At a preliminary stage, steel seismic design is conventionally carried out by assuming idealized member connections. Only after the definition of the main structural members, one proceeds with the sizing of the connections, whose actual geometry/behaviour may or not invalidate the preliminary assumptions. The present study investigates the implications of connection-related design assumptions for steel concentrically-braced frames (CBFs) designed to Eurocode 8. This goal is achieved via an automated seismic analysis and design framework applied to define a comprehensive group of code-compliant CBF archetypes. The effect of conventional idealizations, at the frame design stage, of the diagonal-to-frame gusset connections is examined. Due to the combined effects of member length reduction and boundary condition flexibility on the member’s normalized slenderness, full compatibility of the aforementioned assumption is shown to be unassured. A regression-based modified normalized slenderness factor is proposed to adress this incompatibility.