1. A method for extending a schematized XML protocol, comprising: generating, by a client application on a client processor, a request message specifying first data supported by a first version of the XML protocol schema, and comprising an annotation specifying second data supported by a second version of the XML protocol schema, wherein the client application implements the first version of the XML protocol schema and the second version of the XML protocol schema; transmitting the message to a server application, wherein the server application implements the first version of the XML protocol schema and a third version of the XML protocol schema, wherein the third version of the XML protocol schema supports third data, and wherein at least a portion of the second data supported by the second version of the XML protocol schema is the same as at least a portion of the third data supported by the third version of the XML protocol schema; receiving a response to the request message from the server application; identifying returned data in the response, the returned data corresponding to the first data and the portion of the second data that is the same as a portion of the third data.