The post method that creates a new filter is defined in CustomInboundFilterDetailsEndpoint, which is routed at /custom-inbound-filters/{filter_id}/. The collection endpoint CustomInboundFiltersEndpoint, routed at /custom-inbound-filters/, declares only "GET" in publish_status and has no post method, so a standard REST POST /custom-inbound-filters/ create request returns 405 Method Not Allowed. Creating a filter is only reachable by issuing POST /custom-inbound-filters/<arbitrary_id>/, where the filter_id path segment is accepted but never read inside post. A frontend built against the documented "Create a Custom Inbound Filter" collection URL would break, and the generated OpenAPI schema documents the create operation under the detail path with a meaningless required filter_id parameter. Fix: move the post method and its @extend_schema decorator into CustomInboundFiltersEndpoint, drop the unused filter_id parameter from its signature, and add "POST": ApiPublishStatus.PRIVATE to that class's publish_status (removing POST from CustomInboundFilterDetailsEndpoint).