Fix: preserve stream_item_type when using include_router for SSE routes

fastapi/routing.py

@@ -1794,6 +1794,14 @@ class APIRouter(routing.Router):
                         self.strict_content_type,
                     ),
                 )
+                new_route = self.routes[-1]
+                if (
+                    isinstance(new_route, APIRoute)
+                    and isinstance(route, APIRoute)
+                    and getattr(route, "stream_item_type", None) is not None
+                ):
+                    new_route.stream_item_type = route.stream_item_type
+                    
             elif isinstance(route, routing.Route):
                 methods = list(route.methods or [])
                 self.add_route(

tests/test_sse.py

@@ -264,6 +264,23 @@ def test_sse_on_router_included_in_app(client: TestClient):
     ]
     assert len(data_lines) == 2
 
+def test_include_router_copies_stream_item_type():
+    sub_router = APIRouter()
+
+    @sub_router.get("/stream_item_type", response_class=EventSourceResponse)
+    async def stream_item_type_route() -> AsyncIterable[Item]:
+        yield items[0]
+
+    assert getattr(sub_router.routes[-1], "stream_item_type", None) == Item
+
+    main_app = FastAPI()
+    main_app.include_router(sub_router)
+
+    route = next(r for r in main_app.routes
+     if r.path == "/stream_item_type")
+    assert getattr(route, "stream_item_type", None) == Item
+
+
 
 # Keepalive ping tests