benwaffle Slack Copy — paste formatted content into Slack messages
Slack's macOS desktop message composer is picky about what it renders from a pasted clipboard. Empirically verified behavior:
| Format | Slack message composer? | How to put it on the clipboard |
|---|---|---|
| Bold, italic, strikethrough | ✅ via HTML | <b>, <i>, <s> on public.html |
| Bulleted / numbered list | ✅ via HTML | <ul><li>, <ol><li> |
| Inline code | ✅ via HTML | <code> |
| Code block | ✅ via HTML | <pre> (line breaks preserved; <pre><code> is equivalent — no syntax highlighting either way) |
| Blockquote | ✅ via HTML | <blockquote> (renders with the left bar; <code> inside it works) |
| Links | ✅ via HTML | <a href="…">…</a> |
| Table | ❌ HTML <table> is stripped |
Use tab-separated plain text (TSV). Slack renders TSV as a table. |
| Headings, colors, custom fonts | ❌ | Not supported in the composer; use a canvas. |
Three big gotchas:
<meta charset="utf-8">prefix breaks Slack's HTML paste handler — strip it. Inline-formatting HTML must start directly with the first element (<b>,<ul>, etc.).Tables: Slack ignores HTML
<table>entirely. The way to get a table is plain-text TSV (cells separated by\t, rows by\n). Put that onpublic.utf8-plain-text. (HTML can still be on the same pasteboard with a separate marker; Slack will pick plain text for tables.)Slack collapses block-level whitespace —
<p>and the implicit block break around<ul>/<ol>are not honored. To get visible section breaks you must insert explicit<br>tags:<br>after a header before its list, and<br><br>for a blank line between sections. Without them, "What changed" runs straight into the first list item.Inline whitespace from your source HTML is honored — opposite problem from #3. Slack preserves:
- whitespace immediately after
<br>(a newline-then-indent in your heredoc shows up as a leading space on the next line) - the trailing newline at the end of the HTML (adds an empty line at the bottom)
Easiest fixes: keep HTML on a single line with no whitespace between
<br>and the next token, and write the file withprintf '%s'(no trailing newline) instead ofcat <<'EOF'(which always appends one). If you must use a heredoc for readability, post-process:tr -d '\n' < raw.html > clean.htmlorsed -E 's/<br>[[:space:]]+/<br>/g'.- whitespace immediately after
When the user asks to "put X on the clipboard for Slack", "copy a table for Slack", "format for Slack", or invokes /slack-copy, use this skill.
How to put data on the macOS clipboard
Two channels matter for Slack: public.html (rich) and public.utf8-plain-text (fallback). Use scripts/set-clipboard.py to set both at once with the right UTIs:
uv run --with pyobjc-framework-Cocoa \
python ~/dev/skills/skills/slack-copy/scripts/set-clipboard.py \
<html-file> <plain-text-file>
The script clears the pasteboard, writes both representations to one NSPasteboardItem, and prints the resulting types so you can verify.
For TSV-only (tables), pbcopy < table.tsv is enough — Slack only needs the plain channel for tables.
Recipes
Rich text (bold / italic / list / links / code)
Generate an HTML fragment with no <meta> or <html> wrapper, and a clean plain-text fallback:
Note the <br> after Affected: — without it, "Affected:" runs straight into the first bullet.
cat > /tmp/msg.html <<'EOF'
<b>Heads up:</b> the rollout is delayed.<br><br>
<b>Affected</b><br>
<ul>
<li>service <code>provisioner</code> in <i>prod2</i></li>
<li>see <a href="https://example/ticket">ticket 1234</a></li>
</ul>
EOF
cat > /tmp/msg.txt <<'EOF'
Heads up: the rollout is delayed. Affected:
- service `provisioner` in prod2
- see ticket 1234 (https://example/ticket)
EOF
uv run --with pyobjc-framework-Cocoa \
python ~/dev/skills/skills/slack-copy/scripts/set-clipboard.py /tmp/msg.html /tmp/msg.txt
Table (TSV)
Just tabs and newlines. Header row optional. pbcopy is enough:
{
printf 'Time (UTC)\tService\tStatus\n'
printf '23:22:16\tprovisioner\tdegraded\n'
printf '23:36:10\trgw\trecovered\n'
} | pbcopy
Or pass through column -s$'\t' -t first only if you want it human-readable in a terminal — don't do that for the clipboard, Slack needs literal tabs.
Mixed message (text + table)
Slack doesn't render an HTML table even when other HTML works, so a single paste can't include both styled text and a table. Options:
- Paste the text first, then the table on a second paste.
- Or put the leading text in plain Slack markup (
*bold*,_italic_,\code``) typed inline, and only paste the TSV. - For a single artifact, paste into a Slack canvas instead — canvases render markdown tables natively, so a pipe-style
| a | b |table works.
Debugging "did Slack honor my HTML?"
When experimenting with what Slack accepts, always use different content for HTML vs plain-text fallback so you can tell which one ended up in the message. If you put the same text in both and a "table" renders, you can't tell whether Slack honored your HTML or just rendered tab-separated plain text. Use a sentinel:
printf '<b>HTML-BRANCH</b>' > /tmp/h
printf 'PLAIN-BRANCH' > /tmp/p
uv run --with pyobjc-framework-Cocoa \
python ~/dev/skills/skills/slack-copy/scripts/set-clipboard.py /tmp/h /tmp/p
After pasting, "HTML-BRANCH" bolded = HTML honored. "PLAIN-BRANCH" plain = HTML rejected, fallback used.
To inspect what's actually on the pasteboard:
osascript -e 'clipboard info' # lists UTIs and sizes
pbpaste # plain text
osascript -e 'try
return (the clipboard as «class HTML») as string
on error e
return "no HTML: " & e
end try'
Things that DON'T work (so don't try them)
- Heredocs with newlines between elements — newlines after
<br>show as leading spaces; trailing newline shows as an empty final line. Useprintf '%s' '…all-on-one-line…'or post-process. - HTML
<table>in a message composer — always stripped. - RTF (
textutil -convert rtf) onpublic.rtf— Slack ignores it for tables. - HTML with
<meta charset>prefix — breaks even simple<b>/<i>rendering. - HTML with
<thead>/<tbody>wrappers — doesn't help (and<table>is stripped anyway). - Word-flavored HTML with
<!--StartFragment-->markers — Slack still strips tables. <p>for paragraph breaks — Slack collapses them. Use<br>/<br><br>instead.- Relying on
<ul>/<ol>to introduce vertical space — Slack jams the preceding text right against the first list item. Insert<br>between them.
Slack canvases (different surface)
If the user is pasting into a Slack canvas rather than a message, the rules differ — canvases render markdown natively, including pipe-style tables. For canvas paste, generate markdown:
| col1 | col2 |
|---|---|
| a | b |
and pbcopy it.