There are methods and apparatus, including computer program products, for preventing data loss. For example, there is a client agent that automatically performs a data loss prevention action in response to detecting (i) a client navigation event and (ii) a difference between a client data state and a base data state, wherein the detection of the difference between the client data state and the base data state occurs without transmitting data to and from a server. The data loss prevention action can include generating a user prompt. The navigation event can include a navigation that replaces a user interface that the user had used to modify data associated with an application program. The data loss prevention action can be performed without receiving instructions corresponding to the action from the application program.