I needed an MQTT client to repeatedly send some commands to devices I build, with Postman-like variable substitutions as I was testing different device IDs with similar commands. MQTT Explorer is nice, but I had to write the commands again and again, with different payloads, and I don't really use the visualization feature. I wanted to save the connection, have a client that opens instantly, parameterizes some common variables, and saves the commands I wrote, so here's MQTT Topic Lab.
MQTT Topic Lab allows you to save your most-used commands, send messages with repetition, and use variables to switch commands on the fly easily. It also has a message viewer so you can also see messages on the broker. After doing the hard work of building your commands, you can export it to share with colleagues so they can get started fast. Also supports keyboard shortcuts if you want to move around like that.
It's built with Tauri (Rust backend, React frontend), so it's cross-platform, and quite fast. Also binaries are quite small (6-7 MB). This is my daily driver so I am going to maintain and update it, and maybe add some features you might want to see in the app. It's coded with Claude, but it's not an "AI-slop" software. The code might be rough on some edges, but it's quite well on how it works.
Looking forward to hear your thoughts, you can check it out at https://github.com/alsoftbv/topic-lab and download built binaries for your device to test it out.