MQTT Tester (over WebSockets)
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Enter the broker’s WebSocket URL — e.g. wss://broker.emqx.io:8084/mqtt (a public sandbox broker, handy for a first try).
Adjust the client ID if your broker enforces naming rules, add credentials if required, and click Connect.
Subscribe to a topic — wildcards work, so sensors/+/temperature or devices/# capture whole trees.
Publish a payload to any topic with your chosen QoS and optional retain flag.
Watch the live feed: incoming messages show their topic, payload and whether they were retained.
Every MQTT debugging session starts the same way: is the broker reachable, are the credentials right, and what is actually flowing through that topic? This client answers all three from a browser tab. Connect to your broker’s WebSocket listener, subscribe with + and # wildcards, publish test messages with QoS 0–2 and the retain flag, and watch the live feed — client ID, username and password are all under your control.
Browsers cannot open raw TCP connections, so the classic ports 1883/8883 are out of reach — but practically every modern broker (Mosquitto, EMQX, HiveMQ, AWS IoT Core, Azure IoT via its gateway) also exposes MQTT over WebSockets, usually at a path like /mqtt on port 8083/8084. That listener is exactly what this tool speaks to, using the same mqtt.js library most Node.js backends use.
100% private“Your broker URL, credentials and every message travel directly between your browser and the broker over an encrypted WebSocket — this site never sees, relays or stores any of it.”
Those are raw TCP listeners, and browsers simply cannot open raw TCP sockets. Enable your broker’s WebSocket listener instead — in Mosquitto that is two lines in mosquitto.conf (listener 8084 + protocol websockets); EMQX and HiveMQ ship with one already running.
The three usual suspects: wrong credentials (brokers often close the socket instead of returning an error), a duplicate client ID kicking the older session off, or an ACL rejecting the connection. The broker log tells you which.
It is sent only to the broker you specify, over the encrypted wss:// connection — the same thing any MQTT client does. Nothing is transmitted to this site; still, prefer a test account where possible, as you would with any client tool.
MQTT 3.1.1, the most widely supported version. Retained messages, QoS 0–2 subscriptions and publishes, and wildcard topic filters are all supported.