This study investigates an asymmetry in (Mandarin) Chinese-English word-internal code-switching: while Chinese inflectional morphemes readily attach to English verbs, Chinese derivational morphemes are consistently rejected when combined with English lexical bases. This pattern raises questions about how the free morpheme constraint should be formulated in bilingual grammar. Building on the phonetic form (PF) interface condition, as proposed in MacSwan’s [Generative approaches to codeswitching. In B. E. Bullock & A. J. Toribio (Eds.), The Cambridge handbook of linguistic code-switching (pp. 309–335). Cambridge University Press (2009)], we argue that the asymmetry is best explained in a lexicalist model that permits post-syntactic affixation. In this approach, inflectional morphology may attach at PF through PF merger, whereas derivational morphology is assembled prior to syntax. The observed asymmetry thus follows from the distribution of morphology across components and the conditions governing the mapping from syntax to phonology. The findings show that derivational timing shapes code-switching, support the viability of lexicalist models that permit post-syntactic affixation, and indicate that word-internal code-switching is permitted under specific interface conditions.