Which airline's carryon weight limit applies - operated by or codeshare partner for direct flight?

Should say the limits on your ticket. Sometimes airlines have agreements, sometimes not. Even varies by destination, I have been told. Lacking anything on your ticket, I would go with the most restrictive cause no matter who tells you what, if the agent at the gate says no, it's no ... unless your ticket says otherwise ... maybe.

EDIT: OH, it's direct on United .... United will apply. Sorry read it wrong. Was thinking you had a code shared transfer. My bad. And I am about 98% sure I'm correct.

United.

It's United's plane you are going to step on, right? You will be dealing with their employees at the check-in counter and at the gate, not Lufthansa's. It's a flight operated by United, so you'll get treated as a United passenger, because that's what you are.

Think about it: Plenty of other people will getting on the plane and few if any of them will know or care about the fact that the flight is codeshared with Lufthansa (and probably a dozen other airlines, too, for that matter - none of that matters for your carry-on). They're not going to subject different passengers to different bag limits based on who issued their ticket (good luck trying that!).

It's fairly simple for you: You are dealing with United, you are a United customer on a United flight. Lufthansa won't be anywhere nearby, and not involved in anything. They just got paid for it, United does all the work. The only interactions you have with Lufthansa are probably over - unless something goes wrong with your flight, in that case you may have to deal with Lufthansa, but once you have your boarding pass in hand or on your phone, Lufthansa is out of the picture. Auf Wiedersehen, Lufthansa!

Go with the rules of the airline whose name is on the outside of the plane.

I travel frequently on actual UA flights with LH codeshares, UA rules apply, so don't worry. But it was a good question. Thank goodness because Lufthansa's carry-on limit is crazy low! However, I don't know the class of service you're flying - If it's 'basic economy' you have to 1) Pay $150 for a bag check (horror emoji) 2) Can't get seat assignments until 24 hours ahead at least. I just checked these fares for a neighbor going to Lisbon.... Good luck!

I agree with the responses above that you use United rules for weigh limits.

But keep in mind that if you were to have a layover and would have to switch to another airlines, you would be bound by their rules. For example, this year I'm flying nonstop from MSP to Heathrow. The flight was booked using miles via Air France, but I will be flying Delta, which codeshares with KLM and Air France.

However, on the return journey, I will fly from Heathrow to AMS on a KLM flight, then will switch to a Delta flight in Amsterdam. I will be bound by the KLM rules for both checking and carry ons.