1. A computer-implemented method performed by a data processing apparatus, the method comprising: determining search hypotheses for a search query, each search hypothesis defining a search type and respectively corresponding to a resource corpus of a type that matches the search type, and wherein: the search type defined by each search hypothesis is different from the search type defined by each other search hypothesis, and each resource corpus of a particular type is different from a resource corpus of another, different type; for each search hypothesis: generating a hypothesis search query based on the search query and the search type defined by the hypothesis; submitting the hypothesis search query to a search service that performs a search operation on a resource corpus of the type that matches the search type of the search hypothesis and receiving, in response to the search of the resource corpus, data describing resources that are determined to be responsive to the hypothesis search query; determining, from the data describing the resources, a search hypothesis score for the search hypothesis; for each search hypothesis having a search hypothesis score meeting a search hypothesis threshold, providing search results for the search operation performed for the hypothesis search query determined for the search hypothesis; and for each search hypothesis not having a search hypothesis score meeting a search hypothesis threshold, not providing search results for the search operation performed for the hypothesis search query determined for the search hypothesis; wherein a plurality of search hypothesis scores are generated for the search hypotheses and a corresponding plurality of different corpora are searched by corresponding hypothesis search queries.