Functional Portability, Geometric Limits: Cross-Construction Number Interventions

Cross-construction transfer reveals reusable output control, but which properties of the underlying mechanism does it identify? We fit grammatical-number directions on subject–verb agreement and apply them to disjoint reflexive outputs. Across 21 held-out Pythia size–seed models, all three target constructions reach persistent onset (63/63 target trajectories), versus 0/63 for a fixed orthogonal control; this original comparison uses unmatched edit amplitudes. A separate equal-edit-norm follow-up on exposed original materials retains an advantage over random and orthogonal directions, but not a uniform advantage over a simple lexical-number direction. Further boundary tests show that selected supports concentrate the effect while broader random supports recover much of it, and that changing the writing site changes a fixed direction's structural response. On a prospective BLiMP subset, target gain meets its requirement but joint collateral preservation does not; lowering dose reduces both gain and measured collateral. Supplementary reorientation on exposed materials offers no demonstrated advantage over a diagonal approximation. Together, these studies distinguish portable number control from evidence for a uniquely syntactic, compact, position-invariant, or selectively low-collateral mechanism.