HttpProxy generates the query string by concatenating all the properties in the loadConfig. In case of BaseFilterPagingLoadConfig , the filters are stored in the "filters" key as a List, so when HttpProxy creates the query string it implicitly calls List.toString() which results in meaningless value such as: [com.extjs.gxt.ui.client.data.BaseStringFilterConfig@17663b8, com.extjs.gxt.ui.client.data.BaseStringFilterConfig@18d0147] .

There should be a way to define how the filters are converted into strings when HttpProxy is used, with a default reasonable implementation, maybe a JSON structure in like filters=[{id1:value1}, {id2:value2}] .