A quick note: the App Engine development server doesn’t add an User-Agent header for URL fetch requests.
As I commented in a previous post, the App Engine production environment automatically sets an User-Agent (listed below) to all URL Fetch requests. If you set a custom user agent, App Engine will append the below text to your custom header.
AppEngine-Google; (+http://code.google.com/appengine; appid: YOUR_APPLICATION_ID_HERE)
However, the development server doesn’t add this header automatically. If you set a custom User-Agent header, that’s all that will be sent – no other identifying information. If you don’t set an user agent, URL fetches from the development server will not have any user agent information.
This can be an issue while developing applications in the dev server; some APIs require the existence of this header, and will refuse to respond or heavily rate limit requests if this header is missing. For instance, the NewsBlur API requires an user agent header for all requests. If the request doesn’t contain an user agent header, the API will refuse the request even if it’s authenticated.
Always set a custom user agent header which accurately describes your application to all URL fetch requests. If your application does a lot of URL fetches to the same API/server, it may be a good idea to list your email address or a web page with more information about your application.