As someone who has never been in this situation, some of the replies in this thread are very disappointing. They basically seem to amount to, "eh, go pound sand, quitter."
You've put in a ton of work during the hardest period for a startup. That's not worth nothing.
In opposition to these other people, I'd say maybe a frank but congenial discussion with your co-founder is in order. Something along the lines of being burnt out or not agreeing with the current direction of the company, those details don't matter so much. If think you'd want to ultimately fall on you wanting to "pass quietly into the night."
Sometimes people come up with solutions on their own that would surprise you. You just gotta not pigeon hole them into a particular type of response from the start.
The issue has nothing to do with whether he worked hard, it has to do with the fact that the work the company did ended up being not very valuable, so much so that the company is doing something completely different.
Ultimately he created very little value and therefore is entitled to very little value. The company can just go out of business and start fresh!
Raising money is not value.
2-5% could be appropriate. 10% is completely insane.
Bullshit. He helped build a company that could even continue past a year. That's better than literally 80% of the startup market. Look up BLS statistics. Not only that, he helped build a company that could even bother to attract the attention of investors. Prima facia, someone thinks there's value there.
The world is full of jokers talking about starting a business and jokers who have "started" a business but are faking-it-to-make-it off of credit cards. You can't do that for a 18 months. OP has built something. It may not be the software he started out to make, but it's certainly a business that some investor thinks has a chance of being a going concern.
The investors invested on a previous idea.
The fact that the other founders are deciding to pivot is just a luxury of the fact that the company has inertia, not value, which is why they are switching ideas.
Our OP doesn’t want to work on the new idea, the one that might have value, therefore he is entitled to very, very little.
You do not understand the economics of VC backed startups.
None of this has anything to do with anything. In the immortal words of William Munny, "deserve's got nothing to do with it". Any properly papered company has a legal structure based on 20+ years of SFBA lawyers predicting that founders are going to leave their companies, which will need to remain going concerns for any startup ever to be investable.
If you're an exiting founder in an LLC without an operating agreement, you can kill the company. Otherwise: you leave with whatever you vested. That's really all there is to the discussion, for now!
Attracting investors is hard, but it's still the easiest part of building a company. If the company pivots, then it's pretty likely that the vast majority of the previous work no longer has any (monetary) value.