On Tue, Dec 15, 2015 at 2:37 PM, Thomas Steiner <tomac@google.com> wrote:

On a more technical note—trying to be a good citizen [3]—the client
library sets an identifying User-Agent header in Node.js mode.
However, trying to set the corresponding X-User-Agent (note the "X-")
header from a browser context (XMLHttpRequest cannot override the
browser's intrinsic User-Agent for security reasons), this fails with
an error message "Request header field X-User-Agent is not allowed by
Access-Control-Allow-Headers in preflight response". Maybe you could
change your CORS settings and include X-User-Agent in your
Access-Control-Allow-Headers?!