New girlfriend and custody swap days??

Asking for a friend... but not really lol. Been dating someone for 4 months, nothing serious yet, but my 11yo daughter asked why she was at pickup last week. My ex immediately texted asking if I'm "bringing new people around the kids" which like... fair point but also we're adults. I haven't brought her to my house during custody time, she was just grabbing coffee with me before I got the kids. What's the protocol here? When did you guys introduce new partners and how did you handle it with your ex?