You are probably using Enterprise WSDL, which contains stubs for all objects. If you expect changes to objects (as you do here), you should use the partner WSDL. The WSDL partner does not contain stubs for objects; instead, the partner's WSDL file defines a single, shared object (sObject) that represents all the objects.
For more information, see the "Using the WSDL Partner" section of Salesforce.com API documents.
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