Currently jboss-jaxrs-api_2.0_spec has a modified RESTEasy value for the default implementation of javax.ws.rs.ext.RuntimeDelegate. It should likely keep the default Jersey / JDK values and load JBoss/RESTEasy implementations of both javax.ws.rs.ext.RuntimeDelegate and javax.ws.rs.client.ClientBuilder using JBoss Modules when running in-container. This is basically what the jaxws-api already does for the javax.xml.ws.spi.Provider.