Get off plane in Madrid when booked to Lisbon

You can always ask, but they probably won't do that unless you have a ticket that allows such flexibility.

Nope.

What you're describing is well-known to airlines as a way of faking your way to a lower fare, and they do not look kindly on it.

You bought a ticket from A to C. You are not allowed to just bail out at point B. If you're checking a bag, it's going to where your ticket says you're going, period.

Tell them you plan to get off in Madrid rather than your ticketed destination and yes, they'll probably cancel your ticket (probably with no refund). Airlines view this as a scam against them.

If you want to change your ticket so that it ends in Madrid, you'll need to be up front about that.

I didn't want to complicate matters with my explanation, but here goes. I tested positive for Covid 5 days ago and in 3 days I have this plane ticket. Even though I feel better there is a good chance I will still test positive. Portugal requires US citizens to show a negative test, unlike Spain and many other countries. (I am double-boosted but Portugal doesn't recognize the CDC card.) So if I test positive, Portugal won't let me in! I am not trying to cheat or outsmart anyone. Trying to make the best of a crap situation.

With the added explanation -- you need to show all necessary documents that you require for your final destination at the beginning of the itinerary, and if you don't have them all, you are likely to be denied boarding.

(This is because the airline does not want to transport you partway through the trip, only to have you stuck halfway through when you don't have the necessary documents to enter the final destination. In this case the distances are short, but e.g. in a New York - London - Johannesburg trip, an airline does not want to discover that you are inadmissible to South Africa only when you get to London).

So I would expect that if you can't meet Portuguese entry requirements that you will not be able to board your plane in Santiago de Compostela. Of course, it can always be the case that you get an unattentive agent who lets you board anyway. But if you show up and the airline denies you boarding, it's their right to do so -- you bought a ticket from Santiago de Compostela to Lisbon, it's your responsibility as a passenger to have the necessary entry documents for Portugal (you'll find you agreed to this in the conditions of carriage!) and if you don't they can deny you boarding.

All that to say -- you should plan on formally changing the reservation or making a new one.

You need to change your ticket to end in Madrid, or change the date so you can test negative. Or cancel the ticket entirely and re-book later. If you have trip insurance, it may cover the extra cost under the “trip interruption” provisions.

Rebook to Madrid.

Cash in your plane ticket. You might just get a voucher, or you might have to eat the fare. Take a train to Madrid.

Perhaps you should not travel until you test negative just in case, as not to potentially infect others. Santiago de Compostela is not a bad place to spend a few extra days ;-)

What Carlos said! Please don’t expose others.

This may sound like preaching, or moralizing, but if someone who has recently tested Covid-positive wanted to switch from visiting a country with more stringent entry requirements, to a country with less stringent ones, I for one would not be too inclined to help. The "crap situation" you referred to above, will probably be experienced by some unlucky Spanish people, if you bring Covid into their country. As a tourist, I like to follow the physician's code: "at least do no harm."

He OP is in Spain, quarantine, from what I understand. It will be day 8 of quarantine, and the OP says he has no more symptoms.

I still say to take a train but when the Spanish quarantine days are finished. What is required: 10, 7, 5??

https://www.gov.uk/foreign-travel-advice/spain/coronavirus

"You are not required to self-isolate if you have mild or no symptoms but you should wear a mask and avoid crowded spaces."

See, this is why I didn't want to explain my predicament. To the moralizers: I get it. We don't want to spread Covid. I got off my Camino six days before the end, took a cab for 100 Euros to Santiago (wearing my N95 mask and the driver knew I had Covid), and have isolated in a hotel room. I venture outside with a mask away from others in order not to go stir-crazy. Some thoughtless person got me sick and I want to protect others. Now I am trying to get home to the US and have to change all my flights. I have travel insurance. It will be okay. What I would have appreciated is a little understanding rather than moralizing. It's not helpful and I am doing my best to protect my fellow humans.

None of that was clear from your two posts. The second of the two implied that you would still go into Portugal, if you could.

Hey, thanks for your empathy.

Is your flight home from Lisbon?

Yes, from Lisbon. Thus the issue! I will figure it out.

Have you tried changing your return flight to go from Madrid-home? Most major American carriers will let you change for no fee and if you have travel insurance they will cover the difference if the cost is more (probably up to some limit.) With United, it can easily be done online.

When? As mentioned Santiago de Compostela is not the worst place to spend a few extra days. And if you have time to wait for your boosted immune system to fight it, you might just wait there before you continue to Portugal. But, flying from Santiago de Compostela to Lisbon via Madrid is a bit silly in my opinon, take the train.