View the Custom Fields you have access to in a Folder. Get Folder Custom Fields only returns Custom Fields created at the Folder level. Custom Fields created at the List level are not included.

Custom Fields scoped to specific custom task types include an applied_objects array on each field object. Each entry has object_type (19 for custom task type) and object_id (the custom task type ID, matching custom_item_id on tasks). Use Get Custom Task Types to resolve IDs to names. Fields without task-type scoping omit applied_objects. Workspace-scoped Custom Fields are included when querying a Folder.