That makes sense. Perhaps a `ClassCastCoercionException extends ClassCastException`.
Will there be a record in the log of the bad request, even if the exception stacktrace isn't there?

Also worth bearing in mind that this response could be to a programmatic invocation of the REST api. So to debug things like that it could be useful to include the exception stacktrace (in case the caller doesn't record exactly what they were doing when they got back the 400, so we can't tell which arg was wrong without the stacktrace.

But no strong feelings...