slack-block-kit

Slack Block Kit

UI framework for building rich, interactive layouts in Slack messages, modals, and App Home.

CRITICAL: Two Markup Systems

Text inside Block Kit text objects uses Slack mrkdwn syntax (*bold*, <url|text>), NOT standard Markdown. The only exception is the markdown block which uses standard Markdown.

Quick Decision Trees

"Should I use blocks?"

Response type?
├─ Conversational reply, short answer, <3 lines   → text only (no blocks)
├─ Multi-section summary, report, dashboard        → blocks
├─ Two-column key-value data                       → blocks (section fields)
├─ Tabular data                                    → blocks (table)
├─ Code with heading or surrounding context         → blocks
├─ Visual separation needed between topics          → blocks
└─ Feedback buttons or interactive elements         → blocks

"Which block type?"

What am I rendering?
├─ Large section title                → header (plain_text, 150 chars max)
├─ Body text or key-value pairs       → section (text + fields + accessory)
├─ Small metadata or secondary info   → context (images + text, 10 max)
├─ Horizontal separator               → divider
├─ Buttons, menus, date pickers       → actions (25 elements max)
├─ Standalone image                   → image (image_url or slack_file)
├─ Formatted text with lists, quotes  → rich_text (nested sub-elements)
├─ Tabular data                       → table (100 rows, 20 cols, 1 per msg)
├─ LLM-generated markdown content     → markdown (standard MD, messages only)
├─ Embedded video player              → video (requires links.embed:write)
├─ Remote file reference              → file (read-only, source: "remote")
├─ Feedback thumbs up/down            → context_actions (messages only)
├─ Collecting user input (modals)     → input (label + element)
├─ AI agent task steps                → plan (sequential tasks, messages only)
└─ Single task with status            → task_card (inside plan or standalone)

"mrkdwn or markdown block?"

Content source?
├─ Short formatted text, labels, fields     → mrkdwn in section/context
├─ Long-form LLM-generated content          → markdown block (standard MD)
├─ Need tables inside blocks                → mrkdwn in section (manual layout)
├─ Need headings                            → markdown block or header blocks
└─ Mixed: structured layout + prose         → section/header blocks + markdown block

Block Types Overview

header

Large bold text for section titles. plain_text only. Max 150 chars.

{ "type": "header", "text": { "type": "plain_text", "text": "Section Title", "emoji": true } }

section

Primary content block. Supports text, two-column fields, and one accessory element.

{
  "type": "section",
  "text": { "type": "mrkdwn", "text": "*Project Status*\nAll systems operational." }
}

Two-column fields layout:

{
  "type": "section",
  "fields": [
    { "type": "mrkdwn", "text": "*Status:*\nActive" },
    { "type": "mrkdwn", "text": "*Owner:*\nChris" },
    { "type": "mrkdwn", "text": "*Priority:*\nHigh" },
    { "type": "mrkdwn", "text": "*Due:*\nFriday" }
  ]
}

Either text or fields required (or both). Text max 3000 chars. Fields max 10 items, each max 2000 chars. Set expand: true to force full text display without "see more" truncation.

Compatible accessories: button, overflow, datepicker, timepicker, select menus, multi-select menus, checkboxes, radio buttons, image.

divider

{ "type": "divider" }

context

Small, muted text for metadata. Elements: mrkdwn text objects or image elements. Max 10 elements.

{
  "type": "context",
  "elements": [
    { "type": "mrkdwn", "text": "Last updated: Feb 9, 2026" },
    { "type": "mrkdwn", "text": "Source: deploy-bot" }
  ]
}

actions

Interactive elements: buttons, select menus, overflow menus, date pickers. Max 25 elements.

{
  "type": "actions",
  "elements": [
    {
      "type": "button",
      "text": { "type": "plain_text", "text": "Approve", "emoji": true },
      "style": "primary",
      "action_id": "approve_action",
      "value": "approved"
    }
  ]
}

Button styles: primary (green), danger (red), or omit for default. Use primary sparingly — one per set. Action IDs must be unique within the message.

image

Standalone image with alt text. Provide either image_url (public, max 3000 chars) or slack_file object. Formats: png, jpg, jpeg, gif.

