Step 5 Preview

Step 5 Preview is the flagship model for agentic work from StepFun. StepFun announced it on 20 September 2026 and opened its API and studio the same day. The architecture is sparse, a mixture of experts with 600 billion parameters in total, of which 27 billion are active on each token. Its documented context window is one million tokens.

Features

• Image and video input: accepts images and video natively, with text output.
• Long context: documented as a one million token window.
• Measured speed: Artificial Analysis clocks it at 99.8 output tokens a second, with 2.96 seconds before the first token, on StepFun's own API.
• Measured intelligence: Artificial Analysis places it 44 on its Intelligence Index, 24th of the 200 models it tracks.

Who it serves

Developers building agentic applications that need a long context window and image or video input.

Key facts

• Verbosity: Artificial Analysis counts 160M output tokens across its index, which matters because output is the costlier side of the bill.
• Open weights: StepFun's materials give a BF16 release date of 15 October 2026, but the repository held no weights at listing.
• Release date: Artificial Analysis dates the model 18 September, the day it measured it.
• Window: some third-party configurations list 350,000 tokens, while the vendor's documentation states one million.