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The Congress of Westphalia, established to put an end to the Thirty Years’ War, has long been viewed in historiography as a major step in the modernisation of European diplomacy. This article reassesses the congress’s role in building international dialogue in the mid-seventeenth century by analysing how this assembly coexisted with diplomatic practices from preceding centuries. By studying, through the lens of the New Diplomatic History, the Franco-Spanish bilateral negotiations that took place on the margins of the Peace of Westphalia, our objective is to determine how that congress influenced these negotiations, authorising and constraining them at the same time. It emerges that although the bilateral talks between France and Spain were partly the heirs of the Westphalian assembly, the congress also hindered the peace between Catholic monarchies because of the role it took in managing international crises. This research provides a new insight into the complexity of the European system in the mid-seventeenth century, demonstrating that the increasingly strong codification of residential diplomatic missions and congress diplomacy is the visible part of a dialogue that still comprises a multitude of meeting practices, which are both age-old and constantly changing.