StepFun releases Step 5 Preview

Background

StepFun (阶跃星辰) is a Chinese AI company that develops large language models and related systems. By September 2026 the company was positioning its work around agents, which are AI systems that carry out multi-step tasks, as well as around general-purpose foundation models that many applications can build on.

What happened

In September 2026 StepFun released Step 5 Preview as its new flagship foundation model.^[1][2] A foundation model is a large general-purpose model that other applications and tasks are built on. The model is a sparse Mixture-of-Experts design with 600 billion total parameters and 27 billion active parameters.^[3]

The model is offered through the StepFun Open Platform API under the model name step-5-preview.^[4] Its weights were published under the StepFun Model License 1.0.^[3]

Key details

Step 5 Preview supports up to 1 million tokens of context. A context window of that size lets the model take in very long inputs, such as large codebases or many documents, in a single request.^[4] It accepts text, image and video input.^[4]

StepFun aimed the model at agentic software engineering and at professional knowledge work, with a particular focus on finance.^[4] Agentic software engineering means having the model take on multi-step programming work, not only answer questions about code. The license is not the standard Apache 2.0 license; it has its own terms, so anyone using the weights needs to read it.^[3]

Impact

The release adds another large, long-context multimodal model to the choices available to developers, and it gives researchers a set of open weights to examine under a specific license. The company's claims about its strengths are its own, so the release should be judged against independent tests. The sources used for this entry do not describe how the model has been adopted, and no conclusions are drawn here about that.