I've already asked this question in the WELD forums, but it's been suggested that I ask it here too:
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I am running WELD 2.3.1.Final inside Tomcat 8, along with RESTeasy 3.0.13.Final and Hibernate Validator 5.2.2.Final, and I have discovered that constraint violations from Hibernate Validator are always being reported as MIME type "text/plain". My understanding of Hibernate Validator is that the ValidationReport is supposed to be returned as JSON if the HTTP Accept request header is set to "application/json".

Specifically, from Chapter 51. Validation

If any constraint violations are detected, Resteasy will return a report in one of a variety of formats. If one of "application/xml" or "application/json" occur in the "Accept" request header, Resteasy will return an appropriately marshalled instance of org.jboss.resteasy.api.validation.ViolationReport

This problem only happens when I use Resteasy + WELD + Hibernate Validator. I have attached two example projects containing the same Arquillian test, except that weld-validator also contains WELD whereas resteasy-validator contains only Resteasy + Hibernate Validator.

Is this a bug, please? It looks like there is a problem with the WELD / Resteasy / Validator bridging.