Publishing Messages

Overview

MQTT Explorer provides a powerful publishing interface that makes it easy to send messages to any topic. Whether you’re testing IoT devices, triggering automations, or sending configuration updates, the Publish tab gives you complete control over your MQTT messages.

Publish Tab Interface

Access the Publish tab from the sidebar to send messages: Key Components

Topic input: Specify the destination topic path
Payload editor: Enter your message payload
Editor mode selector: Choose payload format
Retain switch: Control message retention
QoS selector: Set Quality of Service level
Publish button: Send the message
Publish history: Access recently sent messages

You must be connected to an MQTT broker before you can publish messages.

Topic Input

Specify where your message should be published: Topic Entry

Type or paste the full topic path (e.g., home/livingroom/light/command)
Auto-complete based on existing topics (when available)
Copy topic path from selected topics in the tree
Topic validation ensures valid MQTT topic syntax

Quick Topic Selection

Click any topic in the tree view
The topic path is automatically copied to the publish topic field
Edit the path as needed before publishing

Use the forward slash (/) separator to create topic hierarchies. Avoid using wildcards (# or +) in publish topics.

Payload Editor

The integrated code editor provides a comfortable writing experience:

Editor Features

Syntax Highlighting

Color-coded syntax for JSON, XML, and other formats

Line Numbers

Easy reference for long payloads

Auto-Indentation

Automatic indentation for structured data

Multi-line Support

Full editor for complex payloads

Keyboard Shortcuts

Shortcut	Action
Ctrl/Cmd + Enter	Publish the message
Tab	Insert indentation
Shift + Tab	Remove indentation

The editor automatically adjusts to light or dark mode based on your system theme.

Editor Modes

Choose the appropriate format for your message payload:

JSON Mode

Best for: Structured data, configuration objects, device statesFeatures:

JSON syntax highlighting
Format validation
Format JSON button to pretty-print
Auto-complete for braces and quotes

Example:

{
  "state": "ON",
  "brightness": 75,
  "color": {
    "r": 255,
    "g": 128,
    "b": 0
  }
}

Plain Text Mode

Best for: Simple values, commands, plain text messagesFeatures:

No formatting constraints
Line breaks preserved
Fastest input method

Example:

ACTIVATE

Base64 Mode

Best for: Binary data, encoded files, encrypted contentFeatures:

Base64 encoding/decoding
Binary data support
Large payload handling

Example:

SGVsbG8gV29ybGQh

Hex Mode

Best for: Low-level protocols, raw bytes, hardware communicationFeatures:

Hexadecimal encoding
Byte-level control
Raw binary representation

Example:

48656c6c6f

Format JSON Feature

When working in JSON mode, use the Format JSON button to:

Write JSON

Enter your JSON payload (formatted or unformatted)

Click Format

Click the Format JSON button (align icon)

Get Pretty-Printed Output

Your JSON is automatically formatted with proper indentation and spacing

Error Handling

Invalid JSON shows an error message
Syntax errors are highlighted
Error message indicates what went wrong

Paste minified JSON from any source and click Format JSON to make it readable instantly.

Load from File

Publish file contents as MQTT messages:

Click Attach File Button

Click the paper clip icon in the editor toolbar

Select File

Choose a file from your system (up to 16MB)

File Loaded

File contents are loaded into the payload editor

Edit & Publish

Modify if needed, then click Publish

Supported File Types

Text files (.txt, .log, .csv)
JSON files (.json)
XML files (.xml)
Configuration files (.yaml, .conf, .ini)
Any text-based format

Binary files are encoded as base64. For browser mode, files are uploaded to the server and may be limited by server configuration.

Retain Flag

Control message retention on the broker: Retain Off (Default)

Message is delivered to current subscribers
Message is not stored by the broker
New subscribers won’t receive this message

Retain On

Message is stored by the broker
New subscribers immediately receive the retained message
Overwrites any previous retained message on the topic
Useful for state values and configuration

When to Use Retain?

Use retained messages for:

Device state values (“light is ON”)
Configuration settings
Status updates (“device online”)
Last known values

Don’t use retained messages for:

Temporary commands
Event notifications
Time-sensitive data
Rapidly changing values

To clear a retained message, publish an empty payload with the retain flag enabled to the same topic.

