Supported markdown

slackmark accepts CommonMark, GFM (tables, task lists, strikethrough, and autolinks), footnotes, Slack entity tokens, Mermaid charts, GitHub-style 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

The converter enforces Slack's documented Block Kit limits plus a small number of explicitly labeled defensive choices. It truncates or falls back with a degradation rather than emitting a payload it knows exceeds those limits:

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.