ccheney/slack-mrkdwn

Slack mrkdwn

Slack's custom text formatting syntax for messages and text objects. Not standard Markdown.

CRITICAL: Two Markup Systems

Slack has two completely different markup syntaxes. Using the wrong one is the most common formatting mistake.

| System | Used In | Bold | Link | Heading | |--------|---------|------|------|---------| | Slack mrkdwn | text field, text objects (type: "mrkdwn"), section fields | *bold* | <url\|text> | Not supported | | Standard Markdown | markdown block only | **bold** | [text](url) | # Heading |

Standard Markdown syntax (**bold**, [text](url), # Heading) renders as literal text in mrkdwn contexts. Slack mrkdwn syntax (*bold*, <url|text>) renders as literal text in markdown blocks. Never mix them.

The markdown block (type: "markdown") accepts standard Markdown and translates it for Slack rendering. Supports: headings, bold, italic, strikethrough, lists, links, blockquotes, code blocks with optional syntax highlighting, horizontal rules/dividers, tables, task lists, and images (rendered as link text). A single input block may produce multiple output blocks. Cumulative limit across all markdown blocks in one payload: 12,000 characters.

mrkdwn Syntax

| Format | Syntax | Notes | |--------|--------|-------| | Bold | *bold* | Not **bold** | | Italic | _italic_ | Not *italic* | | Strikethrough | ~strikethrough~ | Not ~~strikethrough~~ | | Inline code | `code` | Same as standard Markdown | | Code block | ```code``` | No syntax highlighting in mrkdwn; use a markdown block with language-tagged fences for highlighting | | Blockquote | > quoted text | Prefix each line | | Link | <https://example.com\|display text> | Not [text](url) | | Emoji | :emoji_name: | Standard or custom. Direct Unicode also works | | Newline | \n | Literal newline in string | | Ordered list | 1. item | Plain text, no special rendering | | Bullet list | - item | Rendered properly in rich_text blocks only |

Inline code disables all other formatting within it — use it to display literal text like *not bold*.

Nested Formatting

Combining adjacent format markers without spaces (e.g., *bold*_italic_) is unreliable and may not render correctly. Always add a space between differently-formatted segments:

*bold* _italic_              ← works reliably
*bold*_italic_               ← may fail to render

For reliable combined formatting on a single word, use rich_text blocks with explicit style objects ({"bold": true, "italic": true}).

Links

<https://example.com>                            Auto-detected URL
<https://example.com|Display Text>               URL with custom text
<mailto:[email protected]|Email Link>             Email link

URLs posted in text are auto-linked by Slack. Use <url|text> for custom display text. Spaces in URLs will break parsing — remove them. mrkdwn formatting inside link labels (e.g., <url|*bold*>) works for basic styles.

Link Unfurling

Slack previews ("unfurls") linked content. Control this per message:

| Parameter | Controls | Default (API) | |-----------|----------|---------------| | unfurl_links | Text-based content previews | false | | unfurl_media | Media (images, video, audio) previews | true |

Set both to false to suppress all previews. These are chat.postMessage parameters, not mrkdwn syntax.

Mentions and References

User Mentions

<@U0123ABC456>

Triggers a notification for the mentioned user. Auto-converts to display name.

Channel References

<#C0123ABC456>

Auto-converts to channel name. Users without access see "private channel".

User Group Mentions

<!subteam^SAZ94GDB8>

Notifies all members of the user group.

Special Mentions

| Syntax | Scope | Caution | |--------|-------|---------| | <!here> | Active members in channel | Use sparingly | | <!channel> | All channel members | Triggers push notifications for everyone | | <!everyone> | All non-guest workspace members | Very disruptive |

Best Practice

Always use IDs, not names. IDs are stable; names change:

  <@U0123ABC456>       (user ID)
  @chris                (name — may not resolve)

  <#C0123ABC456>       (channel ID)
  #general              (name — may not resolve)

To enable name-based parsing, set link_names: 1 in the API call. This is fragile and discouraged.

Date Formatting

Displays dates/times localized to the reader's device timezone (not their Slack preference timezone).

Syntax

<!date^{unix_timestamp}^{token_string}^{optional_link}|{fallback_text}>

Tokens

| Token | Example Output | |-------|---------------| | {date_num} | 2014-02-18 | | {date} | February 18th, 2014 (omits year if within ~6 months) | | {date_short} | Feb 18, 2014 | | {date_long} | Tuesday, February 18th, 2014 | | {date_pretty} | Yesterday / February 18th, 2014 | | {date_short_pretty} | Yesterday / Feb 18, 2014 | | {date_long_pretty} | Yesterday / Tuesday, February 18th, 2014 | | {time} | 6:39 AM (12h) or 06:39 (24h) | | {time_secs} | 6:39:42 AM | | {ago} | 3 minutes ago / 4 hours ago |

_pretty variants use relative terms ("yesterday", "today", "tomorrow") when applicable.

