/slack-block-kit

Composition Objects — Complete Property Reference

Sources:

• Block Kit Composition Objects — Slack
• Text object — Slack

Composition objects are reusable JSON patterns used inside blocks and elements.

Contents

• Text Object
• Option Object
• Option Group Object
• Confirmation Dialog Object
• Conversation Filter Object
• Dispatch Action Configuration Object
• Slack File Object
• Slack Icon Object
• Trigger and Input Parameter Objects
• Workflow Object

Text Object

The most common composition object. Appears in nearly every block and element.

Type Rules

Verbatim Behavior

When verbatim: false (default):

• URLs auto-convert to clickable links
• Channel names auto-convert to channel links
• Mentions auto-parse

When verbatim: true:

• mrkdwn formatting still processes
• Explicit manual parsing strings still process
• Slack does not modify otherwise plain-text content (for example, it does not auto-link a channel name)
• Useful for displaying raw URLs or text containing @ or # that aren't mentions

{ "type": "mrkdwn", "text": "Check the log at http://example.com/debug", "verbatim": true }

Option Object

Represents a single selectable item.

Used in: static_select, multi_static_select, external_select, multi_external_select, overflow, checkboxes, radio_buttons.

{
  "text": { "type": "plain_text", "text": "High Priority" },
  "value": "high",
  "description": { "type": "mrkdwn", "text": "Critical issues only" }
}

Option Group Object

Groups related options in select menus for visual organization.

Used in: static_select, multi_static_select, external_select, multi_external_select.

Max 100 option groups per menu.

{
  "label": { "type": "plain_text", "text": "Engineering" },
  "options": [
    { "text": { "type": "plain_text", "text": "Backend" }, "value": "backend" },
    { "text": { "type": "plain_text", "text": "Frontend" }, "value": "frontend" }
  ]
}

Confirmation Dialog Object

Adds a confirmation step to an element that exposes a confirm property.

Desktop: danger = red background, primary = green background. Mobile: danger = red text, primary = blue text.

{
  "title": { "type": "plain_text", "text": "Delete item?" },
  "text": { "type": "plain_text", "text": "This cannot be undone." },
  "confirm": { "type": "plain_text", "text": "Delete" },
  "deny": { "type": "plain_text", "text": "Keep" },
  "style": "danger"
}

Used in: Any element that accepts a confirm property (buttons, select menus, overflow, datepicker, timepicker, checkboxes, radio buttons).

Current Slack-doc inconsistency: the field table calls text a plain_text object, while Slack's official example uses mrkdwn. Prefer plain_text when formatting is unnecessary; if following the example, use mrkdwn only for this explanatory field, never for title, confirm, or deny.

Conversation Filter Object

Filters the options in conversation select menus.

At least one property must be supplied.

{
  "include": ["public", "private"],
  "exclude_bot_users": true
}

Used in: conversations_select, multi_conversations_select (via filter property).

Known issues: iOS shows "0 selected" instead of placeholder text when nothing is selected. iOS also has UI inconsistencies when users interact with multi-select menu items.

Dispatch Action Configuration Object

Controls when supported text-like inputs (plain_text_input, rich_text_input, number_input, email_text_input, and url_text_input) return block_actions payloads during input. The composition-object page still describes plain-text input specifically, while each of the other live element pages exposes the same property.

Trigger values:

Requires dispatch_action: true on the parent input block.

{
  "type": "input",
  "dispatch_action": true,
  "element": {
    "type": "plain_text_input",
    "action_id": "search_input",
    "dispatch_action_config": {
      "trigger_actions_on": ["on_character_entered"]
    }
  },
  "label": { "type": "plain_text", "text": "Search" }
}

Slack File Object

References a Slack-hosted file for use in image blocks or image elements.

Provide exactly one of url or id. The file must be an image, the posting user must have access to it, and Slack currently supports Slack-hosted PNG, JPG/JPEG, and GIF files. Supplying both properties rejects the payload.

{ "url": "https://files.slack.com/files-pri/T0123-F0123/image.png" }

{ "id": "F0123ABC456" }

Used in: image block (slack_file property), image element (slack_file property).

Slack Icon Object

References a built-in Slack icon for use in card blocks. Renders next to the card's title/subtitle in place of a custom icon image (the two are mutually exclusive).

Icon names: archive, book, bookmark, bot, bug, calendar, call, caret-left, caret-right, check, clipboard, code, comment, compass, copy, cube, download, edit, email, eye-closed, eye-open, file, flag, folder, gear, globe, heart, help, image, info, key, lightbulb, link, map, mobile, new-window, pin, plus, refine, refresh, rocket, save, screen, share, sparkle, star, star-filled, tag, thumbs-down, thumbs-up, trash, upload, user, warning.

{
  "type": "card",
  "slack_icon": { "type": "icon", "name": "rocket" },
  "title": { "type": "mrkdwn", "text": "Deploy complete" }
}

Used in: card block (slack_icon property).

Trigger Object

Contains trigger information for workflow buttons.

Values can be visible to end users. Never put secrets or other sensitive information in these parameters.

{
  "trigger": {
    "url": "https://slack.com/shortcuts/Ft0123ABC/launch",
    "customizable_input_parameters": [
      { "name": "input_param_a", "value": "Value for param A" }
    ]
  }
}

Input Parameter Object

Supplies a customized workflow input inside trigger.customizable_input_parameters.

{ "name": "input_parameter_a", "value": "Value for input param A" }

Slack's composition-object index links to a dedicated input-parameter page, but as of the documentation audit that Markdown endpoint returns 404 and the browser route falls back to the docs home page. The trigger-object reference above is therefore the authoritative field description currently available.

Workflow Object

Wraps a trigger object for workflow buttons.

Used in: workflow_button element (workflow property).