MCP Server for Slack

Instructions

Implementation Reference

• index.ts:433-447 (handler)

  Executes the slack_post_message tool by validating input arguments and invoking the SlackClient.postMessage method to send the message.

  case "slack_post_message": {
    const args = request.params.arguments as unknown as PostMessageArgs;
    if (!args.channel_id || !args.text) {
      throw new Error(
        "Missing required arguments: channel_id and text",
      );
    }
    const response = await slackClient.postMessage(
      args.channel_id,
      args.text,
    );
    return {
      content: [{ type: "text", text: JSON.stringify(response) }],
    };
  }

• index.ts:74-91 (schema)

  Defines the input schema and metadata for the slack_post_message tool.

  const postMessageTool: Tool = {
    name: "slack_post_message",
    description: "Post a new message to a Slack channel",
    inputSchema: {
      type: "object",
      properties: {
        channel_id: {
          type: "string",
          description: "The ID of the channel to post to",
        },
        text: {
          type: "string",
          description: "The message text to post",
        },
      },
      required: ["channel_id", "text"],
    },
  };

• index.ts:567-582 (registration)

  Registers the slack_post_message tool (as postMessageTool) in the ListTools response.

  server.setRequestHandler(ListToolsRequestSchema, async () => {
    console.log("Received ListToolsRequest");
    return {
      tools: [
        listChannelsTool,
        postMessageTool,
        replyToThreadTool,
        addReactionTool,
        getChannelHistoryTool,
        getThreadRepliesTool,
        getUsersTool,
        getUserProfileTool,
        lookupUserByEmailTool,
      ],
    };
  });

• index.ts:273-284 (helper)

  SlackClient helper method that performs the HTTP POST to Slack's chat.postMessage API endpoint.

  async postMessage(channel_id: string, text: string): Promise<any> {
    const response = await fetch("https://slack.com/api/chat.postMessage", {
      method: "POST",
      headers: this.botHeaders,
      body: JSON.stringify({
        channel: channel_id,
        text: text,
      }),
    });
  
    return response.json();
  }