ITS OWN ROUTE, not a branch inside create-story above. The two answer different questions with different shapes: that one builds synchronously from an outline and returns the story, this one starts a build on the bus and returns a STATUS, because a deck can carry a hundred slides at two Cosmos commands each — see CreateSlideshowStory for the transport reasoning. Folding them together would also mean softening SlideshowStoryCreationRefusal, which is the guard keeping a slideshow out of the outline path in the first place.

200 FOR EVERY OUTCOME THAT IS NOT A REFUSAL, including the two that did not build anything: a story that already exists (the double click, answered with its id) and one already being built (the second tab). Both are the mechanism working, and both leave the caller with the same next move the storyboard's poll would give it.

422 carries text meant for the user, which is why there is no failure-reason enum beside it. Every refusal on this route — no presenter, a presenter who has left their slot, a deck still rendering, slides whose images were deleted — is something the person pressing the button can act on, and the message names the action. See SlideshowStoryReadiness, which holds the same four clauses the client's own gate holds so the two cannot disagree.

GATED ON DeckToSlideshow AT THE ENDPOINT as well as in the handler, for the reason the presenter route below gives: an action filter short-circuits before MediatR is entered, so with the flag off this endpoint must not exist rather than 422 from a validator that has already run its lookups.