Step 3.5 Flash
StepFunReleased Jan 29, 2026
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....
- Context window
- 262K tokens
- Max output
- 66K tokens
- Input
- text
- Output
- text
- Tokenizer
- Other
- Released
- Jan 29, 2026
- Reasoning
- always on
Pricing
Per 1M tokens. The provider price and our flat 3% fee are separate columns — what you pay is their sum.
| Per 1M tokens | Provider | + 3% fee | You pay |
|---|---|---|---|
| Input | $0.100 | $0.0030 | $0.103 |
| Output | $0.300 | $0.0090 | $0.309 |
Supported parameters
- frequency_penalty
- include_reasoning
- max_tokens
- reasoning
- temperature
- tool_choice
- tools
- top_k
- top_p
Call it
OpenAI-compatible: point your SDK at api.openkey.ai/v1 and use model stepfun/step-3.5-flash.
curl https://api.openkey.ai/v1/chat/completions \
-H "Authorization: Bearer $OPENKEY_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"model": "stepfun/step-3.5-flash",
"messages": [{"role": "user", "content": "Hello"}]
}'import os
from openai import OpenAI
client = OpenAI(
base_url="https://api.openkey.ai/v1",
api_key=os.environ["OPENKEY_API_KEY"],
)
completion = client.chat.completions.create(
model="stepfun/step-3.5-flash",
messages=[{"role": "user", "content": "Hello"}],
)
print(completion.choices[0].message.content)import OpenAI from "openai";
const client = new OpenAI({
baseURL: "https://api.openkey.ai/v1",
apiKey: process.env.OPENKEY_API_KEY,
});
const completion = await client.chat.completions.create({
model: "stepfun/step-3.5-flash",
messages: [{ role: "user", content: "Hello" }],
});
console.log(completion.choices[0].message.content);Questions
- How much does Step 3.5 Flash cost via API?
- Through OpenKey, Step 3.5 Flash costs $0.103 per 1M input tokens and $0.309 per 1M output tokens. That is the provider price ($0.100 / $0.300) plus a flat 3% fee — nothing else.
- What is Step 3.5 Flash's context window?
- Step 3.5 Flash accepts up to 262K tokens of context and returns up to 66K tokens per request.
- Is Step 3.5 Flash OpenAI-compatible?
- Yes. Send requests to OpenKey's /v1/chat/completions endpoint with model "stepfun/step-3.5-flash" using any OpenAI SDK — only the base URL and API key change.
- What inputs does Step 3.5 Flash support?
- Step 3.5 Flash accepts text input and produces text output.