Techniques are provided for automatically determining when to trigger instant search. In one technique, while a user is entering text to formulate a search query, and prior to receiving an indication that the user has completed formulation of the search query, a search assistant system determines the most popular queries that include the text the user has already entered. The search assistant system then determines whether to proactively provide, to the user, search results for the top-candidate of those completed queries. That determination may take into account factors such as whether the top-candidate qualifies as a dominant completed query, a richness metric determined for the search results for the particular completed query, and/or a satisfaction metric determined for the search results for the particular completed query. If the search assistant system determines to proactively provide search results for the particular completed query, the search results for the particular completed query are presented to the user prior to receiving any indication that the user has completed formulation of the search query.