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.
View API reference- Input and output price
- Input $0.09, Output $0.30, Per 1M tokens
- 24h uptime
- Loading AI Gateway uptime
import { streamText } from 'ai'
const result = streamText({ model: 'stepfun/step-3.5-flash', prompt: 'Why is the sky blue?'})Copy link to headingPlayground
Try out StepFun 3.5 Flash by StepFun. Usage is billed to your team at API rates. Free users (those who haven't made a payment) get $5 of credits every 30 days.
StepFun 3.5 Flash
StepFun 3.5 Flash
Copy link to headingProviders
Route requests across multiple providers. Copy a provider slug to set your preference. Visit the docs for more info. Using a provider means you agree to their terms, listed under Legal.
Copy link to headingUptime24 hours
Direct request success rate on AI Gateway and per-provider. Visit the docs for more info.
Copy link to headingThroughput24 hours
P50 throughput on live AI Gateway traffic, in tokens per second (TPS). Visit the docs for more info.
Copy link to headingLatency24 hours
P50 time to first token (TTFT) on live AI Gateway traffic, in milliseconds. View the docs for more info.
Getting started
Call StepFun 3.5 Flash through AI Gateway with the AI SDK generateText and streamText functions, or through the OpenAI Chat Completions, OpenAI Responses, and Anthropic Messages APIs by changing the base URL. AI Gateway authenticates the request and routes it to an available provider.
Install the AI SDK (pnpm add ai dotenv), create an API key from the API Keys page, and set it as AI_GATEWAY_API_KEY in your environment. Full setup is covered in the text generation quickstart.
import { generateText } from 'ai';import 'dotenv/config';
async function main() { const result = await generateText({ model: 'stepfun/step-3.5-flash', prompt: 'Why is the sky blue?', });
console.log(result.text);}
main().catch(console.error);Top-level parameters
The same StepFun 3.5 Flash request in each API format AI Gateway supports.
import { generateText } from 'ai';import 'dotenv/config';
async function main() { const result = await generateText({ model: 'stepfun/step-3.5-flash', system: 'You are a concise technical assistant.', prompt: 'Summarize the tradeoffs between static generation and SSR.', maxOutputTokens: 1024, });
console.log(result.text);}
main().catch(console.error);Standard parameters like prompt, messages, temperature, and tools work as documented in the AI SDK docs. These are the parameters with model-specific behavior.
| Parameter | Type | Required | Description |
|---|---|---|---|
model | string | Yes | Model ID in the form creator/model, e.g. stepfun/step-3.5-flash. AI Gateway routes the request to an available provider. |
maxOutputTokens | number | No | Hard cap on generated tokens. StepFun 3.5 Flash supports up to 262,114 output tokens. Reasoning tokens count toward this limit. |
reasoning | 'provider-default' | 'none' | 'minimal' | 'low' | 'medium' | 'high' | 'xhigh' | No | Provider-agnostic reasoning effort, available in AI SDK 7 or later. Maps to the provider’s native reasoning configuration; reasoning settings under providerOptions take precedence when both are set. See the Reasoning section below. |
providerOptions | Record<string, JSONValue> | No | AI Gateway routing options under gateway, plus any provider-native options under the provider’s own namespace — see the table below. |
Input limits
| Input | Formats | Sources | Max count | Max size | Limits |
|---|---|---|---|---|---|
| Text | — | — | — | — | Prompt and response share the 262K-token context window |
| Image | — | URL, base64, Uint8Array | — | — | Sent as image parts in messages; counts as input tokens |
Provider options
Set AI Gateway routing options under providerOptions.gateway. For provider-specific options, pass them under the provider’s namespace as documented by the AI SDK.
Learn more in the AI SDK provider docs.
import { generateText } from 'ai';import 'dotenv/config';
async function main() { const result = await generateText({ model: 'stepfun/step-3.5-flash', prompt: 'Why is the sky blue?', providerOptions: { gateway: { only: ['deepinfra'], }, }, });
console.log(result.text);}
main().catch(console.error);These AI Gateway routing options apply to every model. Provider-specific options pass through under the provider’s own namespace (for example providerOptions.anthropic) exactly as documented by the AI SDK.
| Parameter | Type | Required | Description |
|---|---|---|---|
providerOptions.gateway.only | string[] | No | Restrict routing to these provider slugs. Requests fail over only within the listed providers. |
providerOptions.gateway.order | string[] | No | Preferred provider order. Listed providers are tried first; unlisted providers remain available as fallbacks. |
providerOptions.gateway.sort | 'cost' | 'ttft' | 'tps' | No | Rank candidate providers by price, time to first token, or tokens per second instead of the default routing order. |
providerOptions.gateway.zeroDataRetention | boolean | No | Route only to providers with a zero-data-retention policy for this model. |
Routing across providers
AI Gateway serves the same model through multiple providers and fails over automatically. order expresses a preference while keeping every provider eligible; only is a hard allowlist — if none of the listed providers are available the request fails instead of falling back.
