Slack Post Message MCP API

The Slack Post Message MCP API allows you to send text messages to Slack channels using the Model Context Protocol interface. For more information about MCP, see the official documentation.

Tool Information

slack_post_message

• Tool name: slack_post_message
• Function: send_slack_message
• Description: Send text to the given Slack channel. This is the primary tool for delivering textual notifications to Slack channels on behalf of the user.

Input Parameters

The tool accepts a SlackPostMessageInput object with the following parameters:

Parameter	Type	Required	Description
channel	string	Yes	Slack channel ID (e.g., C12345678) or name with # (e.g., #general)
text	string	Yes	The plain-text message to post (up to 40 kB)

Usage Examples

Basic Message

{
  "channel": "#general",
  "text": "Hello from the MCP server! This is a test message."
}

Channel ID Format

{
  "channel": "C1234567890",
  "text": "Build completed successfully! ✅"
}

Multi-line Message

{
  "channel": "#notifications", 
  "text": "Deployment Status:\n• Database migration: Complete\n• Application deployment: Complete\n• Health checks: Passed"
}

Response Format

The tool returns the raw JSON response from Slack's chat.postMessage API:

{
  "ok": true,
  "channel": "C1234567890",
  "ts": "1234567890.123456",
  "message": {
    "text": "Hello from the MCP server!",
    "user": "U12345678",
    "ts": "1234567890.123456",
    "team": "T1234567890",
    "type": "message"
  }
}

Key Response Fields

Field	Description
ok	Boolean indicating if the operation was successful
channel	The channel ID where the message was posted
ts	Timestamp of the posted message (used as message ID)
message	The complete message object that was posted

Error Handling

This function may raise errors in the following situations:

• Missing or invalid Slack API token
• Invalid channel name or ID
• Insufficient permissions to post in the channel
• Network connectivity issues
• API rate limiting
• Message too long (exceeds 40 kB limit)

Error responses will have "ok": false and include an error message:

{
  "ok": false,
  "error": "channel_not_found",
  "detail": "Value passed for channel was invalid."
}

Common Error Codes

Error Code	Description	Solution
channel_not_found	Channel doesn't exist or bot lacks access	Verify channel name/ID and bot permissions
not_in_channel	Bot is not a member of the channel	Invite the bot to the channel
invalid_auth	Invalid or missing API token	Check SLACK_BOT_TOKEN environment variable
rate_limited	Too many requests sent	Implement rate limiting in your application
msg_too_long	Message exceeds 40,000 characters	Shorten the message or split into multiple messages

Use Cases

Typical scenarios for using slack_post_message:

• Build/Deployment Notifications: Alert team channels about build or deployment status
• Automated Reminders: Send scheduled reminders or summaries after completing tasks
• Event Broadcasting: Share important events like incident reports or announcements
• Status Updates: Provide real-time updates on long-running processes
• Integration Alerts: Forward notifications from external systems to Slack

Authentication

This tool requires a valid Slack bot token set in one of the following environment variables:

• SLACK_BOT_TOKEN (recommended)
• SLACK_TOKEN (fallback)

The bot must have the following OAuth scopes:

• chat:write - Required to post messages
• channels:read - Required to access public channels
• groups:read - Required to access private channels (if needed)

Rate Limiting

Slack enforces rate limits on the chat.postMessage API:

• Tier 1: 1+ message per second
• Tier 2: 20+ messages per minute
• Tier 3: 50+ messages per minute
• Tier 4: 100+ messages per minute

The server will automatically handle retries according to Slack's rate limiting headers.