/slack-mrkdwn

Slack Text Formatting Quick Reference

Authoritative sources: Formatting message text, Markdown block, and Text object.

Pick the Right Renderer

*bold* is italic on standard-Markdown surfaces. Never choose syntax without first checking the destination field.

mrkdwn Syntax

*bold*                          _italic_
~strikethrough~                 `inline code`
```code block```               >quoted line
<https://url|display text>      <mailto:user@example.com|Email>
<@U0123ABC>                    <#C0123ABC>
:emoji_name:                   \n
<!date^1234567890^{date} at {time}|fallback with timezone>

Not mrkdwn: **bold**, [text](url), # heading, true list syntax, or language-tag syntax highlighting.

Standard Markdown Surfaces

markdown block

• 12,000 characters cumulative across all markdown blocks in a payload.
• A block can translate into multiple output blocks; block_id is ignored and not retained.
• Documents bold/italic/strike, links, ordered/unordered lists, block quotes, inline/fenced code, syntax highlighting, headings, dividers, tables, task lists, image-to-link translation, and backslash escapes.
• The block reference says all heading levels render at the same size, but Slack's March 6, 2026 changelog says variable-sized headers are rolling out. Treat heading size as rollout- and client-dependent.

markdown_text

12,000-character standard-Markdown argument on:

• chat.postMessage, chat.postEphemeral, chat.scheduleMessage, chat.update
• chat.startStream, chat.appendStream, chat.stopStream

On the four non-streaming methods, do not combine it with blocks or text (markdown_text_conflict). chat.stopStream may include final blocks, rendered after the streamed content.

Current streaming-method Markdown chunk shape:

{ "type": "markdown_text", "text": "**Result**" }

Streaming messages do not unfurl links.

Work Objects

{
  "type": "string",
  "value": "**Blocked** — [runbook](https://example.com)",
  "format": "markdown"
}

• format: "markdown" is for string fields/custom fields and Work Object comments.
• It cannot be combined with a string field's icon or link.
• Partial-access custom_message uses message_format: "markdown".
• A Work Object reference inside rich_text uses a work_object_mention element, not mrkdwn.

Mentions

<@U0123ABC>                  User mention; notifies the user
<#C0123ABC>                  Conversation link
<!subteam^SAZ94GDB8>         User group mention; notifies the group
<!here>                      Active channel members
<!channel>                   All channel members
<!everyone>                  Non-guest workspace members through #general

Always use IDs. link_names currently links user groups only, not users. Use <@USER_ID> for users and <#CONVERSATION_ID> for conversations.

Dates

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

Slack uses the viewer device's timezone, not the Slack preference timezone. The fallback is required; include its timezone. The optional link must be fully qualified.

Escaping

Only these Slack control characters use HTML entities:

&  ->  &
<  ->  &lt;
>  ->  &gt;

Escape & first. Do not encode the whole string. Escape untrusted text before inserting it into a mrkdwn-capable field; verbatim: true does not neutralize explicit <...> controls.

Text Objects

[
  { "type": "mrkdwn", "text": "*bold*", "verbatim": true },
  { "type": "plain_text", "text": "literal", "emoji": true }
]

• Generic text: 1–3,000 characters; containing fields can impose lower limits.
• Section fields: up to 10 items, 2,000 characters each.
• verbatim: false (default): auto-preprocesses raw URLs, conversation names, and certain mentions.
• verbatim: true: skips that preprocessing but still parses mrkdwn and explicit <...> constructs.
• emoji is only for plain_text and controls conversion into colon emoji form.

Common acceptance rules:

Top-Level Parsing

Keep top-level text under 4,000 characters for best results; Slack truncates over 40,000.

Accessibility and Notifications

For a message with blocks, either:

1. put all necessary content in top-level text, or
2. omit top-level text and let Slack attempt to synthesize it from supported blocks.

Screen readers default to top-level text. Desktop notifications (since July 13, 2026) extract supported block text first and fall back to message.text; mobile notifications exclusively use message.text.

Links and Unfurls

https://example.com
<https://example.com>
<https://example.com|Display Text>
<mailto:user@example.com|Email>

• Spaces break URLs.
• Set both unfurl_links: false and unfurl_media: false to suppress previews.
• For LLM-generated URLs, default both flags to false; Slack warns that the preview crawl can complete prompt-injection data exfiltration. If previews are required, allow-list trusted domains.
• <http://example.com|example.com> is not unfurled because its label is a complete substring of the URL without the protocol.
• Composer unfurl preview elements with type: "mrkdwn" and unfurl Block Kit text objects use mrkdwn; Work Object metadata fields use standard Markdown instead.
• chat.unfurl does not currently support rich-text blocks and may return invalid_blocks for a Block Kit payload containing a rich-text section element.

Rich Text

rich_text is structured, not a syntax string. Use rich_text_section, rich_text_list, rich_text_quote, or rich_text_preformatted and explicit elements/styles. It is Slack's user-composer and rich_text_input representation, and is preferred for user-defined formatted content.

Legacy Attachments

Prefer Block Kit. For existing attachments:

{
  "fallback": "Deployment completed",
  "text": "Deployment *completed*",
  "mrkdwn_in": ["text"]
}

Documented mrkdwn-capable legacy content: text, pretext, and field-object values through fields. Field titles and fallback are plain text. Without attachment blocks, one of fallback or text is required. Maximum 20 attachments per message.