StepFun opens Step 5 Preview's 600B MoE weights with a custom commercial-use license

StepFun has published official downloadable Step 5 Preview checkpoints after describing the model as a flagship for coding, agents and professional knowledge work. Its primary Hugging Face cards identify a sparse mixture-of-experts architecture with 600 billion total parameters, 27 billion activated per token and a context window of up to one million tokens. The published repositories include distinct precision variants rather than a single universally deployable image, and StepFun documents vLLM, SGLang and Transformers-based paths for inference.

The headline numbers need careful interpretation. Active parameters describe the selected computation path, not the amount of model data that must be stored or accessed. FP8, BF16 and NVFP4 packages have different serving characteristics and hardware requirements. A one-million-token supported window is a stated model capability, not a claim that every deployment can afford maximum-context attention, KV cache, multimodal preprocessing and useful throughput. Operators should calculate their own total memory and batching envelope before selecting a variant.

StepFun describes long-horizon tool workflows, software engineering, document analysis and finance as target use cases. The card attributes results to specified evaluation configurations, including a High reasoning setting and named benchmark harnesses. No comparative model test was run for this article. Benchmarks created or summarized by the publisher are helpful leads for evaluation, not independent proof of superior production agents.

The license is especially consequential. The actual weights are governed by StepFun Model License 1.0, not standard Apache 2.0. The text requires additional prominence and attribution after certain very large usage or revenue thresholds and a separate commercial agreement for specified hardware-device products and sufficiently large model-as-a-service businesses. Exceptions and definitions matter, so organizations should read the original grant and conditions before deploying, redistributing or building a commercial product. Step 5 Preview is a meaningful open-weight release, but open weights do not mean unlimited rights or inexpensive local operation.