I’m not sure if this is a bug, or an undocumented feature.
You can add tasks via the API and not specify a time or duration and they are added fine.
HOWEVER,
You can’t mark them as completed.
From the API you get an error Unprocessable Entity.
If you try and complete them from the application under the deal, no error, but the task is not completed.
If you add a deal task via the application it defaults to a 15 minute duration and doesn’t give you the option to set the duration.
When you edit the deal task from the contact, it then defaults the duration to 30 minutes, and allows you to complete it.
Also, the “duration” field is not used. The duration is calculated by the difference between the edate and duedate fields.
I’ve posted this here to help anyone else, and save them the hours I spent trying to work out why I couldn’t complete some tasks.