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 accepts standard Markdown (**bold**, [text](url)). Mixing these up is the single most common Block Kit generation error.

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 / data_table)
├─ Charts or metrics visualization                 → blocks (data_visualization)
├─ 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)
├─ Status callout in a MODAL          → alert (modals only, 200 chars)
├─ Compact entity or summary preview  → card (optional image/actions)
├─ Multiple comparable cards/options  → carousel (1-10 cards)
├─ Grouped/collapsible block set      → container (title + ≤10 child blocks)
├─ Standalone image                   → image (image_url or slack_file)
├─ Formatted text with lists, quotes  → rich_text (nested sub-elements)
├─ Simple static table                → table (100 rows, 20 cols)
├─ Sortable/paginated table           → data_table (caption + 100 data rows)
├─ Pie/bar/area/line chart            → data_visualization (native charts)
├─ 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              → 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)
├─ LLM-generated tables/task lists/code     → markdown block
├─ Programmatic tabular data                → table or data_table block
├─ Need headings                            → markdown block or header blocks
└─ Mixed: structured layout + prose         → section/header blocks + markdown block

"table, data_table, or markdown table?"

├─ LLM already emitted a Markdown table          → markdown block
├─ Static data, no interaction needed            → table
└─ Users need sorting, paging, or clickable cells → data_table (caption required)

Block Types Overview

Full property tables for all 21 blocks: references/BLOCKS.md. Highlights and sharp edges below.

header

Large bold text. plain_text only — mrkdwn is silently ignored. Max 150 chars.

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

section

Primary content block. Supports text (3000 chars), two-column fields (10 items, 2000 chars each), and one accessory element. Either text or fields is required. Set expand: true to prevent "see more" truncation on long AI responses.

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

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 • 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" }, "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.

alert

Callout for status, risk, or urgency. Modals only — putting an alert in a message payload is invalid. Text accepts plain_text or mrkdwn, max 200 chars. level: default, info, warning, error, or success. For message-surface callouts, use a section block with an emoji or a card instead.

{
  "type": "alert",
  "text": { "type": "mrkdwn", "text": "*Dependency conflict detected* before deploy." },
  "level": "warning"
}

card

Compact, scannable preview for entities, summaries, records, or agent results. At least one of hero_image, title, actions, or body is required.

{
  "type": "card",
  "title": { "type": "mrkdwn", "text": "Daily Standup Reminder" },
  "subtitle": { "type": "mrkdwn", "text": "Runs every weekday at *9:00 AM*" },
  "body": { "type": "mrkdwn", "text": "Last run: Today at 9:00 AM. Status: Success" },
  "actions": [
    { "type": "button", "text": { "type": "plain_text", "text": "View Logs" }, "action_id": "view_logs" }
  ]
}

Fields: icon/hero_image (image elements), slack_icon (built-in icon, mutually exclusive with icon), title/subtitle (150 chars), body/subtext (200 chars), actions (max 3 buttons; danger left-aligns, primary/unstyled right-align). No size attribute.

carousel

Horizontal, scrollable group of card blocks for options, recommendations, search results, or next steps. Must contain 1-10 cards.

{
  "type": "carousel",
  "elements": [
    { "type": "card", "title": { "type": "mrkdwn", "text": "Option A" } },
    { "type": "card", "title": { "type": "mrkdwn", "text": "Option B" } }
  ]
}

container

Groups up to 10 child blocks under a required plain_text title (150 chars). Optional subtitle, icon, width (narrow/standard/wide/full), and collapsing (is_collapsible, default_collapsed). Child blocks: actions, context, divider, file, header, image, input, rich_text, section, table, video — no nested containers, cards, or plan blocks.

{
  "type": "container",
  "title": { "type": "plain_text", "text": "Bulk update: 2 records" },
  "is_collapsible": true,
  "child_blocks": [
    { "type": "section", "text": { "type": "mrkdwn", "text": "*DCW-1024*\nStatus: Open → Closed" } }
  ]
}

image

Standalone image with required 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" }

rich_text

Advanced formatted text with nested elements: styled text, lists, code blocks, quotes. See references/RICH-TEXT.md before writing nested lists or inline mentions — the nesting rules are easy to get wrong.

{
  "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%" }] }
      ]
    }
  ]
}

Sub-element types: rich_text_section (paragraph), rich_text_list (style: "bullet" or "ordered"), rich_text_preformatted (code block), rich_text_quote (blockquote). Inline types: text, link, emoji, user, channel, usergroup, broadcast, date, color.

table

Simple tabular data. Each row is a flat array of cell objects (NOT an object with a cells property). There is no columns property — the first row is the header. Cell types: raw_text, raw_number, rich_text. Max 100 rows, 20 columns, 10,000 chars across all cells (per table AND aggregate per message).

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

data_table

Interactive table: pagination, sorting, filtering, clickable cells. Same row shape as table, plus a required caption string. Rows: header + 1–100 data rows; all rows need equal cell counts; header cells cannot be rich_text. Optional page_size (1–100, default 5) and row_header_column_index (default 0). Numeric sorting when a column is all raw_number cells.

