This study investigates whether re-thinking the separation of lexis and grammar in language testing could lead to more valid inferences about proficiency across modes. As argued by Römer, typical scoring rubrics ignore important information about proficiency encoded at the lexis-grammar interface, in particular how the co-selection of lexical and grammatical features is mediated by communicative function. This is especially evident when assessing oral versus written exam tasks, where the modality of a task may intersect with register-induced variation in linguistic output. This article presents the results of an empirical study in which we measured the diversity and sophistication of four-word lexical bundles extracted from a corpus of French proficiency exams. Analysis revealed that the diversity of noun-based bundles was a significant predictor of written proficiency scores and the sophistication of verb-based bundles was a significant predictor of proficiency scores across both modes, suggesting that communicative function as well as the constraints of online planning mediated the effect of lexicogrammatical phenomena on proficiency scores. Importantly, lexicogrammatical measures were better predictors of proficiency than solely lexical-based measures, which speaks to the potential utility of considering lexicogrammatical competence on scoring rubrics.