That is good context - that sounds like an implicit commitment to be working on that project. If a co-founder of mine bailed because they found something that sounds more fun, that doesn't seem like a good working relationship and would discourage me from ever working with them again.

which is why I'm asking before I sign papers

Ah, I may have misunderstood - you are saying you would get a 15% co-founder stake, not that you already have 15%? In that case, no worries, you are good on either project.

thanks

What is your experience level?

How long have they been doing it and is that 50k ARR 200k ARR?

15% seems low.