Maybe it's time for a Wikipost about all the oddities those codeshares bring with them.

I'm afraid some of the info I scribbled in might be outdated, though, since I avoid codeshares like the plague and have very limited recent experience. I hope everyone can contribute and fill in gaps. And feel free to edit the Wikipost in other ways, too, to increase legibility. I'm not very good at formatting...

Really? Sounds good. What was your ticketing carrier, and did you get the seat preassigned or at check-in?

In April 2017, AA codeshare on AY5, got preassigned Y+ and exit row seats. In June 2018, CX codeshare on AY1331, got preassigned Y+ seats.

Also got free wifi (via voucher, though) and free drinks.

Long story short, most things don't work automatically when on codeshare, but most, if not all things can be sorted with manual intervention.

Seat selection: does not work online. CS can assign the requested seat, but you need an agent who knows what (s)he is doing. I have a booking to the Philippines with CX, first and last leg operated by AY. First time I called for seat reservation, agent confirmed my choices, but at the end it did not "stick", Second round, a few weeks later, another agent was able to book the seats for me and now they are visible in MMB. In the past I've gotten free access (with manual help) for Y+ seats on AA marketed, AY operated LH flights.

Free drink & newspaper: you'll have to show your plat card to the FA, as their POS will not have any idea about your status (the hard copy of the passenger manifest shows it correctly). So unless purser checks the paper manifest, you'll not get a newspaper, or personal greeting either.

WiFi access on widebody aircrafts: probably same system behind that as the one that runs the POS apps on their iPhones. Thus, system will not grant you access to internet. So far during all the codeshare flights I had on widebody aircrafts, the crew "fixed" this by providing voucher(s).

Meal pre-selections: no experience

My experiences this year, AA codehares on HEL-ORD-HEL and HEL-JFK-HEL as OWE. Got a BA HEL-LHR and JFK-HEL coming up, preflight stuff applies here, also to AY short haul parts of BA code transatlantic itinerary

Seat selection: not available on MMB, but finnair chat worked, got Y+

Wifi: purser came to say hi all 4 flights. Got full flight coupon after asking 2x, 1 hour the rest, and once it was a night flight so I just used the biz 1 hr (got upgraded)

Only time it worked without a voucher was the 1 hour upgrade

Status recognition: yea on all flights

Extra: 2 our of 4 legs (the overnight transatlantics) had equipment changes which looked like they would make my 21A seem really bad, but I ended up with upgrades on both legs (the overnight ones)

Free drink: was offered at least once, but didn't track them

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How does it work the other way around? If I want to fly BA over the pond, but on AY flight number, can I assign myself a seat? Can I preorder a meal?

And what about AA on AY flight numbers? Seats, meal preorders?

On the BA website, seat selection or meal preorder did not work for an AY-codeshare ticket operated by BA. Did not try the BA app.

The return flight (not yet flown) is an AY codeshare operated by AA. With the AA booking code (which you have to ask for from AY CS) it looks like you can change seats on the AA website. Meal preorder also appears to be available normally.

I have done seat selection also by contacting BA Twitter team.

AY CS was also able to select seats for that flight (ditto for the AA flight).

I've been able to select seat and meal on BA with AY code. It also needs a separate PNR which you can get by calling the AY CS.