In one aspect, a method includes receiving, from a user device, a search query including one or more search terms for a content item. The method identifies a corpus of content items previously delivered to the user device, creates an index of the corpus of content items previously delivered to the user device, and uses the received search terms to search the index of the corpus of content items. The method determines that at least one content item in the index corresponds to the received search terms and delivers, to the user device, a content item search result, where the content item search result includes the at least one content item corresponding to the received search terms. Other implementations of this aspect include corresponding systems, apparatus, and computer programs, configured to perform the actions of the methods, encoded on computer storage devices.