🐛 Fix Server-Sent Event data ignoring Pydantic serialization_alias

`ServerSentEvent(data=<pydantic model>)` was serialized with `model_dump_json()`, which defaults to `by_alias=False` in Pydantic v2, so `serialization_alias`/`alias` were ignored. Serialize with `by_alias=response_model_by_alias` to match the rest of FastAPI's response serialization (honors aliases by default, respects `response_model_by_alias=False`).

Reported in #15703

fastapi/routing.py

@@ -523,7 +523,13 @@ def get_request_handler(
                             data_str: str | None = item.raw_data
                         elif item.data is not None:
                             if hasattr(item.data, "model_dump_json"):
-                                data_str = item.data.model_dump_json()
+                                # Thread `response_model_by_alias` (default `True`)
+                                # so Pydantic `serialization_alias`/`alias` are
+                                # honored, like the rest of FastAPI's response
+                                # serialization. See #15703.
+                                data_str = item.data.model_dump_json(
+                                    by_alias=response_model_by_alias
+                                )
                             else:
                                 data_str = json.dumps(jsonable_encoder(item.data))
                         else:

tests/test_sse.py

@@ -8,7 +8,7 @@ from fastapi import APIRouter, FastAPI
 from fastapi.responses import EventSourceResponse
 from fastapi.sse import ServerSentEvent
 from fastapi.testclient import TestClient
-from pydantic import BaseModel
+from pydantic import BaseModel, Field
 
 
 class Item(BaseModel):
@@ -16,6 +16,10 @@ class Item(BaseModel):
     description: str | None = None
 
 
+class AliasedItem(BaseModel):
+    name: str = Field(serialization_alias="itemName")
+
+
 items = [
     Item(name="Plumbus", description="A multi-purpose household device."),
     Item(name="Portal Gun", description="A portal opening device."),
@@ -87,6 +91,20 @@ async def sse_items_raw():
     yield ServerSentEvent(raw_data="cpu,87.3,1709145600", event="csv")
 
 
+@app.get("/items/stream-sse-event-alias", response_class=EventSourceResponse)
+async def sse_items_event_alias():
+    yield ServerSentEvent(data=AliasedItem(name="Portal Gun"))
+
+
+@app.get(
+    "/items/stream-sse-event-alias-disabled",
+    response_class=EventSourceResponse,
+    response_model_by_alias=False,
+)
+async def sse_items_event_alias_disabled():
+    yield ServerSentEvent(data=AliasedItem(name="Portal Gun"))
+
+
 router = APIRouter()
 
 
@@ -216,6 +234,27 @@ def test_string_data_json_encoded(client: TestClient):
     assert 'data: "plain text data"\n' in response.text
 
 
+def test_sse_event_data_uses_serialization_alias(client: TestClient):
+    """`ServerSentEvent(data=model)` must honor Pydantic `serialization_alias`.
+
+    Regression test for
+    https://github.com/fastapi/fastapi/discussions/15703
+    """
+    response = client.get("/items/stream-sse-event-alias")
+    assert response.status_code == 200
+    assert 'data: {"itemName":"Portal Gun"}\n' in response.text
+    assert '"name"' not in response.text
+
+
+def test_sse_event_data_respects_response_model_by_alias(client: TestClient):
+    """`response_model_by_alias=False` disables aliasing for SSE event data too,
+    consistent with plainly-yielded items."""
+    response = client.get("/items/stream-sse-event-alias-disabled")
+    assert response.status_code == 200
+    assert 'data: {"name":"Portal Gun"}\n' in response.text
+    assert "itemName" not in response.text
+
+
 def test_server_sent_event_null_id_rejected():
     with pytest.raises(ValueError, match="null"):
         ServerSentEvent(data="test", id="has\0null")