Source Poverty and Canonical Admissibility in Fibered Descent Systems

Three independently discovered collapse phenomena — in monadic model theory, categorical semantics, and topos theory — share a common fibered descent structure. In each case, target content is constrained to a canonically determined admissible region computed from a source, and collapse occurs when the source is informationally trivial. We identify two modes of canonical admissibility: a reflective fixed-point mode (available when round-trip transport exists) and a descent-equalizer mode (available for quotient-type transports). Both yield a Level-1 raw capacity law: the number of raw admissible configurations equals v^d, where v is the value base and d is the source slot count; Level-2 coherence constraints remain case-specific. Collapse is the degenerate case d = 0. Two of the three collapse results — from monadic model theory and topos theory — are further unified as instances of the same categorical construction (pushforward in the subobject bifibration). We classify the descent conditions by morphism type and identify a stratification into raw capacity (Level 1, unified) and coherence constraints (Level 2, case-specific).