This paper examines opaque examples of phrase-level phonology taken from Chilean Spanish under the framework of Stratal Optimality Theory (OT) (Rubach 1997; Bermúdez-Otero 2003, 2019) and Harmonic Serialism (HS) (McCarthy 2008a, b, 2016). The data show an interesting double repair of the coda /s/ taking place at word edges. It is argued that Stratal OT is superior in modelling phonological processes that take place at the interface between morphology and phonology because it embraces cyclicity. Under this model, prosodic structure is built serially, level by level, and in accordance with the morphological structure of the input string. In this way, opacity at constituent edges can be solved. Stratal OT also provides insight into word-internal morphological structure and the domain-specificity of phonological processes. It is demonstrated that a distinction in this model is necessary between the word and the phrase levels, and between the stem and the word levels. As illustrated by the behaviour of Spanish nouns, affixation and the resultant alternations inform us about the domains to which both morphological and phonological processes should be assigned. Against this background, Harmonic Serialism embraces an apparently simpler recursive mechanism in which stepwise prosodic parsing can be incorporated. What is more, it offers insight into the nature of operations in OT, as well as into such problematic issues as structure building and directionality. Nevertheless, despite the model’s ability to solve various cases of opacity, the need to distinguish between two competing repairs makes HS fail when confronted with the Chilean data under examination.