Supported markdown

slackmark accepts CommonMark, the GFM extensions (tables, task lists, strikethrough, autolinks, footnotes), Slack entity tokens, mermaid charts, GitHub admonitions, and optional math and frontmatter. Each construct maps to the most native Slack block available; when a capability is off or a limit is hit, it falls down an ordered chain and records a degradation.

Block-level constructs

Markdown	Native output	Fallback chain
Heading 1–4	`header` with `level`	`header` without level (needs `headerLevel`)
Heading 5–6, > 150 chars, or with inline markup	Bold `rich_text` section	— (Slack headers stop at level 4 / plain text)
Paragraph	`rich_text` section	— (adjacent sections coalesce into one block)
Bullet / numbered list	`rich_text_list` (nested, ordered `start` kept)	Indent clamped at 6 levels
Task list (`- [x]`)	`markdown` block with checkboxes (needs `markdownBlock`)	`rich_text_list` with ☑ / ☐ characters
GFM table ≤ 50 rows	`table` (needs `tableBlock`)	`markdown` pipe table → preformatted text
GFM table ≤ 200 data rows	`data_table` (needs `dataTable`)	Truncated `table` → `markdown` → preformatted
GFM table > 20 columns	`markdown` pipe table	Preformatted text
Code fence	Image via matching configured renderer	`rich_text_preformatted` with language → same without language
`mermaid` fence, `pie` / `xychart-beta`	`data_visualization` (needs `dataVisualization`)	Image via configured renderer → preformatted mermaid source
`mermaid` fence, other diagrams	Image via configured renderer	Preformatted mermaid source
Math block `$$…$$` (`enableMath`)	Image via matching configured renderer	`rich_text_preformatted` with `latex` language → same without it
Blockquote	`rich_text_quote`	Nested lists / code inside quotes flatten to text lines
Admonition (`> [!NOTE]` etc.)	`rich_text_quote` with emoji + bold title line	NOTE 📝, TIP 💡, IMPORTANT ❗, WARNING ⚠️, CAUTION 🚫
Image on its own line	`image` block	Link or text unless a public http(s) png/jpg/gif ≤ 3000 chars
Thematic break `---`	`divider`	—
Footnotes `[^1]`	`[n]` markers + trailing `divider` + `context` blocks	Duplicate definitions: first wins
`<details><summary>` HTML	Collapsible `container` (needs `containerBlock`)	Bold title + body `section`; inline content becomes plain text
Other block HTML	Tags stripped; text kept as a `section`	Slack tokens convert but may keep surrounding tags as literal text
Frontmatter (`enableFrontmatter`)	Skipped	Always a degradation record

HTML degradation reporting is incomplete; see Validating output.

Several rows above appear nowhere else in these docs. Here is a sampler of them, all default options:

> Plain quote with a [link](https://example.com).

1. First
   1. Nested
2. Second

---

<details>
<summary>Rollback plan</summary>
Revert the deploy, then page the on-call.
</details>

See the runbook[^1].

[^1]: https://example.com/runbook

Block Kit JSON

…

Inline constructs

Markdown	Output
`bold`, `italic`, `~~strike~~`	Rich text styles (combinable)
`code`	Code-styled text; never scanned for mentions or emoji
`[label](https://…)`, autolinks	`link` element; `http(s)` and `mailto` only, other schemes become text + URL
Reference links `[label][ref]`	Resolved against definitions; unresolved stay text
Inline image	Hoisted to an `image` block; link left in place
`<@U…>`, `<#C…>`, `<!here>`, `<!subteam^…>`, `<!date^…>`	Native mention / broadcast / date elements (details)
`<https://…\|label>`	`link` element with the label
`:emoji:`	`emoji` element, including custom workspace emoji
`@handle`, `#channel`	Mentions via `mentionResolvers`; else literal text
Inline HTML `<b>`, `<i>`, `<s>`, `<code>`, `<a>`, `<br>`	Corresponding styles, links, and line breaks
Other inline HTML	Stripped to text with a degradation
Inline math $x$ (`enableInlineMath`)	Code-styled text with a degradation; literal text unless opted in
Hard break	Newline within the section

Limits enforced during conversion

Slack's documented Block Kit limits are enforced with truncation + degradation rather than API errors:

Limit	Value
Blocks per message	50
Header text	150 chars
Section text	3,000 chars
Image URL / alt text	3,000 / 2,000 chars
Table rows / columns	100 / 20
`data_table` data rows	200
Cumulative `markdown` block text	12,000 chars
Chart segments / series / points	12 / 12 / 20
Chart title / labels	50 / 20 chars
Charts per message	2 (defensive, community-reported)
Notification `text`	40,000 chars

validateBlocks asserts the block limits above offline; the notification-text clamp is applied separately during message assembly.