{
  "type": "image",
  "image_url": "https://example.com/chart.png",
  "alt_text": "Deployment success rate chart",
  "title": { "type": "plain_text", "text": "Deploy Metrics" }
}

rich_text

Advanced formatted text with nested elements. Supports styled text, lists, code blocks, and quotes. See references/RICH-TEXT.md for deep dive.

{
  "type": "rich_text",
  "elements": [
    {
      "type": "rich_text_section",
      "elements": [
        { "type": "text", "text": "Key findings:", "style": { "bold": true } }
      ]
    },
    {
      "type": "rich_text_list",
      "style": "bullet",
      "elements": [
        { "type": "rich_text_section", "elements": [{ "type": "text", "text": "Latency reduced by 40%" }] },
        { "type": "rich_text_section", "elements": [{ "type": "text", "text": "Error rate under 0.1%" }] }
      ]
    }
  ]
}

Sub-element types: rich_text_section (paragraph), rich_text_list (style: "bullet" or "ordered", with indent, offset, border), rich_text_preformatted (code block), rich_text_quote (blockquote).

Inline element types within sections: text (with optional style: { bold, italic, strike, code, underline }), link, emoji, user, channel, usergroup, broadcast, date, color.

table

Tabular data. One table per message (appended as attachment at bottom).

{
  "type": "table",
  "rows": [
    [
      { "type": "raw_text", "text": "Service" },
      { "type": "raw_text", "text": "Status" },
      { "type": "raw_text", "text": "Latency" }
    ],
    [
      { "type": "raw_text", "text": "API" },
      { "type": "raw_text", "text": "Healthy" },
      { "type": "raw_text", "text": "12ms" }
    ]
  ],
  "column_settings": [
    { "align": "left" },
    { "align": "center" },
    { "align": "right" }
  ]
}

Each row is an array of cell objects (NOT an object with a cells property). Cell types: raw_text or rich_text. There is no columns property — the first row is the header. Max 100 rows, 20 columns. Multiple tables trigger invalid_attachments error.

markdown

Standard Markdown rendering for AI app output. Messages only.

{ "type": "markdown", "text": "**Bold**, *italic*, [link](https://example.com)\n\n## Heading\n\n- List item" }

Supports: bold, italic, strikethrough, links, headers, ordered/unordered lists, inline code, code blocks, block quotes. Does NOT support: syntax highlighting, horizontal rules, tables, task lists. Cumulative 12,000 char limit per payload. block_id is ignored.

context_actions

Feedback and icon buttons for message-level actions. Messages only. Max 5 elements. Compatible elements: feedback_buttons, icon_button.

video

Embedded video player. Requires links.embed:write scope, publicly accessible URL in app's unfurl domains.

input

Collects user data in modals, messages, and Home tabs. Requires label (plain_text, 2000 chars) and one compatible element. See references/ELEMENTS.md for all input element types.

plan

Container for sequential task cards, designed for AI agent output. Messages only.

{
  "type": "plan",
  "title": "Thinking completed",
  "tasks": [
    { "task_id": "t1", "title": "Fetched data", "status": "complete" },
    { "task_id": "t2", "title": "Generating report", "status": "in_progress" }
  ]
}

Task status values: pending, in_progress, complete, error. Each task is a task_card block with optional details, output (rich_text), and sources (url elements).

file

Remote file reference. Read-only. Cannot be directly added to messages by apps.

Composition Objects

Option Object

Used in select menus, overflow, checkboxes, radio buttons:

{
  "text": { "type": "plain_text", "text": "Option 1" },
  "value": "opt_1",
  "description": { "type": "plain_text", "text": "Detailed description" }
}

Text max 75 chars. value max 150 chars. description optional, max 75 chars.

Confirmation Dialog

{
  "title": { "type": "plain_text", "text": "Are you sure?" },
  "text": { "type": "plain_text", "text": "This action cannot be undone." },
  "confirm": { "type": "plain_text", "text": "Yes, do it" },
  "deny": { "type": "plain_text", "text": "Cancel" },
  "style": "danger"
}

Conversation Filter

Filters conversation select menus. include: im, mpim, private, public.

Dispatch Action Configuration

Controls when input elements trigger block_actions: on_enter_pressed, on_character_entered.

See references/COMPOSITION.md for full property tables.

