1. A computer-implemented method performed by a data processing apparatus, the method comprising: accessing command input logs storing data defining received input sentences, each input sentence being one or more n-grams; identifying from the input sentences first input sentences that do not successfully parse by any parsing rule in a set of parsing rules, each of the parsing rules being associated with an action, and wherein each parsing rule causes a user device to perform the action associated with the parsing rule in response to an input sentence received from the user device being successfully parsed by the parsing rule; for each first input sentence, determining whether the first input sentence has an underserving signal, the underserving signal being a signal that is indicative of one or more actions being underserved; and selecting first input sentences determined to have an underserving signal as candidate input sentences; providing each candidate input sentence to an action analysis process that determines whether a candidate input sentence is to be associated with one of a plurality of actions and upon a positive determination generates a parsing rule for the candidate input sentence, wherein the generated parsing rule is associated with the one of the plurality of actions and a successful parse of an input sentence received at a user device by the generated parsing rule causes the user device to perform the one of the plurality of actions associated with the generated parsing rule; and persisting each generated parsing rule and data describing the action with which the generated parsing rule is associated to command models data that is accessible by a language processing system that parses command input sentences by the generated parsing rules and determines actions to perform based on a successful parses.