/slack-mrkdwn

Slack Text Renderers

Contents

• Choose the Rendering System
• Standard Markdown Surfaces
• Text Objects
• Structured Rich Text

Choose the Rendering System

Do not translate by punctuation alone. For example, *bold* in mrkdwn is italic in standard Markdown. A rich_text block does not parse either syntax; construct its elements explicitly.

Standard Markdown Surfaces

Markdown Block

{
  "type": "markdown",
  "text": "## Result\n\n**Complete** — see [details](https://example.com)."
}

Slack's markdown block explicitly documents support for:

• bold, italic, nested bold/italic, and strikethrough
• ordered and unordered lists
• links, block quotes, inline code, and fenced code blocks
• language-tagged fenced code blocks with syntax highlighting
• headings, horizontal dividers, tables, and task lists
• images, translated to hyperlink text using the image alt text
• backslash escaping of documented special characters

The current block reference says all heading levels render at the same size, while Slack's March 6, 2026 changelog says variable-sized headers are being rolled out. Treat heading size as rollout- and client-dependent rather than relying on a specific visual hierarchy until the references converge. A single input block may translate into multiple output blocks. The cumulative text limit across all markdown blocks in one payload is 12,000 characters. A supplied block_id is ignored and not retained.

markdown_text Chat Arguments

The following current Web API methods accept a standard-Markdown markdown_text argument with a 12,000-character limit:

Streaming methods also accept chunks. In the current method references, a Markdown chunk is:

{ "type": "markdown_text", "text": "**Streaming** response" }

chunks can also carry task updates, plan updates, and block chunks. Consult the individual streaming method reference for the exact request fields and current limits. Link unfurling is disabled in streaming messages.

Work Objects

Work Objects do not use mrkdwn for their metadata string formatting:

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

• A Work Object fields or custom_fields property of type string can set format: "markdown". This is incompatible with that field's icon or link properties.
• Work Object comments can provide { "value": "...", "format": "markdown" }; comments may alternatively provide structured rich-text blocks.
• A custom partial-access message can set message_format: "markdown" for its custom_message.
• To reference a Work Object inside rich_text, use a structured work_object_mention element (including entity_id, app_id, text, and url), not a made-up mrkdwn token.
• Block Kit text objects embedded in composer or classic link unfurls still follow their own mrkdwn/plain_text rules.

Text Objects

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

The generic text object allows 1–3,000 characters. A containing block or element may impose a smaller limit; for example, section fields allow up to 10 objects of 2,000 characters each.

• verbatim is valid only for mrkdwn.
• emoji is valid only for plain_text and controls whether Slack escapes recognized emoji into colon format. It does not enable mrkdwn.

verbatim

Use verbatim: true when content contains raw @, #, or URLs that should not be automatically rewritten. It is not a way to neutralize deliberate angle-bracket control syntax; escape untrusted < and >.

Common Field Rules

This is a practical summary, not a substitute for the receiving component's reference:

Slack adds Block Kit components over time. Verify the exact field reference before choosing a text type or limit.

Structured Rich Text

rich_text is the structured format produced by Slack's user composer and by rich_text_input. Slack's reference strongly prefers it for user-defined formatted text because it is more flexible than mrkdwn.

Use explicit elements such as rich_text_section, rich_text_list, rich_text_quote, and rich_text_preformatted, with nested elements/styles for text, links, emoji, users, channels, user groups, broadcasts, dates, and Work Object mentions. Do not put a mrkdwn string into a rich_text block and expect it to parse.

Sources

• https://docs.slack.dev/reference/block-kit/composition-objects/text-object/
• https://docs.slack.dev/reference/block-kit/blocks/markdown-block/
• https://docs.slack.dev/messaging/work-objects-implementation/