This paper explores aspects of Opinion 1/17, handed down by the Court of Justice on 30 April 2019, as an example of the specific procedure of Article 218(11) TFEU and the Court’s evolving practice. The prior Opinion procedure serves to prevent the complications that would arise, both internally and externally, if an international treaty were concluded by the EU and then subsequently found to be incompatible with primary EU law. Opinion 1/17 raised issues of institutional and substantive compatibility, in the form of the principle of autonomy and compliance with the Charter of Fundamental Rights. In handling both, the Court’s approach was characterised by its insistence on the reciprocal nature of the CETA relationship, the separation of the CETA system from EU law, the ability of the EU to engage with such independent and reciprocal dispute settlement processes, and its assessment of the way in which the agreement would be implemented. The Opinion represents an example of the explanatory or didactic form of reasoning typical of recent Opinions, as well as breaking new ground in its conception of an ‘envisaged’ agreement.