Leanstral 1.5 release notes / what's new

Mistral AI has released Leanstral 1.5, a specialized large language model designed for Lean 4 formal proof engineering. The model is optimized specifically for automated theorem proving and autoformalization, providing a targeted tool for the formal verification community.

Model Architecture and Specifications

Leanstral 1.5 is a Mixture-of-Experts (MoE) model with a total of 119 billion parameters, of which 6.5 billion are active during any given inference step. This architecture allows the model to maintain a high capacity for specialized knowledge while remaining computationally efficient during execution.

Key technical specifications include:

  • Context Window: 256k tokens
  • Model ID: labs-leanstral-1-5
  • Release Date: June 30, 2026

Supported Features and Capabilities

Leanstral 1.5 integrates with the Mistral AI ecosystem, supporting a wide range of API features including Chat Completions, Function Calling, and Structured Outputs. For developers working with formal proofs, the model supports Fill-In-the-Middle (FIM) completions, which is critical for editing existing proof scripts.

Beyond standard text generation, the model is compatible with:

  • Agents & Conversations: Integration with the Agents API for complex proof workflows.
  • Document Processing: Support for OCR, BBox extraction, and Document Q&A, allowing the model to process visual representations of mathematical notation.
  • Audio Integration: Capabilities for transcriptions and text-to-speech via the Mistral API.

Ecosystem Integration and Community Tools

Leanstral 1.5 is designed to work within the Mistral "Vibe" harness. Third-party developers have already begun integrating the model into agentic frameworks. For example, the OpenATP project—an open-source Python package and CLI for agentic automated theorem provers—has announced updates to point to Leanstral 1.5 following the deprecation of the previous production Leanstral model on May 22nd.

Community Discussion and Feedback

User feedback following the release has highlighted several points of friction regarding accessibility and transparency:

Access and Deployment

Some users reported significant difficulties accessing the model, citing "unspecified errors" when attempting to enable "labs" models in the Mistral console and a lack of responsive customer support.

Weight Availability

There is confusion regarding the model's weights. While documentation suggests an Apache license, some community members have noted the absence of a clear download link for the 1.5 version, noting that Hugging Face profiles only seem to provide earlier snapshots (such as Leanstral-2603).

Strategic Positioning

Discussion among developers has raised questions about the specialization of the model for Lean 4 specifically, rather than other formal languages like Coq. Some contributors noted that languages like Lean 4 and Idris 2 are particularly well-suited for LLM interaction because they provide additional formal guarantees that can be used to verify model outputs.

Sources

Related

  • Dispatch
    Leanstral 1.5 Release Notes: Advancing Formal Verification with Lean 4

    Mistral AI has released Leanstral 1.5, an Apache-2.0 licensed model that achieves state-of-the-art results in formal mathematics and discovers real-world bugs through automated code verification.

  • Dispatch
    Mistral AI Leanstral Release

    Mistral AI has released Leanstral, an open-source code agent with 6B active parameters designed for Lean 4 to enable formally verified code and mathematical proofs.

  • Dispatch
    Leanstral 1.5 Release: State‑of‑the‑art Formal Verification Model

    Mistral AI released Leanstral 1.5, a 119 B‑parameter, Apache‑2.0 model that saturates miniF2F, solves 587 of 672 PutnamBench problems, and discovers previously unknown bugs in open‑source code.

  • Dispatch
    Mistral AI Devstral Release

    Mistral AI has released Devstral, an agentic LLM for software engineering that outperforms open-source models on the SWE-Bench Verified benchmark and is available under the Apache 2.0 license.

  • Dispatch
    Codestral Release Notes

    Mistral AI has released Codestral, a 22B open-weight generative AI model specifically designed for code generation across 80+ programming languages.