MQTT.fx is a desktop-based MQTT client tool designed for testing and debugging IoT communication. It provides a graphical interface where developers can connect to brokers, publish messages, and subscribe to topics. MQTT.fx supports features like QoS levels, retained messages, and connection monitoring. It is especially useful for verifying device behavior during development. While not used in production, it helps developers understand and troubleshoot MQTT workflows. As a testing tool, MQTT.fx has become a standard companion in many IoT projects.