When used in combination with Bean Validation, RESTEasy should wrap validation errors in a ValidationReport which should be included in the REST response, marshalled to JSON or XML, depending on the content type.
This does not work currently in WildFly 8.1.0.Final.
The issue was reported in
RESTEASY-1054. and is marked as fixed in RESTEasy 3.0.9.Final.
However, I can still reproduce the issue with a local build of WildFly 8.2.0.CR1-SNAPSHOT (from the 8.x branch) which includes RESTEasy 3.0.10.Final.
Going back to WildFly 8.1.0.Final, the issue no longer occurs when I add
to the module.xml descriptor of org.jboss.resteasy.resteasy-validator-provider-11.
The same fix works for 8.2.0.CR1-SNAPSHOT.
Looking at the module descriptor, it seems that 9.0.0.Alpha1 is also affected (but I did not test my sample on 9.0.0.Alpha1).