Depends. The way I view it, if one party provides a chord progression and a second party writes a melody, they are co-writers. If a song is written and, during performance, a musician suggests an F instead of a C, that’s not a co-write. But, you are correct: context, interpretation and negotiation

Michael,
That’s one way to do it – the traditional way, where melody and words are what constitutes a song. I think that’s an anachronism at this point; it just doesn’t reflect how many songs are actually written. As I said about the situations described in the post, if you contribute to the song, it ultimately comes down to whether you can negotiate an ‘upgrade’ or ‘add-on’ from writer or producer to songwriter.
One other thing… You mentioned you go by ‘words and music’. Then you say a chord change doesn’t count. But isn’t a chord ‘music’? Or do you really mean ‘words and melody’?
Thanks a lot for reading and writing!
Best wishes,
Tony

Paul,
Thanks for reading and writing. In the post I mostly focused on situations where a co-write isn’t planned but a co-writing situation might arise. I agree that, when planning ahead, an even split (when that’s fair) is the best way to get everybody fully and unreservedly ‘invested’ and contributing.
Best wishes,
Tony

The exception would be if the song made a ton of money – then I would demand credit! 🙂

A very thoughtful commentary. As a fellow producer/engineer/songwriter/session singer, I sometimes find myself in similar situations. My only suggestion is that you never know what bit of brilliance anyone adds to a song makes the difference and puts that song over the top. Something that is almost there… is not there. Consequently, I am a big fan of equal splits on songs. In the grand scheme of things, I think it is a very fair minded way to approach this. This is pretty much the standard in Nashville.