Slack Message Formatting

Context Slice

Recognition

"slack", "post to channel", "DM", "message the team", "announce"
Quick internal communication
Real-time or near-real-time context

Format Selection

Use	When
Block Kit JSON	Announcements, structured updates, anything needing visual hierarchy, metrics, or actions
Plain mrkdwn	Quick replies, thread responses, simple one-liners

Default to Block Kit for any substantive message. Output valid JSON that can be pasted into Slack's Block Kit Builder or sent via API.

Block Kit Structure

Every Block Kit message is a JSON object with a blocks array:

{
  "blocks": [
    { "type": "header", ... },
    { "type": "section", ... },
    { "type": "divider" },
    { "type": "context", ... }
  ]
}

Block Types

Header

Top-level title for the message.

{
  "type": "header",
  "text": {
    "type": "plain_text",
    "text": "🚀 Release v2.4.0 Shipped",
    "emoji": true
  }
}

Section

Primary content block. Supports mrkdwn text and optional accessory (button, image, etc).

{
  "type": "section",
  "text": {
    "type": "mrkdwn",
    "text": "*What's new:*\n• Faster search indexing\n• Dark mode support\n• Bug fixes for auth flow"
  }
}

With accessory button:

{
  "type": "section",
  "text": {
    "type": "mrkdwn",
    "text": "Review the changes before merging."
  },
  "accessory": {
    "type": "button",
    "text": { "type": "plain_text", "text": "View PR", "emoji": true },
    "url": "https://github.com/org/repo/pull/123"
  }
}

Divider

Visual separator between sections.

{ "type": "divider" }

Context

Small metadata line (timestamps, authors, tags).

{
  "type": "context",
  "elements": [
    { "type": "mrkdwn", "text": "Posted by <@U123ABC> • Dec 5, 2024" }
  ]
}

Actions

Row of interactive buttons.

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

Image

Standalone image block.

{
  "type": "image",
  "image_url": "https://example.com/chart.png",
  "alt_text": "Weekly metrics chart"
}

Limits

Slack truncates or rejects content that exceeds these limits:

Element	Limit	Behavior
Section text	~3,000 chars	Truncated with "See more" link
Header text	150 chars	Hard limit
Context elements	2,000 chars combined	Hard limit
Fields per section	10 items	Hard limit
Blocks per message	50	Hard limit

Breaking Up Long Content

When content exceeds ~3,000 characters, Slack shows a "See more" link that hides the rest. Avoid this by splitting content across multiple blocks.

Strategies:

Split at natural breakpoints — paragraphs, topics, or logical sections
Use dividers — separate major sections visually
Prefer multiple short sections — three 1,000-char sections render better than one 3,000-char section
Use headers for structure — break long updates into titled subsections
Consider file upload — for content over ~5,000 chars total, upload as a text snippet instead

Example: Long update split into sections

Instead of one massive section:

{ "type": "section", "text": { "type": "mrkdwn", "text": "[3000+ chars all here]" }}

Split by topic:

{ "type": "section", "text": { "type": "mrkdwn", "text": "*What shipped:*\n• Feature A\n• Feature B" }},
{ "type": "divider" },
{ "type": "section", "text": { "type": "mrkdwn", "text": "*What's next:*\n• Feature C\n• Feature D" }},
{ "type": "divider" },
{ "type": "section", "text": { "type": "mrkdwn", "text": "*Blockers:*\n• Issue X pending review" }}

Templates

Announcement

{
  "blocks": [
    {
      "type": "header",
      "text": { "type": "plain_text", "text": "📢 [Title]", "emoji": true }
    },
    {
      "type": "section",
      "text": { "type": "mrkdwn", "text": "[Main announcement body with *bold* for emphasis]" }
    },
    { "type": "divider" },
    {
      "type": "context",
      "elements": [
        { "type": "mrkdwn", "text": "Questions? Reply in thread 👇" }
      ]
    }
  ]
}

Status Update with Metrics

{
  "blocks": [
    {
      "type": "header",
      "text": { "type": "plain_text", "text": "📊 Weekly Metrics", "emoji": true }
    },
    {
      "type": "section",
      "fields": [
        { "type": "mrkdwn", "text": "*Active Users*\n12,450 (+8%)" },
        { "type": "mrkdwn", "text": "*Revenue*\n$84.2K (+12%)" },
        { "type": "mrkdwn", "text": "*Churn*\n2.1% (-0.3%)" },
        { "type": "mrkdwn", "text": "*NPS*\n72 (+5)" }
      ]
    },
    { "type": "divider" },
    {
      "type": "section",
      "text": { "type": "mrkdwn", "text": "*Highlights:* Strong week driven by product launch. Retention up across all cohorts." }
    },
    {
      "type": "context",
      "elements": [
        { "type": "mrkdwn", "text": "Week of Dec 2–8, 2024 • <https://dashboard.example.com|Full Dashboard>" }
      ]
    }
  ]
}

Action Prompt

{
  "blocks": [
    {
      "type": "section",
      "text": { "type": "mrkdwn", "text": "🙋 *Need approval:* Budget request for Q1 marketing spend ($25K)" }
    },
    {
      "type": "actions",
      "elements": [
        {
          "type": "button",
          "text": { "type": "plain_text", "text": "✅ Approve", "emoji": true },
          "style": "primary",
          "action_id": "approve"
        },
        {
          "type": "button",
          "text": { "type": "plain_text", "text": "❌ Reject", "emoji": true },
          "style": "danger",
          "action_id": "reject"
        },
        {
          "type": "button",
          "text": { "type": "plain_text", "text": "View Details", "emoji": true },
          "url": "https://docs.example.com/budget-q1"
        }
      ]
    }
  ]
}

Team Update

{
  "blocks": [
    {
      "type": "header",
      "text": { "type": "plain_text", "text": "🏃 Sprint 14 Wrap-up", "emoji": true }
    },
    {
      "type": "section",
      "text": {
        "type": "mrkdwn",
        "text": "*Completed:*\n• User onboarding redesign\n• API rate limiting\n• Mobile push notifications\n\n*Carried over:*\n• Dashboard export feature (blocked on design)"
      }
    },
    { "type": "divider" },
    {
      "type": "section",
      "text": { "type": "mrkdwn", "text": "*Next sprint focus:* Performance optimization and Q1 planning" }
    },
    {
      "type": "context",
      "elements": [
        { "type": "mrkdwn", "text": "Sprint 14 • Nov 25 – Dec 6 • Team: Platform" }
      ]
    }
  ]
}

mrkdwn Reference

For text within blocks, use Slack's mrkdwn syntax:

Element	Syntax
Bold	`bold`
Italic	`_italic_`
Strike	`~strikethrough~`
Code	`inline code`
Code block	```code block```
Link	`<https://url.com\|display text>`
User mention	`<@U123ABC>`
Channel	`<#C123ABC>`
Bullet list	`• item` (or `- item`)

Pitfalls

Section text over ~3,000 characters gets truncated with "See more"—split into multiple sections
Don't mix Block Kit with plain text in the same message—use one or the other
header only accepts plain_text, not mrkdwn
fields in section blocks max out at 10 items, 2 columns
Button style only accepts "primary" (green) or "danger" (red)—omit for default gray
Test in Block Kit Builder before sending

Slice Info

Description

Rich Block Kit messages for channels, DMs, announcements

Tokens

2,280

Used By

Document Formatter skill

Slack Block Kit skill

Compose Block Kit Message task

Show 2 more

slice:compose.format.slack