Originally Posted by jsloan

I'm very surprised that UA would sell an AS flight from SAN-SFO. I can't imagine why they'd do that. You said that you booked it on United.com -- did you search for it there? Or did you search for it elsewhere (e.g., Google Flights) and then click a link to buy it on United? If you don't mind sharing, what's the outbound travel date?

The DOT guidance is clear as mud. AS has the right to charge their baggage fees on the itinerary. Whether or not they will do so is entirely up to them; they are allowed to defer to UA's overwater policies if they want. (The DOT tried to make things simple; the problem is, the rules were written by bureaucrats, not by travelers). You should ask on their forum for any relevant experiences.

You will check in at the Alaska desk, not the United desk -- you always check in with the carrier that operates your first flight. (Even if they were code-sharing, you'd still check-in with Alaska if they're operating the flight). Your bags will be tagged to Osaka and transferred automatically.

You will not get the benefit of any United status or credit card waivers or anything when you check in with Alaska.