StepFun open-sources Step 5 Preview, a 600B model built for agent work

Chinese lab StepFun released the weights of its Step 5 Preview model on October 15, keeping the open-sourcing date it promised when the model launched in September. The release includes BF16 and FP8 checkpoints on Hugging Face and ModelScope, a technical blog explaining how the model was built, and supporting open-source developer tools.

Step 5 Preview is a sparse mixture-of-experts model designed for long-horizon agent tasks. It has 600 billion total parameters, 27 billion of them active per token, and supports a context window of up to 1 million tokens. StepFun says it performs well in coding, software engineering and professional knowledge work, with particular strength in finance.

Almost nobody covered it, which is the point of flagging it. Frontier-class open models from China keep arriving with full weights and real documentation, and each one resets what teams can run without a closed-model contract. The technical write-up is also useful on its own for anyone tracking how labs are building agent-focused models.

Our take

This is the quiet kind of release that changes procurement math. A 27B-active model with open weights and a 1M context is a serious baseline for agent workloads.