@app.get("/items/stream-sse-event", response_class=EventSourceResponse)
async def sse_items_event():
    yield ServerSentEvent(data="hello", event="greeting", id="1")
    yield ServerSentEvent(data={"key": "value"}, event="json-data", id="2")
    yield ServerSentEvent(comment="just a comment")
    yield ServerSentEvent(data="retry-test", retry=5000)


@app.get("/items/stream-mixed", response_class=EventSourceResponse)

```python
from collections.abc import AsyncIterable

from fastapi import FastAPI
from fastapi.sse import EventSourceResponse, ServerSentEvent

app = FastAPI()


@app.get("/events", response_class=EventSourceResponse)
async def stream_events() -> AsyncIterable[ServerSentEvent]:
    yield ServerSentEvent(data={"status": "started"}, event="status", id="1")

async def stream_items(
    last_event_id: Annotated[int | None, Header()] = None,
) -> AsyncIterable[ServerSentEvent]:
    start = last_event_id + 1 if last_event_id is not None else 0
    for i, item in enumerate(items):
        if i < start:
            continue

async def stream_items() -> AsyncIterable[ServerSentEvent]:
    yield ServerSentEvent(comment="stream of item updates")
    for i, item in enumerate(items):

from fastapi.sse import EventSourceResponse, ServerSentEvent
from pydantic import BaseModel

app = FastAPI()


class Prompt(BaseModel):
    text: str


@app.post("/chat/stream", response_class=EventSourceResponse)
async def stream_chat(prompt: Prompt) -> AsyncIterable[ServerSentEvent]:
    words = prompt.text.split()
    for word in words:
        yield ServerSentEvent(data=word, event="token")

from collections.abc import AsyncIterable

from fastapi import FastAPI
from fastapi.sse import EventSourceResponse, ServerSentEvent

app = FastAPI()


@app.get("/logs/stream", response_class=EventSourceResponse)
async def stream_logs() -> AsyncIterable[ServerSentEvent]:
    logs = [
        "2025-01-01 INFO  Application started",
        "2025-01-01 DEBUG Connected to database",
        "2025-01-01 WARN  High memory usage detected",
    ]

{* ../../docs_src/server_sent_events/tutorial001_py310.py ln[34:37] hl[35] *}

## `ServerSentEvent` { #serversentevent }

If you need to set SSE fields like `event`, `id`, `retry`, or `comment`, you can yield `ServerSentEvent` objects instead of plain data.

Import `ServerSentEvent` from `fastapi.sse`:

{* ../../docs_src/server_sent_events/tutorial002_py310.py hl[4,26] *}

你也可以省略返回类型。FastAPI 将使用 [`jsonable_encoder`](./encoder.md) 转换数据并发送。

{* ../../docs_src/server_sent_events/tutorial001_py310.py ln[34:37] hl[35] *}

## `ServerSentEvent` { #serversentevent }

如果你需要设置 `event`、`id`、`retry` 或 `comment` 等 SSE 字段，你可以 yield `ServerSentEvent` 对象，而不是直接返回数据。

从 `fastapi.sse` 导入 `ServerSentEvent`：

{* ../../docs_src/server_sent_events/tutorial002_py310.py hl[4,26] *}

`data` 字段始终会被编码为 JSON。你可以传入任何可被序列化为 JSON 的值，包括 Pydantic 模型。

你也可以省略回傳型別。FastAPI 會使用 [`jsonable_encoder`](./encoder.md) 轉換資料並送出。

{* ../../docs_src/server_sent_events/tutorial001_py310.py ln[34:37] hl[35] *}

## `ServerSentEvent` { #serversentevent }

如果你需要設定 `event`、`id`、`retry` 或 `comment` 等 SSE 欄位，你可以改為 `yield` 出 `ServerSentEvent` 物件，而不是單純的資料。

從 `fastapi.sse` 匯入 `ServerSentEvent`：

{* ../../docs_src/server_sent_events/tutorial002_py310.py hl[4,26] *}

`data` 欄位一律會以 JSON 編碼。你可以傳入任何可序列化為 JSON 的值，包括 Pydantic 模型。

        raise ValueError("SSE 'id' must not contain null characters")
    return v


class ServerSentEvent(BaseModel):
    """Represents a single Server-Sent Event.

    When `yield`ed from a *path operation function* that uses
    `response_class=EventSourceResponse`, each `ServerSentEvent` is encoded
    into the [SSE wire format](https://html.spec.whatwg.org/multipage/server-sent-events.html#parsing-an-event-stream)