Within minimalist approaches to syntax, it is often assumed that operations occur only to satisfy interface conditions – legibility requirements imposed by the morphophonological and semantic interfaces on the output of the syntactic derivation (see Chomsky 1995; and in particular, Chomsky's 2000, 2001 'interpretability'-based system). In this talk, I present an argument against this view, based on agreement in the Agent-Focus construction of the Kichean languages (Mayan). I show that the obligatoriness of agreement in this construction cannot be accounted for within a theory that enforces the occurrence of syntactic operations solely through interface conditions. An adequate model of agreement, I argue, requires a syntactic operation that is obligatorily triggered whenever a head with unvalued phi-features is merged, but whose successful culmination is crucially not enforced by the grammar.