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 on public.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 with printf '%s' (no trailing newline) instead of cat <<'EOF' (which always appends one). If you must use a heredoc for readability, post-process: tr -d '\n' < raw.html > clean.html or sed -E 's/<br>[[:space:]]+/<br>/g'.

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. Use printf '%s' '…all-on-one-line…' or post-process.
HTML <table> in a message composer — always stripped.
RTF (textutil -convert rtf) on public.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  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.

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:

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 on public.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 with printf '%s' (no trailing newline) instead of cat <<'EOF' (which always appends one). If you must use a heredoc for readability, post-process: tr -d '\n' < raw.html > clean.html or sed -E 's/<br>[[:space:]]+/<br>/g'.

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?"

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. Use printf '%s' '…all-on-one-line…' or post-process.
HTML <table> in a message composer — always stripped.
RTF (textutil -convert rtf) on public.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  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.

Adoption

benwaffle/slack-copy

$ install --global

Security Scan Results

SKILL.md

Slack Copy — paste formatted content into Slack messages

How to put data on the macOS clipboard

Recipes

Rich text (bold / italic / list / links / code)

Table (TSV)

Mixed message (text + table)

Debugging "did Slack honor my HTML?"

Things that DON'T work (so don't try them)

Slack canvases (different surface)

Related Skills

benwaffle/slideshow

benwaffle/claude-stats

benwaffle/apple-logarchive

benwaffle/adb

benwaffle/slack-copy

$ install --global

Security Scan Results

SKILL.md

Slack Copy — paste formatted content into Slack messages

How to put data on the macOS clipboard

Recipes

Rich text (bold / italic / list / links / code)

Table (TSV)

Mixed message (text + table)

Debugging "did Slack honor my HTML?"

Things that DON'T work (so don't try them)

Slack canvases (different surface)

Related Skills

benwaffle/slideshow

benwaffle/claude-stats

benwaffle/apple-logarchive

benwaffle/adb