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Overview

Postiz integrates with Slack workspaces to send messages to channels using a Slack bot, with support for rich formatting, media, and threaded conversations.

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Authentication

Slack uses OAuth 2.0 with bot installation.
1

Connect Slack

Click “Connect Slack” in Integrations
2

Select Workspace

Choose which Slack workspace to install to
3

Authorize Bot

Grant all requested permissions
4

Select Channel

Choose channel when creating posts

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Required Permissions

  • channels:read - Read public channels
  • chat:write - Send messages
  • users:read - Read user information
  • groups:read - Read private channels
  • channels:join - Join channels automatically
  • chat:write.customize - Custom name and icon
Slack bot token is configured via SLACK_ID and SLACK_SECRET environment variables.

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Supported Features

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Message Features

  • Maximum: 400,000 characters
  • Markdown formatting (mrkdwn)
  • Links, bold, italic, code
  • Emoji support

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Channel Types

Post to:
  • Public channels
  • Private channels (if bot is invited)

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Posting Messages

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Message Structure

Messages use Slack’s Block Kit format: 
{
  channel: "C12345678",
  username: "Your Bot Name",
  icon_url: "https://example.com/bot-icon.png",
  blocks: [
    {
      type: "section",
      text: {
        type: "mrkdwn",
        text: "Your message text with *formatting*"
      }
    },
    {
      type: "image",
      image_url: "https://example.com/image.jpg",
      alt_text: "Image description"
    }
  ]
}

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Auto-Join Channels

Postiz automatically joins channels before posting:
1

Join Request

Bot joins the selected channel
2

Post Message

Send message with blocks and media
3

Get Permalink

Retrieve permanent link to message
The bot must be invited to private channels manually before posting.

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Threading

Create threaded conversations:
1

Main Message

Post initial message to channel
2

Thread Reply

Reply using thread_ts of main message
3

Additional Replies

Continue thread with same thread_ts
Thread replies maintain the same formatting and media support as main messages.

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Media Handling

Images are added as separate blocks: 
blocks: [
  {
    type: "section",
    text: { type: "mrkdwn", text: "Message" }
  },
  {
    type: "image",
    image_url: "https://example.com/image1.jpg",
    alt_text: ""
  },
  {
    type: "image",
    image_url: "https://example.com/image2.jpg",
    alt_text: ""
  }
]
Multiple images are supported, each as a separate image block.

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Channel Discovery

List available channels: 
GET /channels

Returns:
[
  {
    id: "C12345678",
    name: "general"
  },
  {
    id: "C87654321",
    name: "announcements"
  }
]
Includes both public and private channels the bot can access. Each message receives a permalink: 
https://workspace.slack.com/archives/C12345678/p1234567890123456
Permanent links work even if message is old or in archived channels.

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Rate Limits

Slack Rate Limits:
  • 3 concurrent posting jobs
  • Moderate API rate limits
  • Tier-based limits per workspace
  • No strict message count limits

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Best Practices

Markdown Formatting

Use Slack’s mrkdwn for bold, italic, and code

Thread Organization

Use threads to keep channels clean and organized

Channel Selection

Post to appropriate channels - use announcements sparingly

Bot Identity

Customize bot name and icon to match your brand

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Troubleshooting

If messages fail:
  • Verify bot is in workspace
  • Check bot has chat:write permission
  • Ensure channel exists and bot can access
  • For private channels, manually invite bot first
If channel doesn’t appear:
  • Bot must be invited to private channels
  • Channel might be archived
  • Verify bot has channels:read permission
  • Check workspace permissions
Media display issues:
  • Ensure image URLs are publicly accessible
  • Check image format is supported
  • Verify URLs use HTTPS
  • Images must be hosted externally
Thread issues:
  • Verify original message exists
  • Check thread_ts is correct
  • Ensure bot has message history access
  • Original message may have been deleted

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