Leanstral 1.5Mistral AI
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Step 3.5 FlashStepFun
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About
Leanstral 1.5 is an Apache-2.0 licensed model for practical proof engineering in Lean 4, built to make formal verification more powerful and accessible. With 119B total parameters and only 6B active parameters, it delivers a major performance upgrade for theorem proving, agentic proof engineering, and real-world code verification. Leanstral 1.5 was trained through a three-stage process: mid-training, supervised fine-tuning, and reinforcement learning with CISPO. In the multiturn environment, the model receives a theorem statement, submits a proof, gets Lean compiler feedback, and refines its approach until the proof compiles or the budget is exhausted. In the code agent environment, Leanstral works like a developer in a raw filesystem: it edits files, runs bash commands, and uses the Lean language server to inspect goals, errors, and type information in real time.
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About
Step 3.5 Flash is an advanced open source foundation language model engineered for frontier reasoning and agentic capabilities with exceptional efficiency, built on a sparse Mixture of Experts (MoE) architecture that selectively activates only about 11 billion of its ~196 billion parameters per token to deliver high-density intelligence and real-time responsiveness. Its 3-way Multi-Token Prediction (MTP-3) enables generation throughput in the hundreds of tokens per second for complex multi-step reasoning chains and task execution, and it supports efficient long contexts with a hybrid sliding window attention approach that reduces computational overhead across large datasets or codebases. It demonstrates robust performance on benchmarks for reasoning, coding, and agentic tasks, rivaling or exceeding many larger proprietary models, and includes a scalable reinforcement learning framework for consistent self-improvement.
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Platforms Supported
Windows
Mac
Linux
Cloud
On-Premises
iPhone
iPad
Android
Chromebook
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Platforms Supported
Windows
Mac
Linux
Cloud
On-Premises
iPhone
iPad
Android
Chromebook
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Audience
Formal methods engineers and Lean 4 researchers who need an open model for theorem proving, proof debugging, and real-world code verification
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Audience
Developers, researchers, and AI engineers who want a powerful open source foundational AI model capable of fast, deep reasoning, coding assistance, and agentic task execution
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Support
Phone Support
24/7 Live Support
Online
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Support
Phone Support
24/7 Live Support
Online
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API
Offers API
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API
Offers API
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Screenshots and Videos |
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Pricing
Free
Free Version
Free Trial
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Pricing
Free
Free Version
Free Trial
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Reviews/
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Reviews/
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Training
Documentation
Webinars
Live Online
In Person
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Training
Documentation
Webinars
Live Online
In Person
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Company InformationMistral AI
Founded: 2023
France
mistral.ai/news/leanstral-1-5/
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Company InformationStepFun
Founded: 2023
China
static.stepfun.com/blog/step-3.5-flash/
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Categories |
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Integrations
GitHub
Hugging Face
ModelScope
arXiv
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