diff --git a/fastapi/routing.py b/fastapi/routing.py
index e2c83aa7b3..c7973aa22d 100644
--- a/fastapi/routing.py
+++ b/fastapi/routing.py
@@ -840,28 +840,6 @@ class APIRoute(routing.Route):
         self.path = path
         self.endpoint = endpoint
         self.stream_item_type: Any | None = None
-        if isinstance(response_model, DefaultPlaceholder):
-            return_annotation = get_typed_return_annotation(endpoint)
-            if lenient_issubclass(return_annotation, Response):
-                response_model = None
-            else:
-                stream_item = get_stream_item_type(return_annotation)
-                if stream_item is not None:
-                    # Extract item type for JSONL or SSE streaming when
-                    # response_class is DefaultPlaceholder (JSONL) or
-                    # EventSourceResponse (SSE).
-                    # ServerSentEvent is excluded: it's a transport
-                    # wrapper, not a data model, so it shouldn't feed
-                    # into validation or OpenAPI schema generation.
-                    if (
-                        isinstance(response_class, DefaultPlaceholder)
-                        or lenient_issubclass(response_class, EventSourceResponse)
-                    ) and not lenient_issubclass(stream_item, ServerSentEvent):
-                        self.stream_item_type = stream_item
-                    response_model = None
-                else:
-                    response_model = return_annotation
-        self.response_model = response_model
         self.summary = summary
         self.response_description = response_description
         self.deprecated = deprecated
@@ -897,27 +875,6 @@ class APIRoute(routing.Route):
         if isinstance(status_code, IntEnum):
             status_code = int(status_code)
         self.status_code = status_code
-        if self.response_model:
-            assert is_body_allowed_for_status_code(status_code), (
-                f"Status code {status_code} must not have a response body"
-            )
-            response_name = "Response_" + self.unique_id
-            self.response_field = create_model_field(
-                name=response_name,
-                type_=self.response_model,
-                mode="serialization",
-            )
-        else:
-            self.response_field = None  # type: ignore
-        if self.stream_item_type:
-            stream_item_name = "StreamItem_" + self.unique_id
-            self.stream_item_field: ModelField | None = create_model_field(
-                name=stream_item_name,
-                type_=self.stream_item_type,
-                mode="serialization",
-            )
-        else:
-            self.stream_item_field = None
         self.dependencies = list(dependencies or [])
         self.description = description or inspect.cleandoc(self.endpoint.__doc__ or "")
         # if a "form feed" character (page break) is found in the description text,
@@ -969,6 +926,49 @@ class APIRoute(routing.Route):
         self.is_json_stream = is_generator and isinstance(
             response_class, DefaultPlaceholder
         )
+        if isinstance(response_model, DefaultPlaceholder):
+            return_annotation = get_typed_return_annotation(endpoint)
+            if lenient_issubclass(return_annotation, Response):
+                response_model = None
+            else:
+                stream_item = get_stream_item_type(return_annotation)
+                if stream_item is not None:
+                    # Extract item type for JSONL or SSE streaming when
+                    # response_class is DefaultPlaceholder (JSONL) or
+                    # EventSourceResponse (SSE).
+                    # ServerSentEvent is excluded: it's a transport
+                    # wrapper, not a data model, so it shouldn't feed
+                    # into validation or OpenAPI schema generation.
+                    if (
+                        isinstance(response_class, DefaultPlaceholder)
+                        or lenient_issubclass(response_class, EventSourceResponse)
+                    ) and not lenient_issubclass(stream_item, ServerSentEvent):
+                        self.stream_item_type = stream_item
+                    response_model = None
+                else:
+                    response_model = return_annotation
+        self.response_model = response_model
+        if self.response_model:
+            assert is_body_allowed_for_status_code(status_code), (
+                f"Status code {status_code} must not have a response body"
+            )
+            response_name = "Response_" + self.unique_id
+            self.response_field = create_model_field(
+                name=response_name,
+                type_=self.response_model,
+                mode="serialization",
+            )
+        else:
+            self.response_field = None  # type: ignore
+        if self.stream_item_type:
+            stream_item_name = "StreamItem_" + self.unique_id
+            self.stream_item_field: ModelField | None = create_model_field(
+                name=stream_item_name,
+                type_=self.stream_item_type,
+                mode="serialization",
+            )
+        else:
+            self.stream_item_field = None
         self.app = request_response(self.get_route_handler())
 
     def get_route_handler(self) -> Callable[[Request], Coroutine[Any, Any, Response]]: