Answer
This is available for your own message sent with @ai off, within 15 minutes, and only if it has no mentions. You can change its text, not replace attachments or edit mentions. AI requests and AI responses cannot be edited this way. Delivered push notifications are not rewritten. Saving closes Edit immediately. If saving fails, reopen Edit to recover the draft during this app session.
On updated builds, saving text that already matches the saved message quietly leaves it unchanged. It does not reset the edit time or remove its saved translation.