@qmx Concerning passing the Exception, tbh, I never saw that either :) but based on the feedback on the mailing list I was pointed to the JAX-RS API Client Specs which do this with its callback interface https://github.com/resteasy/Resteasy/blob/master/jaxrs/jaxrs-api/src/main/java/javax/ws/rs/client/InvocationCallback.java

For the naming, I'm +1 with you , I prefer the onXXX form but again I tried to stick to the the specs mentionned above. I will change that if the rest is also okay.