Third countries' participation in European Union (EU) agencies is attracting increasing attention as venues for differentiated European integration. Research has started to vest into the host of formal agreements regulating this cooperation. However, regulatory outreach by EU agencies may be more important than it is stated de jure, as many forms of cooperation do not necessarily entail formal legal requirements. In this comparative study, we explore the extent to which third countries' de facto participation in EU agencies aligns with what agreements establish de jure. We compare de facto participation and de jure access to EU agencies in four sectoral regimes that differ in their regulatory powers and patterns of EU–third country interdependence: energy, chemicals, public health and border control. We find that agencies with regulatory powers tend to align de facto participation with de jure access. In contrast, agencies that lack regulatory competence are more open to informal de facto cooperation, also in the absence of or beyond formal de jure agreements, in particular when the EU's sectoral interdependence is high. These findings have important implications for the study of external differentiated integration, as they underline the contribution of functionalist technocratic outreach by EU secondary bodies in de facto reinforcing and/or expanding EU centrifugal forces beyond EU borders.