I briefly describe a system for automatically creating an implemented grammar of a natural language based on answers to a web-based questionnaire, then present a grammar of Sahaptin, a language of the Pacific Northwest with complex argument-marking and agreement patterns, that was developed to test the system. The development of this grammar has proved useful in three ways: (1) verifying the correct functioning of the grammar customization system, (2) motivating the addition of a new pattern of agreement to the system, and (3) making detailed predictions that uncovered gaps in the linguistic descriptions of Sahaptin.