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Make your device appear on its own
Publish one retained message and Home Assistant creates the entity for you — no YAML, no restart. Getting the topic and payload right is the whole trick.
The two things that go wrong
The entity vanishes after a restart
The discovery message was not retained. Home Assistant only learns the device exists when it reads that message — if the broker did not keep it, nobody says it again and the entity is gone.
You cannot rename it or put it in an area
No unique_id. Without one, Home Assistant treats the entity as disposable and the interface will not let you edit it.
Questions people ask
Does the message have to be retained?
Yes. If it is not retained, the entity disappears the next time Home Assistant restarts, because nobody is there to tell it the device exists.
What is unique_id for?
Without it the entity cannot be renamed or assigned to an area from the interface. Always set one.
Where does the topic prefix come from?
homeassistant by default. If you changed discovery_prefix in your MQTT setup, use that instead.