Apparently, CookieJar.extract_cookies doesn't process cookies form local domains which explicitly set domain in Set-Cookie header. That means that headers with domain specified, like "Set-Cookie: foo=baz; Domain=localhost;", are ignored. As far as I can tell, this might be actually part of the standard: https://stackoverflow.com/questions/1134290/cookies-on-localhost-with-explicit-domain/32210291#32210291 . However, it looks like other HTTP clients, including modern versions of both Chrome and Firefox do accept cookies from localhost with explicit domain=localhost, and this change doesn't appear to break existing software in any way. (simple POC to test behavior in browsers: https://gist.github.com/keddad/e2ce034f68b77e59077cdb1e887fa4a1). Maybe it would be best to also allow this behavior in Python?