Leantral: First Open Source Code Agent for Lean4, Generates Code While Outputting Formal Proofs

According to Crypto界消息, as monitored by 1M AI News, Mistral AI today released Leanstral, the first open-source code agent specifically designed for the formal verification tool Lean 4. The main bottleneck in AI code generation is manual review; Leanstral allows AI to output formal proofs that can be automatically verified by Lean 4 alongside code generation, bypassing this step. The model adopts a sparse MoE architecture with 120B total parameters and 6B active parameters, licensed under Apache 2.0, and has been specially trained and optimized for lean-lsp-mcp. It can be launched with zero configuration in Mistral Vibe (command /leanstall) or accessed via the free API endpoint labs-leanstral-2603, supporting weight download for self-deployment. Mistral also simultaneously released a new evaluation benchmark, FLTEval, which uses the formalization of Fermat’s Last Theorem by the Lean 4 community as its test case. Cost comparison: Leanstral pass@2 scores 26.3 at $36, surpassing the $549 Claude Sonnet 4.6 (23.7); pass@16 scores 31.9 at $290, leading Sonnet 8 points, while Claude Opus 4.6 requires $1,650 to reach 39.6. Among open-source models, Qwen3.5-397B-A17B needs to run four times to reach 25.4, still below Leanstral’s pass@2.