I gotta disagree with my man Tim on this one (first time for everything, right? haha). I have had a few libraries reach out to me in the past looking for WFH agreements on vocal songs where it was obvious I wasn't the singer, and was claiming 100% ownership. I've never claimed to be a good singer, much less a female singer, lol. So it was clear I had someone else doing the vocals for me.No...they could care less who is singing or playing.
You don't have to show them anything.
In those cases though, I had signed WFH agreements that specifically showed that they were paid a flat-fee for the performance of my lyrics & melodies, and would relinquish all claims to ownership. I customized this WFH agreement from one of Robin Fredrick's books. I sent the library a copy of the form, and from there was cleared to sign the deal.
In a lot of cases, you can probably get away with not having anything, but it's better to err on the side of caution.
And, like Johnny said (and anyone who has ever been divorced can attest to), things can get contentious in a hurry if things go south with the relationship. Not saying your marriage is anything but perfect, but.... well.... there's a reason I'll never get married without a prenup!
~~Matt