Slack MCP Server

Instructions

Implementation Reference

• slack_mcp/server.py:290-307 (handler)

  The main handler function for the 'send_message' MCP tool. It creates a SlackClient instance, parses optional blocks JSON parameter, calls the underlying SlackClient.send_message method, and returns the API response as formatted JSON or an error message.

  @mcp.tool()
  async def send_message(channel: str, text: str, thread_ts: Optional[str] = None, blocks: Optional[str] = None) -> str:
      """
      Send a message to a Slack channel.
  
      Args:
          channel: Channel ID or name
          text: Message text (fallback text for notifications)
          thread_ts: Thread timestamp for replies
          blocks: JSON string of Block Kit blocks for rich formatting
      """
      try:
          client = SlackClient()
          blocks_data = json.loads(blocks) if blocks else None
          result = await client.send_message(channel, text, thread_ts, blocks_data)
          return json.dumps(result, indent=2)
      except Exception as e:
          return json.dumps({"error": str(e)}, indent=2)

• slack_mcp/server.py:102-114 (helper)

  The underlying helper method in SlackClient class that constructs the request payload and calls the Slack chat.postMessage API endpoint via _make_request to send the message.

  async def send_message(
      self, channel: str, text: str, thread_ts: Optional[str] = None, blocks: Optional[List[Dict[str, Any]]] = None
  ) -> Dict[str, Any]:
      """Send a message to a channel."""
      data = {"channel": channel, "text": text}
  
      if thread_ts:
          data["thread_ts"] = thread_ts
  
      if blocks:
          data["blocks"] = blocks
  
      return await self._make_request("POST", "chat.postMessage", json_data=data)