Hi @Frederic_Malenfant, sorry for the confusion here. The filters available in the API are limited by the indexes we have in our database. As the web product evolved, we had a number of separate indexes built to support different kinds of filtering in the UI, and the API added these to the list of filters apps could use. However, because filters are not all built on the same index, they’re not all compatible with each other and, as you’ve noticed, sometimes they’re ignored.
As part of the work outlined in Sections are dead! Long live sections!, we are building and backfilling a new index to support the new canonical structure for tasks, and I anticipate that we will be able to unify the filters that are available in the API. Unfortunately, I don’t yet have a timeline for when the new index/filters will be ready for use.
Notably, the “assignee” filter here is misleading. The “Must specify exactly one of…” message will continue to be true in that you can only specify one container in your filters. “Assignee” wasn’t originally meant to be a filter on the assignee field on tasks, but rather one half of “assignee + workspace” which is a filter on the container. “Assignee + workspace” is how we used to describe “user task lists” before we had full support for them in the API. This is why you get an error when trying to filter on assignee and project—the API interprets this as you trying to filter to two containers. Depending on the cost, we may clean up how these filters work, but I don’t have any concrete plans to share at this time.