{
  "type": "data_table",
  "caption": "Service health",
  "rows": [
    [{ "type": "raw_text", "text": "Service" }, { "type": "raw_text", "text": "Latency (ms)" }],
    [{ "type": "raw_text", "text": "API" }, { "type": "raw_number", "value": 12, "text": "12" }]
  ]
}

data_visualization

Native pie, bar, area, or line charts. Required: title (≤50 chars) and chart. Pie: segments (1–12, labels ≤20 chars, values > 0). Bar/area/line: series (1–12; names ≤20 chars, unique) plus axis_config.categories — every data point label must exactly match a category, and no series may omit a category.

{
  "type": "data_visualization",
  "title": "Weekly Sales",
  "chart": {
    "type": "bar",
    "series": [
      { "name": "Online", "data": [{ "label": "Week 1", "value": 32000 }, { "label": "Week 2", "value": 41000 }] }
    ],
    "axis_config": { "categories": ["Week 1", "Week 2"], "y_label": "USD" }
  }
}

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 (all levels render the same size), lists, code (inline and fenced with syntax highlighting), block quotes, dividers, tables, task lists, and escaping. Images render as hyperlink text. Cumulative 12,000-char limit across all markdown blocks per payload. block_id is ignored. One markdown block may translate into multiple Slack blocks.

context_actions

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

video

Embedded video player. Requires links.embed:write scope and a publicly accessible URL in the 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.

Streaming Agent Output

Use chat.startStream, chat.appendStream, and chat.stopStream for live AI responses (all require chat:write). Start requires channel and thread_ts — streamed messages should reply to a user request. When streaming to channels, recipient_user_id and recipient_team_id are also required.

chunks (accepted by all three methods) can include:

• markdown_text chunks — standard Markdown text
• task_update chunks — timeline-style task progress (same shape as task_card; id not task_id)
• plan_update chunks — update a plan title
• blocks chunks — arrays of Block Kit blocks (max 50 per array; extras are dropped with a warning)

task_update and plan_update chunk fields are limited to 256 characters.

Set task_display_mode on chat.startStream:

• timeline (default): tasks appear individually in sequence
• plan: tasks grouped in one plan; first task placement determines plan placement
• dense: consecutive tool calls collapse into a single summarized task card

chat.stopStream can add final blocks rendered after streamed content. Its 50-block limit is separate from streamed blocks chunks, allowing up to 100 total finalized blocks. Rate limits: start/stop Tier 2 (20+/min), append Tier 4 (100+/min).

Composition Objects

Option object (used in selects, 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 max 75 chars.

Confirmation dialog (add to any interactive element via confirm):

{
  "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"
}

Others: conversation filter (include: im/mpim/private/public), dispatch action config (on_enter_pressed, on_character_entered), Slack file, Slack icon (card blocks), trigger/workflow (workflow buttons). Full property tables: references/COMPOSITION.md.

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
Alert text (modals only)	200 chars
Card title/subtitle	150 chars
Card body/subtext	200 chars
Card action buttons	3
Carousel cards	1-10
Container child blocks	10
Container title/subtitle	150 chars
Table rows / columns	100 / 20
Table chars (per table and per message)	10,000
Data table rows	header + 100 data rows
Data viz series/segments	1-12 (labels/names 20 chars)
Data points per series	1-20
Markdown block text	12,000 chars cumulative per payload
Streaming chunk fields (task/plan update)	256 chars
Modal title / submit / close	24 chars each
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	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
alert block in a message	Alerts are modal-only; payload rejected	Use section + emoji or card in messages
Huge tables in one message	10,000-char cell limit exceeded	Split across multiple messages
Rows as { "cells": [...] } objects	Invalid — rows are flat arrays of cells	Each row = array of cell objects
columns property on table	No such property	First row of rows is the header
Chart labels not matching categories	Runtime validation failure	Every data point label must match axis_config.categories
mrkdwn in header text	Ignored — headers only accept plain_text	Use plain_text type
Standard Markdown in mrkdwn fields	**bold** and [links](url) render literally	Use *bold* and <url|text>, or a markdown block
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; two-column key-value layouts improve readability; a table or chart presents data more clearly than prose; visual separation helps; code needs surrounding context; interactive elements are needed.

Don't use blocks when: the response is conversational ("sure, done"), under ~3 lines, or 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 and return view_submission payloads. private_metadata (3000 chars) persists context between views. Read references/SURFACES.md before building modal flows or App Home views — it covers the view object schema, response actions, and view.state.values extraction paths.

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.

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. Read references/WORK-OBJECTS.md before implementing — Slack silently drops the whole payload if any required field is missing.

Reference Documentation

Read this	Before doing this
references/CHEATSHEET.md	Quick lookup of any block/element/limit at a glance
references/BLOCKS.md	Building any block payload — all 21 blocks with full property tables
references/ELEMENTS.md	Adding buttons, selects, inputs, or pickers — all 20 elements
references/COMPOSITION.md	Writing text objects, options, confirm dialogs, filters, Slack icons
references/RICH-TEXT.md	Writing rich_text blocks — nesting, inline types, styles
references/SURFACES.md	Building modals, App Home, canvases, or split view flows
references/WORK-OBJECTS.md	Implementing link unfurls, flexpanes, editable fields

Sources

• Block Kit Reference — Slack
• Block Kit Blocks — Slack
• Block Kit Elements — Slack
• Composition Objects — Slack
• chat.startStream — Slack
• chat.appendStream — Slack
• chat.stopStream — Slack
• Work Objects — Slack
• Surfaces — Slack
• Modal views — Slack
• App Home — Slack