Unfortunately, we don’t have the sort of filtering in our API where you can query to a subset of tasks that match an arbitrary pattern that you specify (i.e. "only the tasks where
completed=true"). We also don’t have an easy way to only get the completed tasks. You can get all incomplete tasks fairly easily by specifying
completed_since=now on the tasks query endpoint - which is admittedly a bit strange, but works - but its converse (get only completed tasks) doesn’t exist
We are evaluating use cases for this sort of thing, though, so you might see it at some point! For now, however, the only way to go about this is to get all the tasks and filter out the incomplete ones on your side.