> How do we create a guideline then for what messages to use?

I think about it this way: what we want to tell customers is "We will provide you some extra, informative text about the error." We don't want to tell them "We are holding the language about this extra, informative information invariant."

What this means is we assert that it exists, and that it's a string, but we don't assert anything about the message.

Does that sound reasonable?