APIs for extending MP Rest Client functionality - such as new providers. Example:

@RegisterProvider(UnknownWidgetExceptionMapper.class)
public interface MyClientService {
    @GET
    @Path("/myService/{id}")
    Widget getWidget(@PathParam("id") String id) throws UnknownWidgetException;
}
...
public class UnknownWidgetExceptionMapper implements ResponseExceptionMapper {

    @Override
    UnknownWidgetException toThrowable(Response response) {
        String msg = "Could not find widget with ID of " + response.getHeaderString("WidgetId");
        return new UnknownWidgetException(msg)
    }
}