The EU and its Member States both contribute to the informalization of international relations’ tools by concluding bilateral soft law instruments which prepare, implement, and especially replace international agreements. This contribution analyses the EU practice of applying international soft law and focuses on the institutional challenges deriving from external relations’ soft law instruments. It has the three-fold aim of explaining why the informalization or ‘softification’ of EU bilateral instruments has proliferated, categorizing them according to their function and purpose in international law and EU external relations law and finally assessing the legal implications in EU law resulting from their application. The paper will argue that while, in practice, differences between international treaty law and bilateral soft law disappear, more legal challenges arise in the EU system through bilateral soft law measures than international agreements. This is caused by the rules on EU external representation, dispersed among supranational and intergovernmental EU actors and characterized by their general wording. International soft law tools operate in a politically contested area with several actors and a variety of informal instruments. The flexibility and hybrid character of soft law form an advantage for the institutions at the expense of the rule of law, legal certainty, and legal review.