MQTT WebSocket Client
Publish and subscribe through a disclosed secure WebSocket broker.
Session
Broker connection
Egress
Before Connect
Enter a complete wss:// URL to reveal the exact broker destination.
Connecting leaves StackCache.
The broker receives the client ID, username/password when provided, connection IP, subscriptions, published topics/payloads, and normal WebSocket metadata. StackCache does not proxy, log, cache, or background the session. The broker controls authentication, retention, delivery, and its own logs.
Receive
Subscriptions
Up to 20 filters. MQTT + and # wildcards are validated.
Send
Publish message
Memory only
Received messages (0 / 200)
Connected subscription messages appear here. Oversized messages are dropped, and only the newest 200 are retained in memory.
Optional
Encrypted save
Broker URL, client ID, credentials, reconnect setting, and subscription filters remain session-only unless you explicitly save the whole profile. Save disconnects first, then uses a one-minute consume-once memory handoff and commits only after the encrypted vault is created or unlocked.
Working notes
Use MQTT WebSocket Client with the boundary visible.
MQTT WebSocket Client connects the browser directly to a disclosed secure WebSocket broker for bounded publish and subscribe sessions without a StackCache relay.
What is MQTT WebSocket Client?
MQTT (Message Queuing Telemetry Transport) is a lightweight publish-subscribe messaging protocol designed for constrained devices and low-bandwidth networks. It is the dominant protocol for IoT (Internet of Things) communication, used by smart home devices, industrial sensors, fleet tracking systems, and cloud IoT platforms (AWS IoT Core, Azure IoT Hub, Google Cloud IoT). MQTT uses a broker-based architecture: clients publish messages to topics and subscribe to topics to receive messages. The protocol supports three quality-of-service levels (QoS 0: at most once, QoS 1: at least once, QoS 2: exactly once), retained messages, last-will-and-testament messages, and session persistence. This tool connects directly from your browser to an MQTT broker over secure WebSockets (wss://), which is the only transport available in a browser context. It supports MQTT 3.1.1 and 5.0, allowing you to subscribe to topics, publish messages, and inspect incoming messages in real time. StackCache does not proxy or relay any traffic — the connection goes directly from your browser to the broker.
When to use it
- Testing IoT device communication — subscribe to device topics to verify that sensors are publishing data correctly.
- Debugging message routing — publish test messages and subscribe to response topics to trace message flow through your MQTT architecture.
- Prototyping IoT dashboards — subscribe to topics and inspect the message format before building a production consumer.
- Monitoring production topics — connect to a production broker (with read-only credentials) to observe live message traffic.
- Learning MQTT — experiment with publish/subscribe patterns, QoS levels, and retained messages without installing a local broker.
How to use it
- 01Enter the broker endpoint (wss:// URL) and optional credentials (username, password, client ID).
- 02Review the connection details and click Connect.
- 03Add topic subscriptions with the desired QoS level.
- 04View incoming messages in real time. Each message shows the topic, payload, QoS, and retain flag.
- 05Publish messages to topics with custom payloads.
- 06Disconnect when the session is complete.
Common mistakes
- Using mqtt:// instead of wss:// — browsers can only connect via WebSockets. Raw TCP MQTT (mqtt://) requires a native client like mosquitto_sub.
- Wrong port — MQTT WebSocket endpoints often use a different port than the raw MQTT port (e.g., 8883 for MQTTS vs 8084 for WSS). Check your broker documentation.
- Missing client ID — some brokers require a unique client ID. If two clients connect with the same ID, the broker disconnects one of them.
- Wildcard topic confusion — MQTT uses + (single level) and # (multi-level) wildcards for subscriptions. Publishing to a wildcard topic is not allowed.
- QoS mismatch — the effective QoS is the minimum of the publisher's and subscriber's QoS levels. If you publish at QoS 2 but subscribe at QoS 0, delivery is at most once.
Synthetic example
Describe a local test session
Input
Broker: wss://broker.example.test/mqtt Topic: demo/status
Result
No connection occurs until Connect; example host is not contacted by documentation.
Related standards
Limits and data boundary
- Only secure WebSocket MQTT is available; raw TCP mqtt:// requires a native client.
- The broker receives connection metadata, credentials, topics, and payloads and may retain them.
Frequently asked questions
- Which MQTT protocol versions are supported?
- The client connects over secure WebSockets (wss://) and supports MQTT 3.1.1 and 5.0, depending on what the broker accepts.
- Can I connect to a local MQTT broker?
- Yes, if the broker exposes a wss:// WebSocket endpoint accessible from your browser. Raw TCP (mqtt://) connections require a native client.
- Are my messages logged by StackCache?
- No. Messages flow directly between your browser and the broker. StackCache does not proxy, log, or retain any session data.
- What are MQTT QoS levels?
- QoS 0 delivers at most once (fire and forget). QoS 1 delivers at least once (may duplicate). QoS 2 delivers exactly once (highest overhead). Choose based on your reliability requirements.
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