Examples

<!date^1392734382^{date} at {time}|February 18th, 2014 at 6:39 AM PST>
<!date^1392734382^{date_short_pretty} {time}|Feb 18, 2014 6:39 AM>
<!date^1392734382^{ago}|February 18th, 2014>

Tokens can be mixed with literal text in the token string. The optional link (third ^-separated parameter) makes the date a clickable hyperlink. Fallback text (after |) displays for clients that cannot render date formatting.

Escaping

Only three characters require escaping in mrkdwn:

| Character | Escape Sequence | |-----------|----------------| | & | & | | < | < | | > | > |

Do NOT encode other characters as HTML entities. Only these three are control characters in Slack's markup system.

When displaying user-generated content that may contain these characters, always escape them to prevent unintended formatting or link injection.

Text Object

The text object is the most common composition object in Block Kit. It determines how text is rendered.

[
  { "type": "mrkdwn", "text": "*bold* and _italic_", "verbatim": false },
  { "type": "plain_text", "text": "No formatting", "emoji": true }
]

mrkdwn supports Slack mrkdwn syntax. plain_text renders literally. emoji: true converts :emoji: to rendered emoji (plain_text only). Min 1 char, max 3000 chars (section fields max 2000 chars each, max 10 fields).

Where Each Type Is Allowed

| Context | Allowed Types | |---------|--------------| | Header block text | plain_text only | | Section text / fields | mrkdwn or plain_text | | Context elements | mrkdwn or plain_text | | Button text | plain_text only | | Placeholder | plain_text only | | Input label / hint | plain_text only | | Modal title / submit / close | plain_text only | | Option text | plain_text only | | Option description | mrkdwn or plain_text |

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:

• Markdown formatting still processes
• No auto-linking or mention parsing
• 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 }

Auto-Parsing Behavior

The parse Parameter (chat.postMessage)

| Value | Effect | |-------|--------| | "none" (default) | mrkdwn formatting enabled; minimal auto-parsing of names/URLs | | "full" | Disables mrkdwn formatting; auto-parses URLs, channel names, user mentions |

Disabling Auto-Parsing

In text objects: Set verbatim: true (see above).

In message payloads:

• Omit link_names argument (or set to 0)
• Set parse: "none" to disable all auto-parsing

Disabling Formatting Entirely

| Context | Method | |---------|--------| | Text objects | Set type to "plain_text" | | Top-level message text | Set mrkdwn: false | | Attachments | Exclude field from mrkdwn_in array |

Anti-Patterns

| Anti-Pattern | Problem | Fix | |--------------|---------|-----| | **bold** in mrkdwn | Renders literally | Use *bold* | | [text](url) in mrkdwn | Renders literally | Use <url\|text> | | # Heading in mrkdwn | Renders as plain text | Use header block or markdown block | | *bold* in markdown block | Renders as italic | Use **bold** | | link_names: 1 for mentions | Fragile — names change, IDs don't | Use <@USERID> directly | | HTML-encoding beyond &<> | Renders literally | Only escape &, <, > | | Spaces in URLs | Breaks link parsing | URL-encode spaces as %20 | | Combining *bold*_italic_ without space | Rendering unreliable | Add space: *bold* _italic_ |

Secondary Attachments (Legacy)

The attachments array adds secondary content below the main message. One of fallback or text is required (unless using blocks).

| Field | Description | |-------|-------------| | fallback | Plain-text summary for limited clients (always plain text) | | color | Hex color or "good" / "warning" / "danger" | | pretext | Text above the attachment block | | author_name, author_link, author_icon | Small author line (16px icon) | | title, title_link | Large heading with optional hyperlink | | text | Main body (auto-collapses at 700+ chars) | | fields | Array of { title, value, short } objects | | image_url | Full-width image (GIF, JPEG, PNG, BMP) | | thumb_url | Thumbnail (75px max) | | footer, footer_icon, ts | Footer metadata (footer max 300 chars) | | mrkdwn_in | Array of fields to format with mrkdwn: "text", "pretext", "fields" |

Only "text", "pretext", and "fields" are accepted values in mrkdwn_in. Fields not listed render as plain text. fallback is always plain text.

Prefer Block Kit blocks over attachments for new development.

Reference Documentation

| File | Purpose | |------|---------| | references/CHEATSHEET.md | Quick reference: mrkdwn syntax, mentions, dates, escaping at a glance |

Sources

• Formatting Message Text — Slack
• Block Kit Composition Objects — Text Object — Slack
• Markdown Block — Slack
• Legacy Secondary Message Attachments — Slack
• Messaging Overview — Slack