Leanstral: open-source Lean 4 proof-engineering agent (Leanstral-120B-A6B) released under Apache 2.0
Leanstral, an open-source code agent trained for Lean 4 proof engineering and shipped as Leanstral-120B-A6B, is released with Apache 2.0 weights, an agent mode for Mistral vibe, a free API endpoint, a forthcoming tech report, and a new evaluation suite called FLTEval. The model uses a highly sparse architecture (6B active parameters), supports MCPs via vibe (optimized for lean-lsp-mcp), and is benchmarked on realistic repository tasks where it shows cost and efficiency advantages versus several closed- and open-source competitors.