WCF WebFaultException ExceptionDetail - json

Question

I'm creating a WCF service that returns data in JSON Format. I'm trying to figure out how to best handle exceptions and I'm trying to use the WebFaultException class to return an exception detail message in the response, which can later be outputted to the user.
A simple Test of this method I am trying is as follows
The WCF Service method
<WebInvoke(Method:="POST",
ResponseFormat:=WebMessageFormat.Json)>
<OperationContract()>
Public Function Test() As Object
Throw New WebFaultException(Of String)("Message Details", Net.HttpStatusCode.NotFound)
End Function
From what I found searching for answers to this questions, you should give the service a behaviorconfiguartion which sets includeExceptionDetailInFaults to true.
My Web.Config
<service name="WebserviceExceptionTest.Service" behaviorConfiguration="behavior">
<endpoint address="" behaviorConfiguration="WebserviceExceptionTest.ServiceAspNetAjaxBehavior"
binding="webHttpBinding" contract="WebserviceExceptionTest.Service" />
</service>
<serviceBehaviors>
<behavior name="behavior">
<serviceDebug includeExceptionDetailInFaults="True"/>
</behavior>
Unfortunately, this appears to not do the trick for me and the response still does not include the exception detail, the JSON string looks like this:
{"ExceptionDetail":null,"ExceptionType":null,"Message":"Not Found","StackTrace":null}
Does anyone have any idea of what it is I am doing wrong, or am I just entirely on the wrong path? Thanks!
Edit
The Response I'm getting is always "500 Internal server error" Even though I'd expect it to get a 400 not found. The Error message does contain the "No Content" though.

Answer 1

Set your automaticFormatSelectionEnabled to be false, defaultOutgoingResponseFormat to Json (I believe it even ignores ResponseFormat:=WebMessageFormat.Json)
<standardEndpoint name="" helpEnabled="true" automaticFormatSelectionEnabled="false" defaultOutgoingResponseFormat ="Json" />
http://msdn.microsoft.com/en-us/library/system.servicemodel.description.webhttpendpoint.automaticformatselectionenabled.aspx