Streaming Output

Streaming allows the model to return content word by word, enhancing user experience. Enable streaming by setting stream: true.

Basic Information

• API URL: https://fast-api.org/v1/chat/completions
• Authentication: Bearer Token (API Key)
• Request Method: POST

Request Parameters

Parameter	Type	Description
model	string	Model name
stream	boolean	Set to true to enable streaming
messages	array	List of conversation messages

Python Streaming Request Example

import json
import requests

url = "https://fast-api.org/v1/chat/completions"
headers = {
    "Authorization": "Bearer sk-your API Key",
    "Content-Type": "application/json"
}
payload = {
    "model": "gpt-4o-mini",
    "stream": True,
    "messages": [
        {"role": "system", "content": "You are a helpful assistant."},
        {"role": "user", "content": "Please write a poem about spring."}
    ]
}

response = requests.post(url, headers=headers, json=payload, stream=True)
buffer = ""

for chunk in response.iter_content(chunk_size=None):
    if chunk:
        buffer += chunk.decode("utf-8")
        while "\n" in buffer:
            line, buffer = buffer.split("\n", 1)
            if not line.strip():
                continue

            if line.startswith("data: "):
                data_line = line[6:].strip()

                if data_line == "[DONE]":
                    break

                try:
                    data = json.loads(data_line)
                    content = data["choices"][0]["delta"].get("content", "")
                    print(content, end="", flush=True)
                except json.JSONDecodeError:
                    buffer = line + "\n" + buffer
                    break

Node.js Streaming Request Example

const https = require('https');

const options = {
  hostname: 'fast-api.org',
  path: '/v1/chat/completions',
  method: 'POST',
  headers: {
    'Authorization': 'Bearer sk-your API Key',
    'Content-Type': 'application/json'
  }
};

const req = https.request(options, (res) => {
  let buffer = '';
  res.on('data', (chunk) => {
    buffer += chunk.toString();
    const lines = buffer.split('\n');
    buffer = lines.pop();
    for (const line of lines) {
      if (line.startsWith('data: ') && line.trim() !== 'data: [DONE]') {
        const data = JSON.parse(line.slice(6));
        const content = data.choices?.[0]?.delta?.content || '';
        process.stdout.write(content);
      }
    }
  });
});

req.write(JSON.stringify({
  model: 'gpt-4o-mini',
  stream: true,
  messages: [{ role: 'user', content: 'Hello!' }]
}));
req.end();

Notes

• During streaming, the server returns data in chunks using SSE (Server-Sent Events) format
• Each line of data starts with data: and ends with data: [DONE]
• Please keep your API Key safe
• It is recommended to use the HTTPS protocol