This article examines the ways in which EU actors have engaged in incremental changes to the eurozone rules ‘by stealth’ ‒ that is, by reinterpreting the rules and recalibrating the numbers without admitting it in their public discourse. Using the methodological framework of discursive institutionalism to focus on agents’ ideas and discursive interactions in institutional context, the article links EU actors’ reinterpretation of rules to their efforts to ensure greater legitimacy in terms of policy performance and governance processes as well as citizen politics. Using the normative theoretical framework of EU democratic systems theory, it analyses EU actors’ considerations of legitimacy not only in terms of their policies’ ‘output’ performance and citizens’ political ‘input’ but also the ‘throughput’ quality of their governance processes. The article illustrates this by elaborating on the different pathways to legitimation of the European Central Bank and the European Commission.