Donut's glasses, whose power view mode reveals the outline of the otherwise invisible Fopsy.
When Carl and Princess Donut both clocked that Tart was whispering to someone unseen over his left shoulder, Donut already had her glasses on. Switching to power view let her make out a faint outline of an invisibility type she otherwise could not see through, and she identified the figure as Fopsy, the adjutant for The Dream, describing them as a weird alien who looks like a basketball on stilts. That identification is what convinced Carl to call Plan B.
Dark cat-sized sunglasses worn by Donut, switchable into a power view mode.
The detection tool that exposed the invisible Dream adjutant.
Also appears in books you've already read — spoiler-safe to their endings: