Answer
It first considers your text, then a relevant hashtag description, then useful text from a completed AI response. Existing attachment descriptions or transcripts can help with an attachment-only start. A greeting alone does not establish a topic.
After three completed exchanges, an automatically named chat gets one review. It changes only when the conversation supports a clearer topic. If there is too little context, the title stays the same.
Names you enter manually are preserved, as are existing names whose origin cannot be verified. Automatic naming does not add an activity message or notification, change the last-message preview or chat order, or add an AI usage fee. It uses the existing web, phone-browser, iOS and Android controls; availability can differ on older releases.