X-Appengine-Inbound-Appid header not set

I have two AppEngine modules, a default module running Python and a java module running Java. I am accessing a Java module from the default module using urlfetch. According to the AppEngine docs (cloud.google.com/appengine/docs/java/appidentity), I can check in a Java module that a request is being sent from a module in the same app by checking the X-Appengine-Inbound-Appid header.

However, this header is not installed (in a production deployment). I am using urlfetch in Python module like this:

hostname = modules.get_hostname(module="java")
hostname = hostname.replace('.', '-dot-', 2)
url = "http://%s/%s" % (hostname, "_ah/api/...")
result = urlfetch.fetch(url=url, follow_redirects=False, method=urlfetch.GET)

      

        
        
        
      

    

Please note that I am using the notation:

<version>-dot-<module>-dot-<app>.appspot.com

      

        
        
        
      

    

not a designation:

<version>.<module>.<app>.appspot.com

      

        
        
        
      

    

which for some reason results in a 404 response.

In a Java module, I run a servlet filter that loops through all request headers like this:

Enumeration<String> headerNames = httpRequest.getHeaderNames();
while (headerNames.hasMoreElements()) {
    String headerName = headerNames.nextElement();
    String headerValue = httpRequest.getHeader(headerName);
    mLog.info("Header: " + headerName + " = " + headerValue);
}   

      

        
        
        
      

    

AppEngine sets some headers like. X-AppEngine-Country. But the X-Appengine-Inbound-Appid header is not set.

Why can't I see documented behavior? Any suggestions would be much appreciated.