Leanstral 1.5

### TL;DR

Leanstral 1.5 is an open-source code agent model specifically designed for Lean 4, a proof assistant used for formal verification and mathematical research. It utilizes a Mixture-of-Experts (MoE) architecture to provide high-performance proof engineering and code verification capabilities while remaining cost-effective. It features 119 billion total parameters with 6.5 billion active per token; the model is engineered for efficiency and high performance, allowing it to tackle complex tasks like formalizing mathematical objects and verifying software specifications. It supports multimodal inputs, enabling users to process both text and images to generate rigorous, machine-verifiable proofs.

Key Insights & Metrics

Key Features

• MoE architecture with 128 experts (6.5B parameters active per token)
• 256k token context length for long-range reasoning
• Multimodal input support (text and image)
• Optimized specifically for Lean 4 proof engineering and formal verification
• Apache 2.0 open-source license
• Strong integration with Mistral Vibe for agentic workflows

Discussion