As there is no direct API resource for that, I believe you would need to download all messages - i.e. back to the date/time of the thread's "root/parent" message - and check if the parentId value is equal to the root/parent message.
@demule we do not have that unfortunately. You can suggest it here https://ciscocollabcustomer.ideas.aha.io/ so that other users can support the idea, which would help Product Management prioritise it for the future. Thanks
