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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?'
})
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About StepFun 3.5 Flash

StepFun 3.5 Flash is StepFun's open-source reasoning model, released under the Apache 2.0 license with weights published on GitHub and Hugging Face. The architecture is a sparse mixture of experts with 196B total parameters and about 11B active per token. Each token routes through eight of 288 experts plus one shared expert, so inference cost tracks the active subset rather than the full parameter count.

Two design choices keep long-context work affordable. A 3:1 ratio of sliding-window to full attention layers supports the 262.1K tokens context window without quadratic cost across every layer. Multi-token prediction lets StepFun 3.5 Flash draft several tokens per forward pass, which accelerates generation.

StepFun 3.5 Flash posts frontier-level reasoning scores for its size: 97.3 on AIME 2025, 74.4% on SWE-bench Verified, 86.4% on LiveCodeBench-v6, and 88.2% on Tau2-Bench. Deep reasoning, tool calling, and agentic control loops are first-class capabilities rather than add-ons.

Through AI Gateway, you call StepFun 3.5 Flash with a single API key and get provider routing, automatic failover, and built-in observability. Integrate via the AI SDK, the Chat Completions API, the Responses API, the Messages API, or other supported API formats. No StepFun account is required.