Extending res judicata beyond the operative part of a judgement into its reasoning is not a mere technicality—it is a constitutional test. This chapter confronts doctrinal and jurisprudential resistance to granting finality to preliminary determinations, questioning whether such resistance holds up against the dual imperatives of legal certainty and procedural freedom. Far from abstract, the analysis reveals the real consequences of denying stabilising force to issues that have been fully litigated and expressly adjudicated. It begins with a critical reading of constitutional case law, showing how courts have implicitly wrestled with the reach of res judicata without naming the issue directly. The chapter then advances a principled argument: under safeguards such as full litigation, adversarial participation, and explicit adjudication, extending finality to preliminary issues is not only constitutionally permissible but constitutionally required. Drawing on Anglo-American collateral estoppel and its Brazilian adaptations, the chapter reframes the scope of res judicata as a constitutional obligation. For those committed to the integrity of adjudication, the challenge is clear: Can a legal system that values both stability and fairness allow fully decided issues to be relitigated? Or does the rule of law itself falter when the judiciary refuses to bind itself to its own determinations?