There is a lively debate in constructivism today regarding the constitutive and regulatory power of international norms. First-generation studies assume that when states support norms they have fully “internalized” normative ideals. However, the scholarship tends to privilege structure over agency and may not account well for changes in state policies that impact normative architectures. This study extends recent work on agentic constructivism and international law to develop a typology of “norm stewardship” and model of norm change. It conducts a plausibility probe of the new stewardship model by comparing case studies of US policies on exports of sensitive uranium enrichment and reprocessing (ENR) technologies. Case evidence suggests that variations in norm internalization and dissonant norm strains impact multilateral initiatives on nonproliferation. This study concludes with a discussion of wider implications of norm stewardship for theories of international cooperation.