The category of the ‘irregular’ migrant is usually seen as the quintessential non-status under international law, offering states plenty of discretion while providing few practically accessible rights for migrants. At the same time, certain local authorities have struggled to justify more pragmatic responses when dealing with the reception of irregular immigrants. This article explores a recent trend that potentially holds the key to both conundrums: the invocation of international human rights law, in their defence, by local authorities. More specifically, their engagement of human rights can force international institutions to apply and develop norms in this area. Within this story of legal pluralism, nation states are under increasing pressure to live up to the standards that they had previously avoided. Two examples of ‘frontier cities’ operating in very different constitutional and discursive environments will be used to substantiate the argument. The first concerns support by the city of Utrecht of a case concerning emergency social assistance for undocumented migrants before the European Committee of Social Rights. The second example concerns San Francisco as a sanctuary city in the US and a place with a long history of localization of international human rights law. The article closes with a critical reflection on the potential trajectories that this trend might take and what this means for understandings of legal pluralism as well as future research.