This article examines the development of the EU’s biometric Entry-Exit System (EES) to explore how public plans are enacted through testing of prototypes. Drawing on legislative history, technical documentation, and legal analysis, it argues that the EES reframes political decisions as matters of technical optimisation. A process of empirical validation of prototypes has enrolled agencies, contractors, and lawmakers into socio-material practices that stabilise and legitimise the objectives of public planning. The involvement of consulting firms and managerial discourses—emphasising scalability, feasibility, and pre-emptive responsiveness—entrenches this logic and narrows the scope for legal argument. The article situates the EES within broader debates on infrastructural planning and governance by prototype. It reconstructs the system’s legislative trajectory, focusing on the invention of new procedures of evidence-based persuasion. Finally, it compares the EES to EU-funded biometric projects, iBorderCtrl and TRESSPASS, highlighting how public-led prototyping structures regulatory authority and shapes the available channels for contestation.