Slack best practices

Slack Best Practices 💬🐈‍⬛

🛑 CRITICAL RULE: NEVER USE DOUBLE ASTERISKS (**)

Slack does NOT support Standard Markdown bolding. Using ** will show the literal asterisks to the user. ALWAYS use single asterisks (*) for bold text.

FOR AGENTS: You are hard-wired to use **. You MUST run a post-processing step on your final message string to replace ** with * before calling the message tool.

Slack uses a unique flavor of Markdown called mrkdwn. This skill captures the quirks and best practices learned by Yesterday agents to ensure our messages look perfect.

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Formatting Rules (mrkdwn)

Element	mrkdwn Syntax	Standard MD (Avoid!)
Bold	*text* (single asterisk)	**text**
Italic	_text_ (underscore)	*text*
Strike	~text~	~~text~~
Blockquote	> text	same
Inline Code	`code`	same
Code Block	```code block```	same
Link	<URL|Label>	[Label](URL)

Note: Slack does NOT support # headers or Markdown tables. Use ALL CAPS BOLD to simulate headers. Use bullet lists instead of tables.

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Links & URLs

Auto-linking works if you paste a raw URL. To suppress the preview embed, wrap it in angle brackets:

<https://example.com>

To show custom link text:

<https://example.com|Click here>

Email links:

<mailto:[email protected]|Mail Alex>

Special characters that must be escaped in text strings (not in URLs):

Character	Use instead
&	&
<	&lt;
>	&gt;

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Mentions

User mention:

<@U012AB3CD>

Slack auto-converts the user ID to their display name. Get user IDs from their Slack profile ("Copy member ID").

Channel mention:

<#C123ABC456>

Slack auto-converts to the channel name. Get channel ID from the URL: https://app.slack.com/client/E.../C123ABC456

User group mention:

<!subteam^SAZ94GDB8>

Special mentions (use sparingly!):

Mention	Syntax	Who gets notified
@here	<!here>	Active members of channel
@channel	<!channel>	All members of channel
@everyone	<!everyone>	Every workspace member (#general only)

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Date & Time Formatting

Slack can auto-localize timestamps to the reader's timezone:

<!date^UNIX_TIMESTAMP^{date_short} at {time}|Fallback text>

Available tokens:

Token	Example output
{date_num}	2026-03-05
{date}	March 5th, 2026
{date_short}	Mar 5, 2026
{date_long}	Thursday, March 5th, 2026
{date_pretty}	today / yesterday / tomorrow
{time}	2:00 PM
{time_secs}	2:00:00 PM

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Emoji

• Standard Only: Stick to standard Slack emojis (:feet:, :sparkles:, :rocket:).
• No hallucinations: Do not invent emoji names unless sure they exist in the workspace.
• Tone: Use sparingly to soften messages -- one emoji per paragraph max.
• Emoji are stored internally in colon format (:smile:). When posting, you can use native unicode emoji directly and Slack will convert them.

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Cross-Channel Posting (message Tool)

🛑 CRITICAL: Use the Correct Tool Format

The generic message tool's channel parameter does NOT work for cross-channel posting in Slack. It always sends to the current session's DM. Use the Slack skill sendMessage action instead.

Posting to a different channel from within a DM session:

{
  "action": "sendMessage",
  "to": "channel:C0EXAMPLE01",
  "content": "Hello from another channel"
}

Posting to a user's DM:

{
  "action": "sendMessage",
  "to": "user:U012AB3CD",
  "content": "Hello via DM"
}

The to field format:

Target	Format	Example
Channel by ID	channel:C123ABC456	channel:C0EXAMPLE01
User by ID	user:U012AB3CD	user:U0EXAMPLE02

Common mistake: Using message(action=send, channel=C123) -- this ignores the channel parameter and posts to the DM of the current session. Always use sendMessage with to: "channel:...".

Prerequisites:

• The target channel must be in channels.slack.accounts.default.channels with allow: true
• The bot must be invited to the target channel
• groupPolicy: "allowlist" means only explicitly listed channels are allowed

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Communication Etiquette

1. Thread over Flood: Use threads for deep-dives, detailed logs, or long explanations. Keep channels scannable.
2. Mentions: Use @user sparingly. Prefer no mentions for broad updates unless truly urgent.
3. Headers: Simulate with *ALL CAPS BOLD* -- Slack has no # header support.
4. No tables: Slack does not render Markdown tables. Use bullet lists instead.
5. Link embeds: Wrap links in <> to suppress large previews in busy channels.

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Troubleshooting & Common Agent Mistakes

• The "Model Reflex" (CRITICAL): LLMs are pre-trained on millions of Standard Markdown examples and WILL reflexively use **bold** even when they know better. Treat this as a high-risk failure point.
• The "Mental Double-Check": Before any message is sent to Slack, perform a final scan of the text string to ensure NO double-asterisks (**) exist. Convert to * if found.
• Formatting Over Content: High-quality content is useless if the formatting is broken. Prioritize correct mrkdwn syntax over speedy replies.
• Self-Correction Loop: Before calling the message tool, run a search-and-replace: ** → *.

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"Clear is kind. Scannable is better." 🐾📡