International climate governance no longer takes place only in the UNFCCC but is spread across numerous fora that collectively form the international regime complex on climate change. For climate leaders like the EU, the regime complex creates opportunities for strategic activity in its diplomacy across the different fora. This article examines how internal compartmentalisation affected the EU’s diplomacy across the international regime complex on climate change in the negotiation of four climate agreements: the Paris Agreement (UNFCCC), CORSIA (ICAO), the Kigali Amendment (Montreal Protocol), and the Initial Strategy (IMO). It finds that internal compartmentalisation indeed hinders the EU’s pursuit of a comprehensive climate diplomacy, making the regime complex a missed opportunity for the EU. Various combinations of a lack of communication channels, different priorities and policy framings, and a lack of resources and expertise contributed to situations where the EU was limited in using the regime complex.