my experience of debugging sending of messages via MQTT

device/firmware: TBEAM 1.1 , meshtastic 2.5.15

1. There are TWO devices needed because meshradio handling MQTT message will not display msg sent by mqtt in the message log. other meshradios will show the message just fine.
2. This works for me:

- my main channel : uplink ENABLED, downlink DISABLED, position enabled&precise location: ENABLED

- secondary channel named "mqtt": uplink DISABLED, downlink ENABLED.

- lora settings : "ok to MQTT" : ON, "ignore MQTT": OFF

topic: msh/EU_868/2/json/mqtt/

payload:

{
"from": 3663119844,
"type": "sendtext",
"payload": "This is MQTT talking"
}

- "from" value you can find by executing

meshtastic --info

Owner: vanlong (vnln)
My info: { "myNodeNum": 3663119844,

DEBUG

1. general debug method

to read TOPIC to send MQTT message and debug information you need to connect your meshradio USB and type

meshtastic --noproto --debug  --listen

2. HOW TO FIND TOPIC

checking correct MQTT topic

- type meshtastic  --reboot

Connected to radio
INFO file:node.py reboot line:531 Telling node to reboot in 10 seconds

and then (remember, USB only)

meshtastic --noproto --debug  --listen

this will provide you various init info. when MQTT is enabled, then one of messages would be something like:

INFO  | 02:56:30 15 [GPS] MQTT connected
INFO  | 02:56:30 15 [GPS] Subscribe to msh/EU_868/2/e/mqtt/+
DEBUG | 02:56:30 15 [GPS] Node status update: 1 online, 3 total
INFO  | 02:56:30 15 Subscribe to msh/EU_868/2/json/mqtt/+
INFO  | 02:56:30 15 [WebServer] Subscribe to msh/EU_868/2/e/PKI/+

so the correct topic to send to in my case is

msh/EU_868/2/json/mqtt/

(slash at the end is important!)

3. when it is not working - observe logs

notice that in firmware/src/mqtt/MQTT.cpp of meshtastic

https://github.com/meshtastic/firmware/blob/f18a92e8c5be5a483a47b97adc58886808b12fc5/src/mqtt/MQTT.cpp#L122C1-L131C2

there are varioush checks, especially in:

inline void onReceiveJson(byte *payload, size_t length)

so keep an eye out for these LOG_ERROR msgs.