Mobile carriers are struggling to cope with the surge in smartphone traffic, which reflects badly on end users who often experience poor connectivity in densely populated urban environments. Data transfers between mobile client applications and their Internet backend services contribute significantly to the contention in radio access networks (RANs). Client applications, however, typically transfer unnecessary data because (i) backend service APIs do not support a fine-grained specification of the data actually required by clients and (ii) clients aggressively prefetch data that is never used. We describe Edge Reduce, an automated approach for reducing the data transmitted from backend services to a mobile device. Based on source-level program analysis, Edge Reduce generates application-specific proxies for mobile client applications that execute part of the application logic at the network edge to filter data returned by backend API calls and only send used data to the client. Edge Reduce also permits the tuning of aggressive prefetching strategies: proxies replace large prefetched objects such as images by futures, whose access by the client triggers the retrieval of the object on-demand. We show that Edge Reduce reduces the RAN traffic for real-world iOS client applications by up to 8×, with only a modest increase in response time.