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Agent changes Content-Type header for WCF apps (.NET)


With the .NET agent enabled for a Windows Communication Foundation (WCF) application, the response header Content-Type is unexpectedly changed to application/xml.


Basic solutions


Disable the Cross Application Tracing (CAT) feature.

Read about how to change the CAT configuration.

If you need CAT to remain enabled, see the other solutions.

Use the Distributed Tracing (DT) feature instead of CAT.

Read about how to enable the DT configuration.

DT is a new and improved way to accomplish tracing and has enhanced features, compared to CAT, in the New Relic UI.

DT doesn't require modifications to response headers and won't be affected by changing response headers.

Add your desired Content-Type header to both the message and OperationContext.

This solution requires you to modify your application code.


The Cross Application Tracing (CAT) feature is enabled by default in the .NET agent. CAT works by adding headers to both request messages and response messages.

This is what happens in the agent:

  1. The WCF service method implementation executes.
  2. The agent will attempt to add CAT headers to the WCF response by adding an HttpResponseMessageProperty instance, with the CAT headers, to OperationContext.Current.OutgoingMessageProperties.
  3. The custom DispatchMessageFormatter executes and creates a new message where the expected Content-Type header is added directly to the new message.
  4. Microsoft’s WCF implementation will attempt to merge the message properties defined in the response message with the message properties defined in the OperationContext. See this Microsoft reference on ImmutableDispatchRuntime.cs for more details.
  5. The HttpResponseMessageProperty doesn't support merging, so the value defined in the OperationContext is used instead of the value defined in the message.
  6. As a result, the Content-Type header that was originally added to the formatted message is thrown away, and the default Content-Type header is used.

Since the agent must support multiple WCF Bindings, it needs to add our CAT headers to the OperationContext.

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