Step 5 Preview

Step 5 Preview is StepFun's flagship model for agentic work, a sparse Mixture-of-Experts model with 27B active and 600B total parameters. StepFun StepFun Flagship Agent Model, with a 1M token context window.

POST/v1/ai/chat

$0.01/call

Overview

Property	Value
Model ID	`stepfun/step-5-preview`
Context Window	1,048,576 tokens
Max Output	64,000 tokens
Input Price	$1.48 / 1M tokens
Output Price	$3.99 / 1M tokens

Usage

const res = await fetch('https://api.yepapi.com/v1/ai/chat', {
  method: 'POST',
  headers: {
    'x-api-key': 'YOUR_API_KEY',
    'Content-Type': 'application/json',
  },
  body: JSON.stringify({
    model: 'stepfun/step-5-preview',
    messages: [{ role: 'user', content: "Write unit tests for a function that parses ISO 8601 durations." }],
  }),
});
const { data } = await res.json();
console.log(data.message.content);

curl -X POST https://api.yepapi.com/v1/ai/chat \
  -H "x-api-key: YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"model": "stepfun/step-5-preview", "messages": [{"role": "user", "content": "Write unit tests for a function that parses ISO 8601 durations."}]}'

Request Body

Parameter	Type	Required	Description	Default
`model`	`string`	Yes	Model ID (e.g. `stepfun/step-5-preview`)	—
`messages`	`Message[]`	Yes	Array of `{ role, content }` objects	—
`maxTokens`	`number`	No	Maximum tokens in the response	Model default
`temperature`	`number`	No	Sampling temperature (0.0–2.0)	`1.0`
`topP`	`number`	No	Nucleus sampling threshold	`1.0`
`frequencyPenalty`	`number`	No	Penalize repeated tokens	`0`
`presencePenalty`	`number`	No	Penalize tokens already present	`0`
`stream`	`boolean`	No	Enable SSE streaming	`false`

Info

All AI models use the /v1/ai/chat endpoint. Specify the model with the model field.

Response

{
  "ok": true,
  "data": {
    "model": "stepfun/step-5-preview",
    "message": {
      "role": "assistant",
      "content": "I wrote eight tests: plain hours and minutes, days with time, weeks, fractional seconds, zero duration, missing T separator (rejected), negative values (rejected), and an empty string (rejected)."
    },
    "usage": {
      "promptTokens": 24,
      "completionTokens": 180,
      "totalTokens": 204
    }
  }
}

Streaming

Set "stream": true to receive Server-Sent Events. Each chunk contains a delta object:

data: {"delta":{"content":"I"},"model":"stepfun/step-5-preview","index":0}
data: [DONE]

Under the Hood

We handle auth, billing, and response normalization — you just send messages.