MQTT Discovery availability topics not receiving

Configuration
1

Hi!
I am implemeting a device compatible with HASS MQTT discovery feature.
During my experiments I have faced unexpected behaviour: the availability and status topics subscriptions for autodiscovered devices do not receive any messages.

I have added a new device using the following message to topic:
homeassistant/switch/dev1/config

{
   "~":"b/office/dev1",
   "uniq_id":"dev1",
   "cmd_t":"~/set",
   "stat_t":"~/state",
   "avty_t":"~/available",
   "dev":{
      "ids":"b_esp32",
      "name":"B",
      "mf":"B",
      "mdl":"153-e",
      "sw":"1.0",
      "sn":"ea3350945afc",
      "hw":"B5"
   }
}

This creates a new device with an entity and correct subscriptions.

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Then I post the messages:
b/office/dev1/available
online

and
b/office/dev1/state
OFF

But neither of the listed Subsribed topics receives the incoming messages.
The home-assistant.log says:

2024-08-09 14:21:48.705 DEBUG (MainThread) [homeassistant.components.mqtt.client] Received message on homeassistant/switch/dev1/config (qos=0): b'{\r\n   "~":"b/office/dev1",\r\n   "uniq_id":"dev1",\r\n   "cmd_t":"~/set",\r\n   "stat_t":"~/state",\r\n   "avty_t":"~/available",\r\n   "dev":{\r\n      "ids":"b_esp32",\r\n      "name":"B",\r\n      "mf":"B",\r\n      "mdl":"153-e",\r\n      "sw":"1.0",\r\n      "sn":"ea3350945afc",\r\n      "hw":"B5"\r\n   }\r\n}'
2024-08-09 14:21:48.706 DEBUG (MainThread) [homeassistant.components.mqtt.discovery] Process discovery payload {'device': {'name': 'B', 'model': '153-e', 'hw_version': 'B5', 'serial_number': 'ea3350945afc', 'identifiers': 'b_esp32', 'manufacturer': 'B', 'sw_version': '1.0'}, 'unique_id': 'dev1', 'command_topic': 'b/office/dev1/set', 'availability_topic': 'b/office/dev1/available', 'state_topic': 'b/office/dev1/state', 'platform': 'mqtt'}
2024-08-09 14:21:48.706 INFO (MainThread) [homeassistant.components.mqtt.discovery] Found new component: switch dev1
2024-08-09 14:21:48.707 DEBUG (MainThread) [homeassistant.components.mqtt.discovery] Pending discovery for ('switch', 'dev1'): deque([])
2024-08-09 14:21:48.810 DEBUG (MainThread) [homeassistant.components.mqtt.client] t144cc11.ala.eu-central-1.emqxsl.com: register write 15
2024-08-09 14:21:48.810 DEBUG (MainThread) [homeassistant.components.mqtt.client] Subscribing with mid: 24 to topics with qos: [('b/office/dev1/available', 0), ('b/office/dev1/state', 0)]
2024-08-09 14:21:48.811 DEBUG (MainThread) [homeassistant.components.mqtt.client] t144cc11.ala.eu-central-1.emqxsl.com: unregister write 15
2024-08-09 14:22:39.882 DEBUG (MainThread) [homeassistant.components.mqtt.client] t144cc11.ala.eu-central-1.emqxsl.com: register write 15
2024-08-09 14:22:39.887 DEBUG (MainThread) [homeassistant.components.mqtt.client] t144cc11.ala.eu-central-1.emqxsl.com: unregister write 15

register write 15 and unregister write 15 are the only records I get in home-assistant.log when I send messages to the availability or state topics.

Any ideas how to find out why HASS doesn’t receive the messages though everything looks like it should?

2

Turns out the issue is related to using the external third party MQTT Broker. EMQX in my case.

I’ve payed more attention to the MQTT documentation and the warning message saying that the only recommended option is Mosquitto broker.

I’ve updated my HASS instance to use Mosquitto Broker and the issue is resolved.

TLDR for future references:
If you have issues receiving MQTT messages in Home Assistant using external 3rd party MQTT broker – check first if the issue persists when using Mosquitto.