StepFun releases open weights for Step 5 Preview

Chinese lab StepFun released open weights for Step 5 Preview, a sparse mixture-of-experts model with 600 billion total parameters of which 27 billion activate per token, with a 1-million-token context window and native text-and-vision input. StepFun had announced the model in September and promised open weights for 15 October; the release met that date, with BF16 and FP8 weights published on Hugging Face and ModelScope alongside a technical blog post, and an FP4 variant following shortly after.

The weights carry the StepFun Model License 1.0 rather than the Apache 2.0 licence StepFun used for its earlier Step 3.5 Flash model. StepFun’s own documentation describes the new licence as “Apache-2.0-style” but with added attribution and commercial-use conditions — a stricter term than a standard permissive licence, though less restrictive than a fully closed release. StepFun pitched the model at long-horizon agentic work: coding, software engineering, professional knowledge work and finance, and its model card cites internal benchmark results on coding and agentic tasks without giving the comparison models used.

Some secondary coverage reported that Step 5 Preview briefly reached second place on OpenRouter’s usage leaderboard, a claim this entry treats as unverified rather than confirmed. The release continues a run of large open-weight Chinese model launches through 2026, in which StepFun, DeepSeek, Moonshot AI and Zhipu AI have repeatedly used open licensing to compete against closed Western frontier models on headline benchmark scores; StepFun’s shift away from Apache 2.0 for its largest model is a departure from that pattern worth watching rather than a reversal of it, since the licence still permits free weights downloads and most commercial use.