I fail to see how this can be anything other than some form of AR glasses. By now, you can get minimally functional AR glasses that look almost like normal eyeglasses (not unlike the old Google Glass project). If they just act as an I/O device that's dependent on your phone for connectivity and processing power, they can be quite small.
Eyeglasses are the right height and perspective for AI to see and hear what you're seeing and hearing . That's exactly what it needs, in order to be sensitive to the context around you. They also provide ways it can present information to the user (i.e. via heads-up display or bone-conduction audio) that are private to the user and less disruptive than looking at & swiping on a phone.
The AI amulet/pin wearable is the next option, but really a distant second. It can't see or hear what's behind you, it's disadvantaged by being at a lower height and not held as steady as your head, and doesn't really solve the problem of user interaction that glasses would. It would also lack the attention cues that a head-mounted device gets, based on where your head is pointed (even better, if the device can see your eyes, so it knows where you're looking).
The only other options I see are some other kind of head gear. Like, a headband, maybe? I am reminded of old sci fi movies and art, which sometimes featured such accessories. It's not maybe such a crazy idea, for those who don't want to wear eyeglasses and are content to receive only audio feedback.