Server-Sent Events

Reviewed by Proof · Updated

Definition

Server-Sent Events (SSE) is a simple web standard for a server to push a stream of text messages to a client over one open HTTP connection.

Key takeaways

  • SSE is a one-way stream: the server sends, the client listens.
  • Each message is a line starting with data: followed by a blank line.
  • OpenAI-compatible APIs use SSE to deliver streamed replies, with one JSON chunk per data: line.
  • In the OpenAI format, the stream ends with a final data: [DONE] line.
  • Most SDKs parse SSE for you. You only need the details if you read the raw stream yourself.

The format

SSE is defined in the HTML standard. The server responds with the content type text/event-stream and keeps the connection open. It then writes messages like this:

data: {"choices":[{"delta":{"content":"Hel"}}]}

data: {"choices":[{"delta":{"content":"lo"}}]}

data: [DONE]

These chunks are shortened examples. Real chunks carry more fields, such as id, model and finish_reason.

The rules are short:

  • A line that starts with data: holds the message text.
  • A blank line marks the end of one message.
  • Lines starting with : are comments. Servers sometimes send them to keep the connection alive, and clients should ignore them.

Reading a stream by hand

  1. Read the response line by line.
  2. Skip empty lines and lines starting with :.
  3. For each data: line, strip the prefix.
  4. If the rest is [DONE], stop. Otherwise parse it as JSON and take choices[0].delta.content.

A network read can split a line in the middle, so buffer text until you reach a newline before you parse.

On luv13

To ask luv13 for a streamed reply, send "stream": true. See Streaming for the idea and Streaming on luv13 for what's confirmed on luv13.

Example

This Python sketch reads the raw SSE stream from luv13 using the requests library.

import json, os, requests

resp = requests.post(
    "https://api.luv13.ai/v1/chat/completions",
    headers={"Authorization": f"Bearer {os.environ['LUV13_API_KEY']}"},
    json={
        "model": "luv13/glm-5.3-flash",
        "messages": [{"role": "user", "content": "Say hello."}],
        "stream": True,
    },
    stream=True,
    timeout=60,
)
for line in resp.iter_lines(decode_unicode=True):
    if not line or line.startswith(":") or not line.startswith("data:"):
        continue
    data = line[len("data:"):].strip()
    if data == "[DONE]":
        break
    chunk = json.loads(data)
    if chunk.get("choices"):
        print(chunk["choices"][0]["delta"].get("content") or "", end="", flush=True)