I have outlined a basic example in this Gist  showing the behaviour.  Issue part 1: Content-Type on request Steps: Create a simple JAX-RS annotated resource Create a POST endpoint that takes a content type header param and some content and consumes multiple mime types. Using ResteasyClientBuilder build a client and subsequent  ResteasyWebTarget.proxy(Class) create a proxied client for the resource Invoke the resource passing the content type header param Result: The default (first) header param on the @Consumes annotation will be sent to the server, instead of the user provided type. Issue part 2: Content-Type in ClientRequestContext Steps: Follow the steps for part 1 but register a ClientRequestFilter in your ClientBuilder Invoke the resource passing the content type header param Result Inside ClientRequestFilter.filter(ClientRequestContext) the ClientRequestContext will only contain a single header first in the list, the header from the @Consumes annotation on your proxied resource. However, if you pass you params the other way round, the collection will contain both headers... // client filter will only contain one header. doPost(@HeaderParam("Content-Type) String contentType, byte [] content); // client filter will contain two headers. doPost( byte [] content, @HeaderParam("Content-Type) String contentType);   This feels inconsistent, if not a bug, is there any clarification on the expected behaviour?