Hello Mark,
I have had a look at the documentation for getting tasks using a filter and as far as I can see you can’t filter by user defined filters. I am not a Todoist premium user so can’t create any to test with but would assume that they are just a collection of built-in filters.
There are other limitations too, this is from their documentation:
When fetching a list of tasks, the API will do so in the following order: -
filter(with or withoutlang) -ids-label_id/project_id
So if you include a filter and ids, then only the filter will be used, if you include ids and project_id, only ids is used, and so on.
Unfortunately you will need to process the list of returned tasks using just one filter, the project ID is a good one, then loop through that list and filter out what you want to see.
Another example might be all the tasks that you are assigned to and then post process all of those that are due today, or that were due yesterday. Or a list of overdue tasks and then process those for the users that you need to send a reminder to.
Regards Alan