When creating an activity through the API associated to a deal, the following information is added automatically to the description (visible to all participants on their calendars):
Deal: XXXXXdeal
Organization: XXXXX
Participants:
Contact: XXXX
How do I tell the API not to add this information?
Have you tried doing this on the Reference page? I just tested adding an Activity and only included the deal_id (of a Deal connected to a Person & Org) and the Org and Person weren’t added to the Activity.