Studio openapi api-docs crashes with 500 error

[samuelallan72]

Attempting to view the openapi (swagger) api docs for the cms fails. The Swagger web UI loads successfully, but displays a message "Failed to load API definition.". The CMS logs show the following traceback and error:

cms-1  | 2025-07-30 04:47:20,468 ERROR 54 [django.request] [user None] [ip None] log.py:246 - Internal Server Error: /api-docs/
cms-1  | Traceback (most recent call last):
cms-1  |   File "/openedx/venv/lib/python3.11/site-packages/django/core/handlers/exception.py", line 55, in inner
cms-1  |     response = get_response(request)
cms-1  |                ^^^^^^^^^^^^^^^^^^^^^
cms-1  |   File "/openedx/venv/lib/python3.11/site-packages/django/core/handlers/base.py", line 197, in _get_response
cms-1  |     response = wrapped_callback(request, *callback_args, **callback_kwargs)
cms-1  |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
cms-1  |   File "/opt/pyenv/versions/3.11.8/lib/python3.11/contextlib.py", line 81, in inner
cms-1  |     return func(*args, **kwds)
cms-1  |            ^^^^^^^^^^^^^^^^^^^
cms-1  |   File "/openedx/venv/lib/python3.11/site-packages/django/views/decorators/csrf.py", line 56, in wrapper_view
cms-1  |     return view_func(*args, **kwargs)
cms-1  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^
cms-1  |   File "/openedx/venv/lib/python3.11/site-packages/django/views/generic/base.py", line 105, in view
cms-1  |     return self.dispatch(request, *args, **kwargs)
cms-1  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
cms-1  |   File "/openedx/venv/lib/python3.11/site-packages/rest_framework/views.py", line 509, in dispatch
cms-1  |     response = self.handle_exception(exc)
cms-1  |                ^^^^^^^^^^^^^^^^^^^^^^^^^^
cms-1  |   File "/openedx/venv/lib/python3.11/site-packages/rest_framework/views.py", line 469, in handle_exception
cms-1  |     self.raise_uncaught_exception(exc)
cms-1  |   File "/openedx/venv/lib/python3.11/site-packages/rest_framework/views.py", line 480, in raise_uncaught_exception
cms-1  |     raise exc
cms-1  |   File "/openedx/venv/lib/python3.11/site-packages/rest_framework/views.py", line 506, in dispatch
cms-1  |     response = handler(request, *args, **kwargs)
cms-1  |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
cms-1  |   File "/openedx/venv/lib/python3.11/site-packages/drf_yasg/views.py", line 112, in get
cms-1  |     schema = generator.get_schema(request, self.public)
cms-1  |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
cms-1  |   File "/openedx/venv/lib/python3.11/site-packages/drf_yasg/generators.py", line 276, in get_schema
cms-1  |     paths, prefix = self.get_paths(endpoints, components, request, public)
cms-1  |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
cms-1  |   File "/openedx/venv/lib/python3.11/site-packages/drf_yasg/generators.py", line 482, in get_paths
cms-1  |     operation = self.get_operation(view, path, prefix, method, components, request)
cms-1  |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
cms-1  |   File "/openedx/venv/lib/python3.11/site-packages/drf_yasg/generators.py", line 524, in get_operation
cms-1  |     operation = view_inspector.get_operation(operation_keys)
cms-1  |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
cms-1  |   File "/openedx/venv/lib/python3.11/site-packages/drf_yasg/inspectors/view.py", line 45, in get_operation
cms-1  |     responses = self.get_responses()
cms-1  |                 ^^^^^^^^^^^^^^^^^^^^
cms-1  |   File "/openedx/venv/lib/python3.11/site-packages/drf_yasg/inspectors/view.py", line 182, in get_responses
cms-1  |     responses=self.get_response_schemas(response_serializers)
cms-1  |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
cms-1  |   File "/openedx/venv/lib/python3.11/site-packages/drf_yasg/inspectors/view.py", line 268, in get_response_schemas
cms-1  |     serializer = force_serializer_instance(serializer)
cms-1  |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
cms-1  |   File "/openedx/venv/lib/python3.11/site-packages/drf_yasg/utils.py", line 327, in force_serializer_instance
cms-1  |     assert isinstance(serializer, serializers.BaseSerializer), \
cms-1  | AssertionError: Serializer class or instance required, not UnionType
cms-1  | [30/Jul/2025 04:47:20] "GET /api-docs/?format=openapi HTTP/1.1" 500 327998

The environment I was testing in was a local master devstack launched with Tutor (dev mode).

[awais786]

I can look into this.

[sumair-arbisoft]

@awais786 I was able to re-produce the issue and trying to fix this.