If it can be finished in one sitting, it is not the work we mean.
A prompt is a request. A demonstration is a recording. Work that takes days has a schedule, a constraint, and a consequence. The world can change while the system is still running. State has to survive the night.
Horizon is the unit. Not tokens. Not a turn. The question is whether the work continues after interruption — a shift change, a comms gap, a plan that met the world and broke.
That is why the system has to stay inspectable. If you cannot reconstruct why it moved, you cannot hold the work. And when the system meets a decision rather than a computation, it stops. That stop is the design.
Humans still decide.