This paper offers a purely structural account of addiction as a regime of constraint saturation within the framework of Informational Ontology (IO). Working strictly downstream of the completed ontology and its derivative corpus, addiction is analyzed not as a disease, a failure of will, a hidden preference, or a loss of agency, but as a trajectory-level phenomenon in which salience achieves monopoly over an agent’s action space. Under sustained salience pressure, underdetermination collapses upstream of choice, producing reliable path-dependent trajectory convergence despite the persistence of agency and locally intact choice mechanisms. The account explains why agents in addiction act against articulated preferences without invoking executive failure, compulsion, or moral deficit; why freedom (i.e., underdetermination) degrades structurally rather than psychologically; and why responsibility attenuates without disappearing. The resulting picture dissolves the apparent paradox of “choosing what one does not want” by relocating explanation from momentary decisions to the constraint history that produces them.