A Claude Code skill that formats messages for Slack with pixel-perfect accuracy. Converts standard Markdown to Slack-compatible output with two delivery paths:
- Copy-paste — Rich HTML that preserves formatting when pasted into Slack's compose box
- API/Webhook — Slack mrkdwn syntax for bots, automation, and CI/CD
- Slack uses mrkdwn (not Markdown).
**bold**doesn't work — you need*bold*. - Existing tools handle conversion well, but none combine generation + preview + copy-paste in one workflow.
- Programmatic clipboard doesn't preserve tables in Slack. Manual browser copy does.
- This skill gives you both paths: copy-paste for humans, webhook for bots.
Zero dependencies. 232+ tests. Built for Claude Code.
claude plugin marketplace add karanb192/slack-message-formatter
claude plugin install slack-message-formatter@slack-message-formattercurl -sSL https://raw.githubusercontent.com/karanb192/slack-message-formatter/main/install.sh | bash -s codex# Claude Code global
curl -sSL https://raw.githubusercontent.com/karanb192/slack-message-formatter/main/install.sh | bash
# Codex global
curl -sSL https://raw.githubusercontent.com/karanb192/slack-message-formatter/main/install.sh | bash -s codex
# Current project only (auto-detects Claude Code or Codex)
curl -sSL https://raw.githubusercontent.com/karanb192/slack-message-formatter/main/install.sh | bash -s projectThe marketplace tracks the main branch of this repo. Claude Code auto-updates marketplace plugins at startup, so most users will pick up changes on their next restart.
To refresh immediately without restarting:
/plugin marketplace update slack-message-formatter
/reload-plugins
# Claude Code
rm -rf ~/.claude/skills/slack-message-formatter
# Codex
rm -rf "${CODEX_HOME:-$HOME/.codex}/skills/slack-message-formatter"Then in Claude Code, just ask:
- "Write a Slack message announcing our v2.5 release"
- "Format this for Slack"
/slack-message-formatter
echo '**bold** and *italic*' | node src/run.mjs html
# → <b>bold</b> and <i>italic</i>
echo '**bold** and *italic*' | node src/run.mjs mrkdwn
# → *bold* and _italic_
echo '## Announcement' | node src/run.mjs preview
# → Opens browser with Slack-themed preview + copy page- Opens a clean HTML page in your browser
Cmd+A,Cmd+C, thenCmd+Vin Slack- Preserves: bold, italic, strikethrough, links, lists, nested lists, code blocks, blockquotes, headings, task lists, tables (as code blocks), horizontal rules
- Converts to Slack's native mrkdwn format
- Send directly via webhook:
echo '**hello**' | node src/run.mjs send
- Requires
SLACK_WEBHOOK_URLenvironment variable (CCH_SLA_WEBHOOKalso works) - HTTP errors from Slack are reported with status and response body
| Markdown | Slack mrkdwn | HTML (paste) |
|---|---|---|
**bold** |
*bold* |
<b>bold</b> |
*italic* |
_italic_ |
<i>italic</i> |
~~strike~~ |
~strike~ |
<s>strike</s> |
`code` |
`code` |
<code>code</code> |
[text](url) |
<url|text> |
<a href="url">text</a> |
# Heading |
*Heading* |
<b>Heading</b> |
- [x] Done |
:white_check_mark: Done |
✅ Done |
- [ ] Todo |
:black_square_button: Todo |
⬜ Todo |
| Tables | Code block | Code block |
--- |
━━━━━━━━━━ |
<hr> |
:tada: |
:tada: (Slack renders) |
🎉 (Unicode) |
| Command | What it does |
|---|---|
preview |
Opens browser with copy page + dark preview |
send |
Sends via Slack webhook (mrkdwn) |
html |
Outputs raw HTML to stdout |
mrkdwn |
Outputs raw mrkdwn to stdout |
| Env Variable | Default | Description |
|---|---|---|
SLACK_FORMATTER_PREVIEW_DIR |
/tmp/slack-formatter |
Directory for preview HTML files (pruned after 7 days) |
SLACK_WEBHOOK_URL |
(none) | Slack webhook URL for send command (CCH_SLA_WEBHOOK also honored for back-compat) |
JIRA_BASE_URL |
(none) | Jira site URL (e.g. https://yoursite.atlassian.net). When set, bare ticket keys like ENG-12345 become clickable links on every output path |
Set JIRA_BASE_URL and bare Jira keys are turned into links automatically:
export JIRA_BASE_URL=https://yoursite.atlassian.net
echo 'Fixed DEVOPS-14389' | node src/run.mjs mrkdwn
# → Fixed <https://yoursite.atlassian.net/browse/DEVOPS-14389|DEVOPS-14389>Keys inside code spans, code blocks, existing links, or URLs are left alone,
as are common acronyms that look like keys (UTF-8, SHA-256, CVE-…, etc.).
