Southwest to europe

Southwest and Icelandair will partner to serve Customers through an interline agreement, with bookings on connecting service between the two carriers available through Icelandair channels. The first opportunity to book and fly on connecting itineraries will be in February 2025 via the first shared North American gateway, Baltimore-Washington. In the future, Southwest will offer booking for interline travel through Southwest.com and other places where Southwest flights are offered.
https://www.swamedia.com/news-and-stories/news-release/southwest-airlines-and-icelandair-sign-commercial-agreement-MCVSV7AYRZJRHJPESEK7GNS24VV4

Looks like for now you have to do it on the Iceland air website

As Joe pointed out, it won't actually be Southwest flying to Europe but will codeshare with Icelandair who will be operating the aircraft over the Atlantic.

That means all flights to mainland Europe and the UK will connect in Reykjavik.

Note Icelandair's carry on baggage limits are much stricter than Southwest.

Yeah, Southwest is not set up to handle this at all.

It has to all be done through their partner. Keep in mind you can only connect through Baltimore so it’s very limited. You will not earn any southwest reward points nor can you use Southwest points

From what Southwest has said it will be 2026 before they are able to process the booking themselves

I tried to do a test booking with both sites last week using a travel date in the fall and neither site worked, so I suspect they're still working out the process. Perhaps Southwest is busy with their other operational changes.

I'm assuming that it will work like any other codeshare arrangement, with single ticketing and seamless transfer of baggage.

Thank you all. I guess I can forget using SW points to Europe this year.

Icelandair's carry on baggage limits are much stricter than Southwest

Practically every airline's carry on limits are stricter than Southwest's. Icelandair allows a carryon of 21.6"x15.7"x7.8" or 2645 ci. (It's actually 26.84 ci if you use the official limit, 55x40x20 cm = 44 Liters converted to cubic inches.) That's only slightly less than the 2772 ci limit for most US airlines. Biggest problem is the 21.6" length, which, depending on how closely Icelandair checks dimensions, might be too tight for a lot of 22" US sold rolling bags.

However, for everything except "Economy Light", you are allowed one free checked bag up to 62" total inches and 50#, so the "strict" baggage limit shouldn't be a problem for most travelers. In my case, I go strictly carryon, with a 12#, <2000 ci bag for three weeks (and if I can go for three weeks, I could go all summer), so I'd be fine with Economy Light. But if the flight ever included SW, I'd never take it.