Interline agreements do not apply to codeshare flights.

If WN has ZED w/ AF, then just use the AF flight #, if they don't you're kinda stuck.

US and UA codeshare extensively out of MDT, US we have ZED with ($32.88 to PHL), UA we do not (about $100 to IAD). I so want ZED on United, but as of yet, no dice.

Anyway, operating carrier only, even some connection carriers are out for some airlines.