Markdown → Parser → Dual Renderer
├── Rich HTML → Browser → Cmd+C → Slack paste
└── mrkdwn → Webhook → Slack API
- You write Markdown (or Claude generates it)
- The converter transforms it deterministically — same input always produces same output
- Two outputs: Rich HTML for copy-paste, mrkdwn for API
Tables are rendered as aligned code blocks because Slack's paste handler breaks HTML <table> tags when mixed with other rich content.
Through extensive testing, we discovered:
- Programmatic clipboard (Clipboard API,
execCommand,osascript) does not reliably preserve formatting when pasting into Slack - Manual browser copy (
Cmd+A,Cmd+Cfrom a rendered HTML page) works perfectly for all formatting including tables - HTML tables break in Slack paste when mixed with other rich content (bold, lists, blockquotes) — even with manual copy. Tables must be code blocks.
- Slack's paste handler trims spaces around inline formatting inside list items —
<li>with <b>bold</b> and</li>pastes as "withboldand". The converter emits around inline tags in list items; Slack normalizes it back to a regular space. - A
<br>inside<li>makes Slack flatten the whole list to plain paragraphs. Multi-line list items are joined with spaces (Markdown soft-wrap semantics) so lists stay native. - 150+ emoji shortcodes (
:tada:,:rocket:, etc.) are converted to Unicode for browser preview
node test-skill.mjs # from repo rootComprehensive test suite with 232+ tests covering:
- Both HTML and mrkdwn output for every feature
- Emoji shortcode conversion (85+ verified individually)
- Nested formatting, edge cases, unclosed markers
- Real-world messages (deployment, incident, meeting notes, code review, sprint summary)
- Special character escaping, Windows line endings
- Bare URLs (
https://example.comwithout link syntax) are not auto-linked. Use[text](url)syntax. Slack auto-links bare URLs when sent via API anyway. - Relative links (
[Docs](/path)) are ignored — onlyhttp://,https://, andmailto:links are converted. - Deeply nested parenthesized URLs like
(a_(b_(c)))may not parse correctly. Single-level parens (e.g. Wikipedia URLs) work fine. - Tables in copy-paste render as code blocks. Slack's WYSIWYG editor does not reliably accept HTML
<table>tags when pasted alongside other rich content. snake_casetext is safe — underscores inside words are not misinterpreted as italic.
Built on the shoulders of great tools in the Slack formatting ecosystem:
- slackify-markdown — the most popular Markdown-to-mrkdwn converter (207k weekly downloads). Inspired our mrkdwn conversion approach.
- sirkitree/slack-markdown-formatter — a Claude Code skill that pioneered teaching Claude Slack formatting rules.
- ccheney/robust-skills — comprehensive mrkdwn and Block Kit skills for Claude Code.
- slackdown.com — web-based converter with HTML copy support.
- Slack's official docs — the mrkdwn specification.
This tool adds browser preview + copy-paste + table support on top of the conversion these tools pioneered.
MIT