Skip to content
StepFun

StepFun 3.5 Flash

StepFun 3.5 Flash is an open-source sparse MoE reasoning model from StepFun with 196B total parameters and about 11B active per token. It supports a context window of 262.1K tokens and a max output of 262.1K tokens per request.

ReasoningTool UseImplicit Caching
index.ts
import { streamText } from 'ai'


const result = streamText({
  model: 'stepfun/step-3.5-flash',
  prompt: 'Why is the sky blue?'
})
OverviewAboutProvidersThroughputLatencyUptimeStatusSimilarFAQ

Frequently Asked Questions

  • What architecture does StepFun 3.5 Flash use?

    StepFun 3.5 Flash uses a sparse mixture-of-experts design with 196B total parameters and about 11B active per token. Each token routes through eight of 288 experts plus one shared expert, and a 3:1 sliding-window attention ratio keeps long-context inference efficient. See https://deepinfra.com/stepfun-ai/Step-3.5-Flash for details.

  • Is StepFun 3.5 Flash open source?

    Yes. StepFun released StepFun 3.5 Flash under the Apache 2.0 license, with weights available on GitHub and Hugging Face. Through AI Gateway you use hosted inference, so licensing matters only if you also self-host.

  • What is the context window for StepFun 3.5 Flash?

    StepFun 3.5 Flash supports a context window of 262.1K tokens and a max output of 262.1K tokens per request.

  • How well does StepFun 3.5 Flash perform on benchmarks?

    StepFun 3.5 Flash scores 97.3 on AIME 2025, 74.4% on SWE-bench Verified, 86.4% on LiveCodeBench-v6, and 88.2% on Tau2-Bench in StepFun's published evaluations. Validate on your own workload before shipping.

  • How do I call StepFun 3.5 Flash through AI Gateway?

    Use the model identifier stepfun/step-3.5-flash with the AI SDK, the Chat Completions API, the Responses API, the Messages API, or another supported API format. You authenticate with an AI Gateway API key, and no StepFun account is needed.

  • Does AI Gateway support Zero Data Retention for StepFun 3.5 Flash?

    Yes, Zero Data Retention is available for this model. Zero Data Retention is offered on a per-provider basis. See https://vercel.com/docs/ai-gateway/capabilities/zdr for details.

  • Can StepFun 3.5 Flash handle image or video inputs?

    No. StepFun 3.5 Flash is text-only. For visual inputs, pick a multimodal model from the AI Gateway catalog, such as a later Step-series flash release with native image understanding.