No etiquette exists for this yet. The technology got here faster than anyone's shared sense of what it means, so the question carries far more weight than the activity deserves.
What follows isn't a rulebook. It's a few distinctions that make the decision easier.
The test that answers most of it
Drop the argument about whether it's "cheating" or "weird". Nobody wins it, and people mostly use it to reach the verdict they'd already picked.
Try this instead: how would I feel if it were discovered rather than disclosed?
If you'd shrug, you're using a piece of software and there's nothing to announce. If you'd wince, that reaction tells you something. It doesn't mean you're doing wrong, but it does mean you've given it a significance you haven't put into words.
One question like that sorts out the partner situation better than any debate over categories.
With a partner
The best reason to bring it up is a dull one: being found out is worse than owning up, every time. An unlocked phone, a notification on the lock screen, a line on a shared account statement. Payment privacy and using a shared device cover the practical side, but no setup removes the risk completely.
If you do raise it, two things help.
Start with what it is, not with a defence. "I've been trying one of those AI chat apps, it's a bit like a game" opens a conversation. A long unprompted explanation of why it means nothing opens an interrogation.
Listen for the question underneath. It's rarely about the app. It's usually "is something missing between us?", and that deserves a real answer, not reassurance about the technology.
Sometimes the honest answer is yes, something is missing. That's worth knowing, and the app isn't to blame.
With friends
The stakes are lower than you'd think. The typical response is curiosity, along the lines of "what's it actually like?", since most people have read about these apps and never tried one.
A short, honest description does the job: it's entertaining, a bit like a novel you can chat to, and costs about the same as a streaming subscription.
Anyone who reacts badly is usually reacting to a stereotype of lonely people, not to you. That's worth thinking about separately, but it's no reason to fib.
At work
Don't.
It's not about stigma. It's about what an employer can see. A work laptop is managed equipment, the network is monitored, and that isn't yours to argue with. Keep it off work devices altogether; the reasoning is in using a shared device.
Wording that helps
Describe the activity, not the relationship. "I use an AI chat app" gets a very different reaction from "I've got an AI girlfriend", and both are true. The first invites a question and the second invites a verdict.
Also say what you get out of it in everyday terms: it's relaxing, it's fun to write with, it fills a quiet evening. Those are ordinary reasons for doing anything, and presenting it as ordinary is usually accurate as well as effective.
Where the real discomfort sits
One situation deserves separate treatment, because being blunt about it helps more than comfort would.
Suppose you notice you don't want to mention it precisely because you're using it in place of something else, such as returning a call, going out or dealing with a problem. Then the reluctance is about the substitution, not about the app. That's what the boundaries piece is about, and it's far better to spot it early than late.
Most people aren't in that boat. They use one of these apps a few evenings a week, and the only awkwardness is conversational, which fades quickly.
The line that usually works
"It's a chat app with a character, it's good for switching off, and I'd rather you heard it from me than stumbled across the subscription."
It's direct, doesn't apologise, and shows you thought of them, which is generally all the other person is really asking.
