If the first flight in your itinerary is operated by a partner airline,you'll need to check in at the partner airline's website or counter.

For a codeshare flight (a flight sold by one airline but operated by another), the universal rule is that you should check in with the operating carrier—the airline that actually owns and flies the aircraft for the first leg of your journey. For example, if you bought a ticket on the American Airlines website but the flight says "Operated by British Airways," you must go to the British Airways desk or use the BA app to check in. This is because the operating airline is responsible for the weight and balance of the aircraft, the passenger manifest, and the physical handling of your checked baggage. Checking in with the operating carrier ensures that your bag tags are printed correctly for their specific system and that you receive a boarding pass that their gate scanners will recognize. While most major alliances (like oneworld, SkyTeam, or Star Alliance) have integrated systems that allow for some cross-airline check-in, the safest and most reliable method is always to use the app or counter of the airline that is physically providing the airplane for your departure.