Limits

Constraint	Limit
Blocks per message	50
Blocks per modal/Home tab	100
Section text	3000 chars
Section fields	10 items, 2000 chars each
Header text	150 chars
Context elements	10
Actions elements	25
Context actions elements	5
Table rows	100
Table columns	20
Tables per message	1
Markdown block text	12,000 chars cumulative per payload
Modal title	24 chars
Modal submit/close text	24 chars
Modal views in stack	3
Modal private_metadata	3000 chars
Button text	75 chars (displays ~30)
Button value	2000 chars
action_id / block_id	255 chars
Overflow options	5
Select options	100
Option text	75 chars
Placeholder text	150 chars
File input max file size	10MB per file

Anti-Patterns

Anti-Pattern	Problem	Fix
Blocks without text fallback	Empty notifications, no accessibility fallback	Always provide text in chat.postMessage
text and blocks diverge	Confusing: notification says one thing, chat shows another	Keep semantically aligned
Blocks for simple replies	Visual noise for short responses	Use text only for simple replies
2+ tables in one message	invalid_attachments error	One table per message
mrkdwn in header text	Ignored — headers only accept plain_text	Use plain_text type
Long header text	Silently truncated at 150 chars	Keep under 150
Missing alt_text on images	Accessibility failure, API may reject	Always include alt_text

Best Practices

Use blocks when:

• The response has multiple distinct sections (summaries, reports, dashboards)
• Two-column key-value layouts improve readability (metadata, config summaries)
• A table presents data more clearly than prose
• Visual separation between topics helps comprehension
• Code needs a header or surrounding context
• Interactive elements (buttons, menus, feedback) are needed

Don't use blocks when:

• The response is conversational ("sure, done", "hey, good morning")
• The response is under ~3 lines of text
• The content is a simple answer to a direct question

Always:

• Provide a complete text field as the accessible fallback (notifications, threads, search, screen readers)
• Keep the text and blocks semantically aligned
• Use mrkdwn syntax in text objects, not standard Markdown (except in markdown blocks)
• Escape &, <, > in user-generated content

Surfaces Overview

Surface	Max Blocks	Key Methods	Notes
Messages	50	chat.postMessage, chat.update	Primary output surface
Modals	100	views.open, views.update, views.push	Requires trigger_id (3s expiry), up to 3 stacked views
App Home	100	views.publish	Private per-user view, Home/Messages/About tabs
Canvases	N/A	canvases.create, canvases.edit	Markdown only — no Block Kit support
Lists	N/A	lists.* API methods	Task tracking and project management
Split View	N/A	Agents & AI Apps config	AI chat surface with Chat + History tabs

Modals collect input via input blocks, return view_submission payloads. They chain up to 3 views with push/update/clear response actions. private_metadata (3000 chars) persists context between views.

Work Objects

Work Objects render rich entity previews when links are shared in Slack. They extend link unfurling with structured data, flexpane details, editable fields, and actions.

Entity Types

Type	Entity ID	Purpose
File	slack#/entities/file	Documents, spreadsheets, images
Task	slack#/entities/task	Tickets, to-dos, work items
Incident	slack#/entities/incident	Service interruptions, outages
Content Item	slack#/entities/content_item	Articles, pages, wiki entries
Item	slack#/entities/item	General-purpose entity

Work Objects use chat.unfurl with a metadata parameter containing entity type, external reference, and entity payload. See references/WORK-OBJECTS.md for full implementation details.

Reference Documentation

File	Purpose
references/CHEATSHEET.md	Quick reference: all blocks, elements, limits at a glance
references/BLOCKS.md	All 15 block types with full property tables and constraints
references/ELEMENTS.md	All 20 interactive elements with properties and constraints
references/COMPOSITION.md	Composition objects: text, option, confirmation, filters
references/RICH-TEXT.md	Rich text block deep dive: sub-elements, inline types, styles
references/SURFACES.md	Modals, App Home, canvases, lists, split view
references/WORK-OBJECTS.md	Entity types, chat.unfurl, flexpane, editable fields, actions

Sources

• Block Kit Reference — Slack
• Block Kit Blocks — Slack
• Block Kit Elements — Slack
• Block Kit Composition Objects — Slack
• Work Objects — Slack
• Surfaces — Slack
• Modals — Slack
• App Home — Slack
• Canvases — Slack
• Lists — Slack
• Split View — Slack
• App Design — Slack