From eb0f83137c8262f9febf25a454479cc7644dc3e9 Mon Sep 17 00:00:00 2001
From: Alexander Rauhut <alexander.rauhut@adnova.gmbh>
Date: Sun, 8 Mar 2026 17:25:54 +0100
Subject: [PATCH] =?UTF-8?q?=F0=9F=90=9B=20Fix=20stream=20schema=20lost=20w?=
 =?UTF-8?q?hen=20using=20include=5Frouter()?=
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Propagate stream_item_type through add_api_route so that
include_router preserves the already-computed stream item
type for SSE and JSONL endpoints.
---
 fastapi/routing.py |  4 +++-
 tests/test_sse.py  | 55 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 58 insertions(+), 1 deletion(-)

diff --git a/fastapi/routing.py b/fastapi/routing.py
index 48c0c2153..6aa4ff243 100644
--- a/fastapi/routing.py
+++ b/fastapi/routing.py
@@ -926,10 +926,11 @@ def _populate_api_route_state(
         generate_unique_id
     ),
     strict_content_type: bool | DefaultPlaceholder = Default(True),
+    stream_item_type: Any | None = None,
 ) -> None:
     route.path = path
     route.endpoint = endpoint
-    route.stream_item_type = None
+    route.stream_item_type = stream_item_type
     if isinstance(response_model, DefaultPlaceholder):
         return_annotation = get_typed_return_annotation(endpoint)
         if lenient_issubclass(return_annotation, Response):
@@ -1414,6 +1415,7 @@ class _EffectiveRouteContext:
                 include_context.included_router.strict_content_type,
                 include_context.strict_content_type,
             ),
+            stream_item_type=route.stream_item_type,
         )
         return context
 
diff --git a/tests/test_sse.py b/tests/test_sse.py
index 86a67f8f9..474ef9499 100644
--- a/tests/test_sse.py
+++ b/tests/test_sse.py
@@ -96,6 +96,18 @@ async def stream_events():
     yield {"msg": "world"}
 
 
+@router.get("/events-typed", response_class=EventSourceResponse)
+async def stream_events_typed() -> AsyncIterable[Item]:
+    for item in items:
+        yield item
+
+
+@router.get("/events-jsonl")
+async def stream_events_jsonl() -> AsyncIterable[Item]:
+    for item in items:
+        yield item
+
+
 app.include_router(router, prefix="/api")
 
 
@@ -274,6 +286,49 @@ def test_sse_on_router_included_in_app(client: TestClient):
     assert len(data_lines) == 2
 
 
+def test_sse_router_typed_openapi_schema(client: TestClient):
+    """Typed SSE endpoint on a router should preserve itemSchema with contentSchema."""
+    response = client.get("/openapi.json")
+    assert response.status_code == 200
+    paths = response.json()["paths"]
+    sse_response = paths["/api/events-typed"]["get"]["responses"]["200"]
+    assert sse_response == {
+        "description": "Successful Response",
+        "content": {
+            "text/event-stream": {
+                "itemSchema": {
+                    "type": "object",
+                    "properties": {
+                        "data": {
+                            "type": "string",
+                            "contentMediaType": "application/json",
+                            "contentSchema": {"$ref": "#/components/schemas/Item"},
+                        },
+                        "event": {"type": "string"},
+                        "id": {"type": "string"},
+                        "retry": {"type": "integer", "minimum": 0},
+                    },
+                    "required": ["data"],
+                }
+            }
+        },
+    }
+
+
+def test_jsonl_router_typed_openapi_schema(client: TestClient):
+    """Typed JSONL endpoint on a router should preserve itemSchema."""
+    response = client.get("/openapi.json")
+    assert response.status_code == 200
+    paths = response.json()["paths"]
+    jsonl_response = paths["/api/events-jsonl"]["get"]["responses"]["200"]
+    assert jsonl_response == {
+        "description": "Successful Response",
+        "content": {
+            "application/jsonl": {"itemSchema": {"$ref": "#/components/schemas/Item"}}
+        },
+    }
+
+
 # Keepalive ping tests