Quality of Service (QoS)

Control message delivery guarantees:

QoS Levels

QoS 0

At most onceFire and forget. No confirmation. Fastest but least reliable.Use for: High-frequency sensor data, non-critical updates

QoS 1

At least onceConfirmed delivery. May receive duplicates.Use for: Important messages, commands, state changes

QoS 2

Exactly onceGuaranteed delivery without duplicates. Slowest but most reliable.Use for: Critical commands, billing data, irreversible actions

The actual QoS level is the minimum of the publisher’s and subscriber’s requested QoS.

Publishing the Message

Send your message to the broker: Publish Methods

Click the Publish button
Press Ctrl/Cmd + Enter while in the editor

Success Feedback

Message is added to publish history
No error dialog means successful publish
Check topic in tree to verify message received (if subscribed)

Error Handling

Connection errors show an alert
Invalid topic format shows validation error
Broker rejection displays error message

Publish History

Quickly resend or reference previous messages: History Features

Stores last 8 published messages per session
Shows topic, payload preview, and timestamp
Click any entry to load it into the editor
Automatic duplicate removal
Most recent messages first

View History

Scroll down below the editor to see publish history

Select Message

Click any historical message card

Message Loaded

Topic and payload are loaded into the publish form

Modify & Resend

Edit as needed and publish again

Use publish history to quickly test variations of a message by loading and modifying previous publishes.

Common Use Cases

Control IoT Devices

Send commands to smart home devices, lights, thermostats, and sensors

Test Integrations

Verify that your applications correctly handle MQTT messages

Trigger Automations

Send messages to activate automation rules in Home Assistant, Node-RED, etc.

Update Configuration

Push configuration updates to devices via retained messages

Send Commands

Execute device commands like “restart”, “update”, or “reset”

Simulate Sensors

Generate test data for development and debugging

Best Practices

Be cautious when publishing to production systems. Always verify the topic path before clicking Publish.

Publishing Tips

Use descriptive, hierarchical topic names
Include a /cmd or /set suffix for command topics
Test with QoS 0 first, then increase if needed
Use retained messages sparingly (only for state/config)
Validate JSON payloads before publishing
Keep payload sizes reasonable (under 1KB when possible)

Security Considerations

Only publish to topics you have write permission for
Don’t publish sensitive data without encryption
Use authentication and TLS for production brokers
Be aware that published messages may be logged

Message Inspection - View published messages in the tree
Topic Visualization - Navigate to topics before publishing
AI Assistant - Get AI-generated publish proposals

Get Started

Deployment

Features

Guides

Advanced

Overview

Publish Tab Interface

Topic Input

Payload Editor

Editor Features

Syntax Highlighting

Line Numbers

Auto-Indentation

Multi-line Support

Keyboard Shortcuts

Editor Modes

Format JSON Feature

Load from File

Retain Flag

Quality of Service (QoS)

QoS Levels

QoS 0

QoS 1

QoS 2

Publishing the Message

Publish History

Common Use Cases

Control IoT Devices

Test Integrations

Trigger Automations

Update Configuration

Send Commands

Simulate Sensors

Best Practices

Build docs developers (and LLMs) love

Get Started

Deployment

Features

Guides

Advanced

​Overview

​Publish Tab Interface

​Topic Input

​Payload Editor

​Editor Features

Syntax Highlighting

Line Numbers

Auto-Indentation

Multi-line Support

​Keyboard Shortcuts

​Editor Modes

​Format JSON Feature

​Load from File

​Retain Flag

​Quality of Service (QoS)

​QoS Levels

QoS 0

QoS 1

QoS 2

​Publishing the Message

​Publish History

​Common Use Cases

Control IoT Devices

Test Integrations

Trigger Automations

Update Configuration

Send Commands

Simulate Sensors

​Best Practices

​Related Features

Build docs developers (and LLMs) love

Overview

Publish Tab Interface

Topic Input

Payload Editor

Editor Features

Keyboard Shortcuts

Editor Modes

Format JSON Feature

Load from File

Retain Flag

Quality of Service (QoS)

QoS Levels

Publishing the Message

Publish History

Common Use Cases

Best Practices

Related Features