SOLID is part of the standard toolkit of many developers – but is it still relevant today? The principles originate from an era of object-oriented class hierarchies. However, in modern environments with microservices, cloud, and functional programming, they are reaching their limits. Vague wording, unnecessary abstraction layers, and dogmatic application often lead to more complexity rather than less.

In this talk, we discuss why SOLID should be critically questioned, which principles are still relevant, and how we can adapt them to today's practices. As an alternative, we learn about CUPID: Composable, Unix philosophy, Predictable, Idiomatic, Domain-based. CUPID is simpler, more practical, and better reflects what we really need: clear responsibilities at the module level, reliable and understandable systems, and a strong connection to the domain. The result is software that is not only theoretically “clean,” but also easy to change, understand, and develop in practice.