slack-formatting

Slack Message Formatting Reference

MCP Tool Overview

The Slack MCP tools accept a message string parameter. The MCP layer translates standard markdown into Slack's native mrkdwn format before posting. Write messages using the conventions below.

Tool	Purpose	Key Parameters
slack_send_message	Send immediately	channel_id, message, thread_ts, reply_broadcast
slack_send_message_draft	Save as draft	channel_id, message, thread_ts
slack_schedule_message	Send later	channel_id, message, post_at (Unix timestamp)
slack_create_canvas	Create document	title, content (markdown)

Limits: 5000 characters per text element. Cannot post to externally shared (Slack Connect) channels.

Message Formatting

The MCP tools accept standard markdown syntax:

Effect	Syntax	Renders as
Bold	**text**	text
Italic	_text_	text
Strikethrough	~text~	text
Inline code	`text`	text
Code block	```code```	fenced code block
Blockquote	> text	indented quote
Link	[label](url)	clickable link

Lists

Bullet lists and numbered lists work with standard markdown:

- Item one
- Item two
- Item three

1. First
2. Second
3. Third

Code Blocks

Triple-backtick fenced blocks. Language hints are not rendered by Slack but are accepted:

```python
def hello():
    print("Hello")
```

Blockquotes

Prefix lines with >:

> This is a quoted line
> This continues the quote

Section Spacing

Slack collapses blank lines in messages. To create visible blank lines between sections, insert a line containing only a zero-width space (Unicode U+200B: \u200B). Regular empty lines and lines with only normal spaces are stripped.

**Section One**
Content here
\u200B
**Section Two**
More content

This produces a visible gap between sections that Slack does not collapse.

Special Slack Elements

These Slack-native constructs work within MCP message strings.

Mentions

Target	Syntax	Notes
User by ID	<@U012AB3CD>	Always use IDs, never display names
Channel by ID	<#C123ABC456>	Links to the channel
@here	<!here>	Notifies active channel members
@channel	<!channel>	Notifies all channel members
@everyone	<!everyone>	Notifies entire workspace

Use slack_search_users to find user IDs and slack_search_channels to find channel IDs before mentioning.

Emoji

Use Slack colon syntax: :tada:, :white_check_mark:, :warning:, :rocket:, :eyes:.

Useful emoji for status messages:

Emoji	Code	Use for
:white_check_mark:	:white_check_mark:	Success, completed
:x:	:x:	Failed, error
:warning:	:warning:	Warning, caution
:rocket:	:rocket:	Deploy, launch
:eyes:	:eyes:	Needs review
:hourglass:	:hourglass:	In progress, waiting
:bulb:	:bulb:	Idea, tip
:rotating_light:	:rotating_light:	Alert, urgent
:memo:	:memo:	Notes, documentation
:link:	:link:	Reference, URL
:tada:	:tada:	Celebration
:fire:	:fire:	Hot, critical

Date Formatting

Renders timestamps in the viewer's local timezone:

<!date^UNIX_TIMESTAMP^TOKEN_STRING|fallback_text>

Tokens:

Token	Example Output
{date_num}	2024-02-18
{date}	February 18th, 2024
{date_short}	Feb 18, 2024
{date_long}	Tuesday, February 18th, 2024
{date_pretty}	today / yesterday / February 18th, 2024
{time}	6:39 AM
{time_secs}	6:39:45 AM
{ago}	3 minutes ago

Tokens combine: {date_short} at {time} renders as "Feb 18, 2024 at 6:39 AM".

Example: <!date^1392734382^{date_short} at {time}|Feb 18, 2014 at 6:39 AM>

Add a link: <!date^1392734382^{date_short}^https://example.com|Feb 18, 2014>

Message Composition Patterns

Use \u200B lines between sections for visual spacing (see Section Spacing above).

Status Update

:rocket: **Deploy v2.4.1 to production**
\u200B
_Environment:_ production
_Branch:_ `release/2.4.1`
_Status:_ :white_check_mark: Complete
\u200B
> All health checks passing. No rollback needed.

Notification with Action Items

:warning: **Database migration required**
\u200B
The `users` table schema changed in PR #482. Before deploying:
\u200B
1. Run `./scripts/migrate.sh` on staging
2. Verify row counts match
3. Deploy to production
\u200B
_Ping <@U012AB3CD> when ready._

Summary Report

:memo: **Weekly Sprint Summary**
\u200B
_Completed:_ 12 tickets
_In Progress:_ 4 tickets
_Blocked:_ 1 ticket
\u200B
**Highlights:**
- Shipped user profile redesign
- Fixed payment processing timeout
- Onboarded new API partner
\u200B
**Blockers:**
- :rotating_light: Waiting on vendor SSL cert renewal (ETA: Thursday)

Incident Alert

:rotating_light: **P1 Incident: API Latency Spike**
\u200B
_Detected:_ <!date^1710900000^{date_short} at {time}|Mar 20, 2024 at 12:00 AM>
_Service:_ api-gateway
_Impact:_ 95th percentile latency > 2s (normal: 200ms)
\u200B
**Current status:** Investigating. Initial triage points to database connection pool exhaustion.
\u200B
_Incident channel:_ <#C123ABC456>

Code Review Request

:eyes: **Review requested: Refactor auth middleware**
\u200B
_MR:_ [!482 - Refactor auth middleware](https://gitlab.example.com/project/-/merge_requests/482)
_Author:_ <@U012AB3CD>
_Changes:_ 4 files, +120 / -85
\u200B
Key changes:
- Extract token validation into standalone module
- Add refresh token rotation
- Remove deprecated session cookie path

Canvas Formatting

The slack_create_canvas tool uses standard markdown with specific constraints:

• Headers must not exceed depth 3 (### max). Deeper headers are truncated to ###.
• Links must be full HTTP URLs. No relative links.
• Tables: escape | inside cell content with \|.
• Write in full paragraphs, not bullet lists (unless explicitly requested).
• Do not duplicate the canvas title in the content body.

Thread Replies

To reply in a thread, set thread_ts to the parent message's timestamp string (e.g., "1234567890.123456").

Set reply_broadcast: true to also show the reply in the main channel.

Slack's Native mrkdwn Format

The MCP tools handle translation, but when reading Slack content or debugging formatting, note that Slack's native mrkdwn differs from standard markdown:

Feature	Standard Markdown	Slack mrkdwn
Bold	**text**	*text*
Italic	_text_ or *text*	_text_
Links	[label](url)	<url|label>
Images		Not supported inline

Slack mrkdwn does not support: nested formatting, tables, headings, horizontal rules, or images inline.

Characters that must be HTML-escaped in raw mrkdwn: & to &, < to <, > to >.

Limitations

The Slack MCP tools only accept a message string. They do not expose a blocks parameter, so Block Kit JSON cannot be used. All formatting must be done with the markdown syntax and special elements documented above.

Additional Resources

• references/block-kit.md - Full Block Kit reference covering block types, interactive elements, composition objects, rich text blocks, and legacy attachments. This is for direct Slack API usage only; Block Kit is not available through the MCP tools.