StepFun: Step 3.5 Flash✓ Catalog verified
Step 3.5 Flash is StepFun's most capable open-source foundation model. Built on a sparse Mixture of Experts (MoE) architecture, it selectively activates only 11B of its 196B parameters per token. It is a reasoning model that is incredibly speed efficient even at long contexts.
Overview
Step 3.5 Flash is StepFun's most capable open-source foundation model. Built on a sparse Mixture of Experts (MoE) architecture, it selectively activates only 11B of its 196B parameters per token. It is a reasoning model that is incredibly speed efficient even at long contexts.
Access: available through the official Stepfun API. Context window: 262K.
Specifications
| API identifier | stepfun/step-3.5-flash |
| Provider | Stepfun |
| Model type | Text LLM |
| Context window | 262K catalog |
| Input modalities | text |
| Output modalities | text |
| Released | Jan 29, 2026 |
| Moderated | No |
| Architecture modality | text->text |
API defaults
{
"top_p": null,
"temperature": null,
"frequency_penalty": null
}Pricing
Live pricing components for stepfun/step-3.5-flash as published in the catalog.
| Pricing component | Raw unit price | Normalized | Unit |
|---|---|---|---|
| Prompt tokens | $1e-7 | $0.10 / 1M | Per input token |
| Completion tokens | $3e-7 | $0.30 / 1M | Per output token |
| Request fee | N/A | N/A | Per request |
| Image fee | N/A | N/A | Per image unit |
| Web search fee | N/A | N/A | Per search request |
Cost calculator
Estimate monthly spend from your own token volumes.
Estimated Total Cost
$0.11Calculated from current list pricing in the ModelsAtlas catalog.Capabilities
| Capability | Status | What it means |
|---|---|---|
| Visual Understanding | Not advertised | Image and document analysis support |
| Audio Processing | Not advertised | Speech and voice aligned flows |
| Tool Calling | ✓ Supported | Supports tools / function calling |
| Self-Hosting | Not advertised | Deploy outside managed APIs |
Capability signals
Directional signals derived from model metadata and capability tags — not official benchmark submissions.
Quick start
Call StepFun: Step 3.5 Flash through an OpenAI-compatible client.
from openai import OpenAI
client = OpenAI(
base_url="https://openrouter.ai/api/v1",
api_key="YOUR_API_KEY"
)
response = client.chat.completions.create(
model="stepfun/step-3.5-flash",
messages=[{"role": "user", "content": "Explain quantum physics."}]
)
print(response.choices[0].message.content)Alternatives
Closest models by context window from a different provider.
| Model | Provider | Context | Input /1M | Actions |
|---|---|---|---|---|
| ByteDance Seed: Seed 1.6 | Bytedance Seed | 262K | $0.25 | View → |
| ByteDance Seed: Seed 1.6 Flash | Bytedance Seed | 262K | $0.07 | View → |
Sources & attribution
Pricing and metadata are maintained in the ModelsAtlas catalog.
Frequently asked questions
How much does StepFun: Step 3.5 Flash cost?
$0.10 per 1M input tokens and $0.30 per 1M output tokens on the official Stepfun API.
What is the context window of StepFun: Step 3.5 Flash?
StepFun: Step 3.5 Flash supports up to 262K tokens of context.
Does StepFun: Step 3.5 Flash support tool / function calling?
Yes, StepFun: Step 3.5 Flash supports tool / function calling — you can register tools and the model will emit structured tool calls.
How do I access StepFun: Step 3.5 Flash?
Use the official Stepfun API with the model id `stepfun/step-3.5-flash`. See the Quick start section above for code examples.