I guess the problem is that you try to return a json. Spring is converting every response of a @ExtDirectMethod method into json. In this case Spring is converting your json into json. Don't know what the result is of such a conversion. How is the response looks like in Firebug?

To prevent the conversion into json you could try and return an instance of ExtDirectRawJsonStoreResponse. Or create your own class based on ExtDirectRawJsonStoreResponse.