You could do this reasonably straightforwardly within an existing programming language: make a string wrapper type that had the standard I/O functionality as a

You could do this reasonably straightforwardly within an existing programming language: make a string wrapper type that had the standard I/O functionality as a "friend" or equivalent, so that printing the string to standard error prints its actual contents, but any other use turns into "The value of this string is not part of a stable API." Or more simply, provide an exception type such that getting its string description isn't a public function, but if it's uncaught from main(), whatever in the language runtime invokes main() is allowed to get the description. That said, it sounds like the "real" problem here is that there's no programmatic way to investigate details an exception, and the answer should likely be to make an extensible structure (to which you can add new fields later) that contains the meaningful information in the exception string.... Ideally the exception string should be a static, probably translatable template string that you can use with the language's printf equivalent to fill in the per-exception data in the structured fields.