mistralai/Leanstral-2603

Leanstral 119B A6B

Leanstral is the first open-source code agent designed for Lean 4, a proof assistant capable of expressing complex mathematical objects such as perfectoid spaces and software specifications like properties of Rust fragments.

Built as part of the Mistral Small 4 family, it combines multimodal capabilities and an efficient architecture, making it both performant and cost-effective compared to existing closed-source alternatives.

For more details about the model and its scope, please read the related blog post.

Key Features

Leanstral incorporates the following architectural choices:

• MoE: 128 experts, 4 active per token
• Model Size: 119B parameters with 6.5B activated per token
• Context Length: 256k tokens
• Multimodal Input: Accepts text and image input, producing text output

Leanstral offers these capabilities:

• Proof Agentic: Designed specifically for proof engineering scenarios
• Tool Calling Support: Optimized for Mistral Vibe
• Vision: Can analyze images and provide insights
• Multilingual: Supports English, French, Spanish, German, Italian, Portuguese, Dutch, Chinese, Japanese, Korean, and Arabic
• System Prompt Compliance: Strong adherence to system prompts
• Speed-Optimized: Best-in-class performance
• Apache 2.0 License: Open-source license for commercial and non-commercial use
• Large Context Window: Supports up to 256k tokens

Recommended Settings

• Temperature: 1.0
• Reasoning Effort:
• 'none' → Do not use reasoning
• 'high' → Use reasoning (recommended for complex prompts)

Use reasoning_effort="high" for complex tasks

• Context Length: ≤ 200k tokens recommended

Usage

Mistral-Vibe

Use Leanstral 119B A6B with Mistral Vibe. Install the latest version (2.5.0):

uv pip install mistral-vibe --upgrade

# make sure it's >= 2.5.0

Leanstral can be added by starting vibe and simply running:

/leanstall

This will add leanstral as an additional model, add a system prompt (see LEAD.md) as well as ensure leanstral can be used as a subagent.

!Screenshot 2026-03-16 at 18.03.39

Then just press "tab+shift" a couple times until you see the new "lean" mode and leanstral model.

!Screenshot 2026-03-16 at 18.17.04

Local server

If instead of pinging the Mistral API, you want to use your local vLLM server, you can do the following:

• 1. Spin up a vllm server as explained in [Usage - vllm](#vllm-recommended)
• 2. Create a new agent file called lean.toml in ~/.vibe/agents:

mkdir ~/.vibe/agents/ && touch ~/.vibe/agents/lean.toml

And then copy-paste the following config into ~/.vibe/agents/lean.toml

display_name = "Lean (local vLLM)"
description = "Lean 4 mode using local vLLM"
safety = "neutral"

system_prompt_id = "lean"
active_model = "leanstral"

[[providers]]
name = "vllm"
api_base = "http://:8000/v1"
api_key_env_var = ""
backend = "generic"
reasoning_field_name = "reasoning_content"

[[models]]
name = "mistralai/Leanstral-2603"
provider = "vllm"
alias = "leanstral"
thinking = "high"
temperature = 1.0
auto_compact_threshold = 168000

[tools.bash]
default_timeout = 1200

Note: Make sure to overwrite `` with your server's url.

Then restart vibe and "tab-shift" to "lean" mode.

Give it a try on some "lean" code such as, *e.g.*: PrimeNumberTheoremAnd

Local Deployment

The model can also be deployed with the following libraries, we advise everyone to use the Mistral AI API if the model is subpar with local serving:

• `vllm (recommended)`: See [here](#vllm-recommended).
• `transformers`: WIP ⏳ - follow updates on this PR.
• `SGLang`: WIP ⏳ - follow updates on this PR

vLLM (recommended)

We recommend using this model with the vLLM library to implement production-ready inference pipelines.

_Installation_

1. Make sure to install vllm nightly:

uv pip install -U vllm \
--torch-backend=auto \
--extra-index-url https://wheels.vllm.ai/nightly

Doing so should automatically install `mistral_common >= 1.11.0`.

To check:

python -c "import mistral_common; print(mistral_common.__version__)"

You can also make use of a ready-to-go docker image or on the docker hub.

2. Install transformers from main:

uv pip install git+https://github.com/huggingface/transformers.git

_Launch server_

We recommend that you use Leanstral in a server/client setting.

vllm serve mistralai/Leanstral-2603 \
--max-model-len 200000 \
--tensor-parallel-size 4 \
--attention-backend FLASH_ATTN_MLA \
--tool-call-parser mistral \
--enable-auto-tool-choice \
--reasoning-parser mistral

_Client_

from openai import OpenAI
from huggingface_hub import hf_hub_download

# Modify OpenAI's API key and API base to use vLLM's API server.
openai_api_key = "EMPTY"
openai_api_base = ""

client = OpenAI(
api_key=openai_api_key,
base_url=openai_api_base,
)

TEMP = 1.0
MAX_TOK = 32000
REASONING = "high" # switch to 'none' for faster answers

models = client.models.list()
model = models.data[0].id

prompt = """Define the transition rules as an inductive proposition.

This choice provides better support for proving properties about valid transitions and is generally more natural for modeling state machines in Lean, where you want to express logical rules rather than just computing a yes/no vale…