I'm using the AutoCompleteExtender from the AJAX control toolkit on my aspx page - I have it wired up to a WCF service that is returning a string array and everything works happily.

If I change my service definition to include a demand for the caller to be authenticated, like so:

<OperationContract(), PrincipalPermission(SecurityAction.Demand, Authenticated:=True)> _
Public Function GetLookupValues(ByVal prefixText As String, ByVal count As Integer, ByVal contextKey As String) As String()

Then the autocomplete extender stops working, and I get an authentication error in the service. The service is set up to use ASPNetCompatibility mode, and I was hoping that the extender would pass the authentication credentials for my logged in user - does anyone know how to make this work?