Something a-la ChatGPT, as in it has to be able to conversational and open domain. Google Assistant doesn't count primarily because you can't ask it stuff like "write a Shakespearean sonnet about Bill Gates".

Has to be accessible from google.com homepage. Has to be open to general public, and easily accessible from Google search home page. Not something you have to get whitelisted for, be in a small experiment or meticulously look for in some settings dialog.

Separate app counts, as long as it's promoted on Google.com homepage to the general public. "Random 1% of users get an invite" doesn't count. "All US users get access but it's not in other locales" does count.

If Assistant (as it's integrated into phones, smart speakers etc.) were to just quietly get this capability, but it weren't promoted on Google.com, that wouldn't count. If it were promoted, it would.