Since direct scanout or sampling can be done interchangeable at any point in time send the formats + modifiers in both results (intersection).

If it helps at all, for fd.o & Collabora, we avoid using arcanist at all, and use git-phab. That ignores untracked files by default. ;) You could also ship this patch for Arcanist to at least allow you to configure --allow-untracked as the default.