This work, set in the context of the apparel industry, proposes an action-oriented disclosure tool to help solve the sustainability challenges of complex fast-fashion supply chains (SCs). In a search for effective disclosure, it focusses on actions towards sustainability instead of the measurements and indicators of its impacts. We applied qualitative and quantitative content analysis to the sustainability reporting of the world's two largest fast-fashion companies in three phases. First, we searched for the challenges that the organisations report they are currently facing. Second, we introduced the United Nations' Sustainable Development Goals (SDGs) framework to overcome the voluntary reporting drawback of 'choosing what to disclose', and revealed orphan issues. This broadened the scope from internal corporate challenges to issues impacting the ecosystems in which companies operate. Third, we analysed the reported sustainability actions and decomposed them into topics, instruments, and actors. The results showed that fast-fashion reporting has a broadly developed analysis base, but lacks action orientation. This has led us to propose the 'Fast-Fashion Sustainability Scorecard' as a universal disclosure framework that shifts the focus from (i) reporting towards action; (ii) financial performance towards sustainable value creation; and (iii) corporate boundaries towards value creation for the broader SC ecosystem.