I could not find anywhere in the proposal that was explicit about this and I know it came up in a Working Group meeting so I am adding this issue to make sure it is tracked. My expectation is that if a service returns a First Party Cookie with the Partitioned attribute, the effect should be to essentially ignore the attribute. The Partitioned attribute should indicate how the cookie should behave in a Third Party context not an indicator that this cookie should only work in a Third Party context. Point being, I would not expect the browser to discard cookies with the Partitioned attribute if they are First Party cookies. For our service, it is very difficult / impossible to tell if the request came from an embedded context and change our behavior at all levels of our code and I would expect that to be similar for others.

If I missed the section in the doc that discusses this, sorry