In June 2018, Directive 2018/822/EU introduced the mandatory disclosure of tax arrangements as the 6th amendment to the Directive on administrative cooperation in the field of taxation (DAC 6). Although presented as a uniform European framework, its broad and vague terminology will lead to differing domestic implementations, immediately challenging the Commission’s argument for a uniform application: secure the proper functioning of the internal market. This article illustrates this tension throughout a profound analysis of DAC 6: although invoking the internal market, it primarily broadly fights against tax avoidance and aggressive tax planning, improving the internal market being only its indirect underlying goal. Given this primary aim, the contribution also assesses the Directive’s relation to other existing tools and confronts it with the freedoms of establishment and services. Developed to enhance the internal market, the question arises whether its additional restrictions can be justified when analysed under EU primary law.