Steps to reproduce:

Ubuntu 24

Install ** and configure Mosquitto MQTT broker in docker container on host

Install zabbix agent2 with a following MQTT configuration in  /etc/zabbix/zabbix_agent2.d/plugins.d/mqtt.conf

Plugins.MQTT.Sessions.docker.Topic=watermeter/main/json
Plugins.MQTT.Sessions.docker.Url=tcp://192.168.196.10:1883
Plugins.MQTT.Sessions.docker.User=zbxuser
Plugins.MQTT.Sessions.docker.Password=PASSWORD

subscribes to topic. 

Result:

upon starting I get an only message that MQTT plugin is loaded in zabbix_agent2 log file (DebugLevel=5)

2026/01/09 12:08:11.277992 using plugin 'MQTT' (built-in) providing following interfaces: watcher, configurator, maximum capacity: 1000, active checks on start enabled: true

zabbix agent2 does not even try to connect to a MQTT broker (verified with tcpdump), MQTT broker (mosquitto) log show no connection attempt. However there are no error messages too.

All credentials then tested with cli tool on same machine are working as expected so permissions and network should be OK.

mosquitto_sub -h 192.168.196.10 -p 1883 -i moskits -u 'zbxuser' -P 'PASSWORD'  -t 'watermeter/main/json

mosquitto log show  connection

1767956902: New connection from 192.168.196.10:50356 on port 1883.
1767956902: New client connected from 192.168.196.10:50356 as moskits (p2, c1, k60, u'zbxuser').
1767956902: No will message specified.
1767956902: Sending CONNACK to moskits (0, 0)
1767956902: Received SUBSCRIBE from moskits
1767956902:     watermeter/main/json (QoS 0)
1767956902: moskits 0 watermeter/main/json
1767956902: Sending SUBACK to moskits

Expected:
zabbix_agent2 on startup should connect to MQTT broker , subscribe to a topic and log attempt of such connection or show an error message in case settings are not correct.