1. A method implemented by data processing apparatus, the method comprising: determining, at a user device, that one or more first search queries received in a user interface during a search session are provided to a first search engine; determining, at the user device, that for each of the one or more first search queries, a plurality of respective search results determined by the first search engine as satisfying the respective first search queries are displayed in the user interface; determining, at the user device, that the respective search results determined by the first search engine as satisfying the respective first search queries do not satisfy an informational need of a user that input the one or more first search queries at the user device; and in response to determining that the respective search results determined by the first search engine do not satisfy an informational need: displaying, in the user interface, a second search query and a third search query that is based on the first search queries, wherein the third search query is different from the second search query; and displaying, in the user interface, a message informing the user of an ability to submit either the second search query or the third search query to a second search engine.