This paper examines how the EU’s AI regulatory sandboxes test the boundaries of constitutional principles in experimental governance. It assesses whether current frameworks adequately uphold the constitutional principle of transparency while respecting legitimate confidentiality interests. Drawing on EU primary law and comparative constitutional perspectives, the paper argues that transparency should function as a substantive constitutional safeguard in digital regulation. It explores practical mechanisms, such as differentiated transparency levels and anonymised reporting, to reconcile openness with the protection of trade secrets and intellectual property. Comparative case studies across Member States reveal divergences in access criteria and information-sharing practices, and the paper suggests a principled and balanced transparency regime to secure both accountability and innovation, reinforcing an autonomous European model of digital constitutionalism.