The disclosure is directed to a method carried out on a wireless device capable of communicating over a first radio-access network (“RAN”) using a first radio-access technology (“RAT”) and over a second RAN using a second RAT. According to various implementations, the device proxies a data communication session between an application executing on the device and a remote host over the first RAN, requests data content from the remote host during the session, transfers the session from the first RAN to a second RAN based on the amount of the data content that remains to be received from the remote host during the session and based on a radio-access condition of the first RAN or of the second RAN.