4 ideas
An Outside Context Problem is an event so far beyond a civilization's experience that its existing categories, plans and defences cannot even frame it, as when a society meets invaders with technology it has no concept of. Most civilizations meet one only once, and it usually ends them, because they respond with tools built for problems they already understood.
When an inexplicable black-body artifact, the Excession, appears near the star Esperi, a faction within the Interesting Times Gang, an informal cabal of Culture ship Minds, secretly uses it as bait. They let the aggressive Affront civilization move to seize it so the Culture gains a pretext for a war to cripple the Affront. Meanwhile the eccentric GSV Sleeper Service, long presumed a harmless recluse storing people in suspended tableaux, reveals that it has covertly built a war fleet. Human agents like the diplomat Byr Genar-Hofoen are moved around as pieces in a game they cannot see.
In a post-scarcity society with no laws, money or formal government, real power flows through private conversations among the most capable agents. Influence depends on reputation and trust within that peer network. Collective decisions are therefore made in back channels that the nominal citizens never see, and a small conspiracy can steer the whole civilization by controlling what the others believe.
The Excession behaves less like an object than like an examiner. It watches how the civilizations around it react, and it withdraws after seeing them scheme, grab and fight over it. The lens reframes a first-contact crisis: the question is not what the unknown is but what the encounter reveals about those responding. A civilization is judged by whether it meets the unprecedented with curiosity or with opportunism.