Abstract Patent applicants and examiners do not always have the same views about what constitutes a patent's relevant prior art. We propose that the processes of categorization and classification variably shape the interface between applicants and examiners by influencing assessments of similarity between new and existing technologies. Some inventions sit in technological domains that cut across the categorical boundaries implied by examiners’ patterns of specialization. Some sit in domains wherein the classification system that guides examiner searches is more volatile. In either of these circumstances, heightened ambiguity leads to more examiner-added citations on patents that are granted. We test and confirm our predictions in a sample of patents granted to semiconductor firms in 2005.