Options under a provider's own namespace (for example providerOptions.anthropic) are forwarded to that provider with the request. Providers ignore option namespaces that don't apply to them, so it is safe to set provider options alongside gateway routing options.
Reasoning
AI Gateway bridges reasoning across every API format. The AI SDK exposes a provider-agnostic top-level reasoning level (none, minimal, low, medium, high, or xhigh); the Chat Completions and Responses formats take the same effort under reasoning.effort; and the Anthropic Messages format uses a native thinking token budget. Whichever you send, the gateway maps it to the target model’s native configuration, converting between effort levels and token budgets as needed. Reasoning-related settings under providerOptions take full precedence over the top-level reasoning value and are never merged. Reasoning tokens typically count toward your output-token usage, though how they’re reported and billed varies by provider.
Learn more in the AI Gateway reasoning guide.
import { generateText } from 'ai';import 'dotenv/config';
async function main() { const result = await generateText({ model: 'stepfun/step-3.5-flash', prompt: 'Explain the Monty Hall problem step by step.', reasoning: 'high', });
console.log(result.text);}
main().catch(console.error);Image input
Send images alongside text as message parts. Images count as input tokens.
import { generateText } from 'ai';import 'dotenv/config';
async function main() { const result = await generateText({ model: 'stepfun/step-3.5-flash', messages: [ { role: 'user', content: [ { type: 'text', text: 'Describe this image.' }, { type: 'image', image: 'https://example.com/photo.jpg' }, ], }, ], });
console.log(result.text);}
main().catch(console.error);Tool calling
Expose tools the model can call. Define each tool’s inputs with a Zod schema.
import { generateText, tool } from 'ai';import { z } from 'zod';import 'dotenv/config';
async function main() { const result = await generateText({ model: 'stepfun/step-3.5-flash', prompt: 'What is the weather in San Francisco?', tools: { getWeather: tool({ description: 'Get the current weather for a location', inputSchema: z.object({ location: z.string() }), execute: async ({ location }) => ({ location, temperatureC: 18 }), }), }, });
console.log(result.text);}
main().catch(console.error);Copy link to headingMore models by StepFun
| Model |
|---|
| 256K | 6.9 s | 94 tps | $0.20/M | $1.15/M | Read$0.04/M | — | +1 | | 05/28/2026 |
Copy link to headingAbout 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.
Copy link to headingWhat To Consider When Choosing a Provider
- Configuration: StepFun 3.5 Flash is a reasoning model, so responses include thinking tokens before the final answer. Budget output tokens accordingly, and tune reasoning depth where your harness allows it.
- Configuration: StepFun 3.5 Flash is text-only. Route requests that carry images or video to a multimodal model instead. Benchmark figures above come from StepFun's published evaluations, so validate on your own workload before committing production traffic. For current throughput and latency numbers, see live metrics on this page.
- Zero Data Retention: Zero Data Retention is available for this model. It is offered on a per-provider and model basis. See the documentation for details.
- Authentication: AI Gateway authenticates requests using an API key or OIDC token. You do not need to manage provider credentials directly.
Copy link to headingWhen to Use StepFun 3.5 Flash
Best for
- Agentic Tool Loops: Pipelines that chain deep reasoning with multi-step tool calls
- Math and Reasoning Workloads: Competition-style problems backed by a 97.3 AIME 2025 score
- Cost-Efficient Coding Agents: Software engineering tasks served by a 74.4% SWE-bench Verified model with only 11B active parameters
- Long-Context Analysis: Repository or document review that uses the full 262.1K tokens window
- High-Volume Production Traffic: Workloads where sparse MoE activation keeps per-request costs low
Consider alternatives when
- Multimodal Inputs: StepFun 3.5 Flash is text-only, so route image or video requests to a vision-capable model
- Maximum Benchmark Headroom: Larger frontier models still lead on the hardest coding and research suites
- Simple Single-Turn Chat: Reasoning tokens add output overhead that lightweight conversation doesn't need
Copy link to headingConclusion
StepFun 3.5 Flash gives you open-weight frontier reasoning without dense-model inference costs. Strong AIME, SWE-bench, and Tau2-Bench results make StepFun 3.5 Flash a practical default for agents that think before they act. Route it through AI Gateway and you get failover, observability, and one key for every model in the catalog.
Copy link to headingFrequently 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-flashwith 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.