Formatting MQTT Topics

In working with Insteon-MQTT, I can now publish a packet, turn on a light, and see the response, using the MQTT integration this way:
topic: insteon/3e.ab.a7/set
payload: on

Now I am trying to get all fancy and make sure I understand MQTT by simply trying to set the dim level. I’ve tried multiple variations of:
topic: insteon/3e.ab.a7
payload: {“state”: “ON”,“brightness”: 50}

I must not be getting the message formatting of the topic/payload correct - any suggestions would be most appreciated!

What software are you using? Is it this? insteon-mqtt/ at 28ca2b80215f3935d77b54125151f403c60002dc · TD22057/insteon-mqtt · GitHub

Yes. Insteon-mqtt. It works correctly as a device in ha, so I could not worry about it, but I’d really like to understand what I’m doing wrong in this manual approach.

Does it expect / allow json payloads ?
Also you have missed off the /set topic suffix

If it works as a device, use mqtt explorer to spy on the mqtt messages.

I was able to get it to work with this topic/payload combo (within the MQTT integration):

topic: insteon/3e.ab.a7/level
payload: {“state”: “ON”,“brightness”: 50}