StepFun Launches Step 5 Preview, Open-Weights on Oct 15

On September 20, StepFun released its new-generation flagship model Step 5 Preview, positioned for agentic work and claimed to reach frontier-level performance in software engineering and knowledge work—especially financial analysis—with weights to be open-sourced on October 15.

Confirmed

Spec-wise, Step 5 Preview is a MoE architecture with 600B total parameters and 27B active, supporting 1M context and vision input.
For long-horizon tasks, the official demo showed the model running continuously for up to 24 hours across software engineering, web apps, 3D workflows, and programmable hardware, including tasks like GPU kernel optimization; the model can remember prior results, use execution feedback to decide next steps, and keep iterating.
In knowledge work, a single agent action orchestrated 950 web fetches, integrating 1,000 locations with 300,000 monthly records spanning 25 years; another case produced a 17-sheet analytical workbook with source alignment, formulas, and trend models.
On the finance front, StepFun built the FinStepBench benchmark, covering LiveSearch, CorporateValuation, and DeepResearch task types to test the model's ability to identify reliable information and cross-check financial statements.
On openness, the model is available for trial with documentation, and a Discord community is live; the platform also offers a limited-time free Step subscription plan and the Step audio realtime 3 voice model, with a single API key covering language, voice, and other multimodal calls.

Why it matters

Step 5 Preview is a rare flagship from a Chinese vendor whose core selling point is "ultra-long-horizon agentic execution"; the 24-hour continuous task and 950-fetch single action show how agents can move from short conversations to long-running workflows.
The October 15 open-weights pledge puts it in direct competition with open-source flagship models, and the launch of the finance-focused benchmark FinStepBench signals StepFun is betting on knowledge work as its commercialization path.

2026-09-20 ~ 2026-09-20 · 6 related posts

Primary sources

StepFun Launches Step 5 Preview: 600B/27B MoE Flagship, Open Weights Oct 15 — StepFun_ai · 2026-09-20 11:15 (UTC+08:00)
StepFun's Step5Preview: 600B-param MoE with 27B active, top-3 open-source on AA Index — APPSO · 2026-09-20 12:00 (UTC+08:00)
Step 5 Preview Sustains 24-Hour Agent Runs, From GPU Kernels to Post-Training — StepFun_ai · 2026-09-20 11:15 (UTC+08:00)

[source] Step 5 Preview Sustains 24-Hour Agent Runs, From GPU Kernels to Post-Training — StepFun_ai · 2026-09-20
[source] StepFun Launches Step 5 Preview: 600B/27B MoE Flagship, Open Weights Oct 15 — StepFun_ai · 2026-09-20
Step 5 Preview: 950 Web Fetches and 17-Sheet Workbooks in One Agent Action — StepFun_ai · 2026-09-20
StepFun's FinStepBench Puts Step 5 Preview Through Financial Scrutiny — StepFun_ai · 2026-09-20
Step 5 Preview Now Open to Try, Step Plan Subscription Free for Now — StepFun_ai · 2026-09-20
[source] StepFun's Step5Preview: 600B-param MoE with 27B active, top-3 open-source on AA Index — APPSO · 2026-09-20