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According to Jeff Atwood,

… any reasonably large site that attempts discovery (XRDS-style) that can be proven to work with a few well behaving providers and does not accept our provider should definitely be reported on

I'm not entirely sure that Google App Engine counts as a reasonably large site, or that it works XRDS-style, but I'm asking anyhow.

I'm setting up a new App Engine application with federated login, and I was able to implement login using Google as a provider, as well as using Yahoo!, WordPress, and the username flavour of MyOpenID, but StackExchange ( isn't working. (Neither is the non-username flavor of MyOpenID (, but I'm not sure that's related.).

When using either of the latter sites, the provider authenticates me and asks if I want to share information with the App Engine application, and then when I'm redirected back, App Engine throws up a 500 Server Error, and there's nothing in the application's log.

Here's my handler, just to show that I'm not doing anything different between say Yahoo! and StackExchange:

class TryLogin(webapp2.RequestHandler):
    def get(self):
        providers = {
            'Google'   : '',
            'MyOpenID' : '',
            'Blair Conrad\'s MyOpenID' : '',
            'Blair Conrad\'s Wordpress' : '',
            'Yahoo' : '',
            'StackExchange': '',

        user = users.get_current_user()
        if user:  # signed in already
            self.response.out.write('Hello <em>%s</em>! [<a href="%s">sign out</a>]' % (
                user.nickname(), users.create_logout_url(self.request.uri)))
        else:     # let user choose authenticator
            self.response.out.write('Hello world! Sign in at: ')
            for name, uri in providers.items():
                self.response.out.write('[<a href="%s">%s</a>]' % (
                    users.create_login_url(dest_url= '/trylogin', federated_identity=uri), name))

handlers = [
    ('/trylogin$', TryLogin),

Just in case it's a problem on the Google side, I've filed an issue with them too.

In case it's helpful, the sample will be live at for a little while.

Any ideas?

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