Forms act when their configuration changes the range of actions available to others. PlasticAgency names this relational capacity of materials, objects, spaces and institutional arrangements to modify behaviour without requiring intentional agency in the human sense. Material agency examines how physical properties participate in outcomes, actor-network theory distributes action across heterogeneous networks, and design theory shows how form anticipates and channels use. Socioplastics treats agency as an effect of relation. A barrier redirects movement, a seating arrangement changes conversation, a software default alters decision-making, and a regulatory form determines which situations can be administratively recognized. None of these objects acts independently, yet each contributes actively to the configuration in which action occurs. Plasticity adds a second dimension: forms can themselves be modified through use, weathering, repair and reinterpretation. PlasticAgency therefore describes both the capacity to shape relations and the capacity to be reshaped by them. Social form is never purely imposed from above or produced solely by human intention. It emerges through continuous negotiation between bodies, materials, rules and environments whose capacities become visible through what they enable, resist and transform.