Abstract: Methods, systems, and apparatus, including computer programs encoded on computer storage media, for implementing a policy layer for controlling how applications interact with internal or external machine learning models. One of the methods includes receiving an original input from an application. One or more input matching processes are performed to identify one or more matching input policy routines. One or more actions are performed according to the one or more matching input policy routines to generate a modified input. The modified input is provided instead of the original input to the machine learning system.