StepFun Step 5 Preview: announced Sept 20, weights visible on the Oct 15 plan
StepFun’s official news dated September 20, 2026 said flagship Step 5 Preview (sparse MoE ~600B total / ~27B active, up to ~1M-token context) was fully opened via products and API and that model weights would be released on October 15. As of this check, Hugging Face listings for stepfun-ai/Step-5-Preview and Step-5-Preview-BF16 remain visible under StepFun Model License 1.0 (with extra commercial conditions); deployment needs high-end GPUs, and the standalone Transformers full BF16 file-shard conversion/generation workflow is not yet validated end to end.
StepFun’s news post announcing Step 5 Preview is dated September 20, 2026 (the site JS news payload uses date=2026-09-20 and dateLabel “2026年9月20日”). It describes a flagship agentic MoE base with about 600B total and 27B active parameters and up to a 1M-token context with native text and vision, says the model is fully open from that day via products/API, and states that model weights would be released on October 15. The English product page likewise says it is available today through products and API, with open weights on October 15.
As of this round’s check, Hugging Face org stepfun-ai still lists Step-5-Preview (including FP8-oriented examples) and Step-5-Preview-BF16 model cards and file listings. The BF16 card’s vLLM examples include FP8 on roughly one node with 4×B300 or one node with 8×H200, and BF16 on roughly one node with 8×B300 or two nodes with 16×H200. The card notes that actual memory, usable context, and concurrency depend on the framework and KV-cache settings.
Licensing: weights use StepFun Model License 1.0, described as Apache-2.0-style with additional attribution and commercial-license conditions for model use, modification, and distribution—not plain Apache 2.0. Teams planning hardware-product or model-service use should read LICENSE.txt and its exceptions. Separately, the model card states that the standalone Transformers full BF16 `convert.py → generate.py --tp-dir file-shard` workflow has not yet been validated end to end (while checkpoint names/shapes, small-model conversion, load/generation, and full-model streaming-weight inference have been validated in parts).
Date note: the article peg is the promised weight-release day 2026-10-15; the announcement day is StepFun’s 2026-09-20 news date. Hugging Face repository createdAt alone is not treated as a launch date. The HTML news URL is a client-rendered shell; body claims were cross-checked against the same-site JS news payload and the product-page copy.
Sources
- 阶跃发布旗舰模型 Step 5 Preview：向前一步，智能效率的新一代“帕累托前沿” — 阶跃星辰
- StepFun website bundle containing Step 5 Preview news entry — 阶跃星辰
- Step 5 Preview: Advancing the Pareto Frontier — StepFun
- stepfun-ai/Step-5-Preview-BF16 · Hugging Face — Hugging Face / StepFun
- stepfun-ai/Step-5-Preview · Hugging Face — Hugging Face / StepFun