home-assistant.io/source/_docs/mqtt/service.markdown

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MQTT Publish service	Instructions on how to setup the MQTT Publish service within Home Assistant.	mqtt.png

The MQTT integration will register the service mqtt.publish which allows publishing messages to MQTT topics. There are two ways of specifying your payload. You can either use payload to hard-code a payload or use payload_template to specify a template that will be rendered to generate the payload.

Service mqtt.publish

Service data attribute	Optional	Description
topic	no	Topic to publish payload to.
payload	yes	Payload to publish.
payload_template	yes	Template to render as payload value. Ignored if payload given.
qos	yes	Quality of Service to use.
retain	yes	If message should have the retain flag set. (default: false)

You need to include either payload or payload_template, but not both.

{
  "topic": "home-assistant/light/1/command",
  "payload": "on"
}

{% raw %}

{
  "topic": "home-assistant/light/1/state",
  "payload_template": "{{ states('device_tracker.paulus') }}"
}

{% endraw %}

payload must be a string. If you want to send JSON then you need to format/escape it properly. Like:

{
  "topic": "home-assistant/light/1/state",
  "payload":"{\"Status\":\"off\", \"Data\":\"something\"}"
}

Example of how to use qos and retain:

{
  "topic": "home-assistant/light/1/command",
  "payload": "on",
  "qos": 2,
  "retain": true
}