Python rich not formatting markup in custom exception

I'm trying to add formatting in a custom exception with rich. The same string when printed as a string works (the carat becomes bold red), but as an exception it doesn't. It's stranger because the foo=123 gets highlighted automatically in both situations. What am I missing here?

In [7]: class InvalidSpec(ValueError):
   ...:     def __init__(self, args):
   ...:         loc = args.find("=")
   ...:         hdr = "invalid spec: "
   ...:         self.args = (f"{hdr}{args}\n[red][bold]{' ' * (len(hdr) + loc)}^", )
   ...:         

In [8]: rich.print(InvalidSpec("foo=123").args[0])
invalid spec: foo=123
                 ^

In [9]: rich.print(InvalidSpec("foo=123"))
invalid spec: foo=123
[red][bold]                 ^

Looks like the solution is to convert to string first. Not sure why:

In [102]: try:
     ...:     raise InvalidSpec("foo=1")
     ...: except Exception as err:
     ...:     c.print(err)
     ...:     c.print(str(err))
     ...:     
invalid spec: foo=1
 [red][bold]                ^
invalid spec: foo=1
                 ^

Update: thanks to the comment by @InSync, I understand it. when it's not a str already rich will only do some simple highlighting, not full console markup. For full markup, you have to implement __rich__.

In [19]: class InvalidSpec(ValueError):
    ...:     def __init__(self, args):
    ...:         super().__init__(args)
    ...:         hdr = "invalid spec: "
    ...:         loc = len(hdr) + args.find("=")
    ...:         self.args = (f"{hdr}{args}\n{' ' * loc}^", )
    ...:     def __rich__(self):
    ...:         loc = self.args[0].find("^")
    ...:         return self.args[0][:loc] + "[red][bold]" + self.args[0][loc:]
    ...:         

In [20]: pprint(InvalidSpec("foo=123"))
invalid spec: foo=123
                 ^

In [21]: print(InvalidSpec("foo=123"))
invalid spec: foo=123
                 ^