Unless there's a workaround that I'm not aware of, one disadvantage of buying AA codeshares for JL flights is that you will never be able to pull up your reservation and view it to confirm things or make changes, I've found, even though AA does give you the code for the JL ticket.

You are at least able to call JL and get seats in case AA hasn't assigned them for you. (Or in any case I've been able to as a One World Emerald.)

But regarding accessing your booking info on the JL website, they throw the tickets issued by "other carriers" into the same category as those issued by travel agents and advise you to contact them, so you have to book with JL directly in order to access your flight info on their website, unlike most of AA's other OW partners.

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