Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for managing data deliver for data constrained user devices. In one aspect, a method includes the actions of receiving, from a user device, requests for resources. For each of the requests, determining whether the user device is currently subject to a data constraint for receiving data responsive to the request. For each request for which the user device is determined to be subject to the data constraint, provide data that causes the user device to display a prompt to choose whether a resource responsive to the request is to be provided in the unrestricted data form or a restricted data form. A resource can be provided in an unrestricted data form has a first data size and provided in a restricted data form has a second data size that is less than the first data size.