Slack MCP Server

Instructions

Implementation Reference

• src/tools.ts:235-288 (handler)

  Main execution logic for conversations_add_message tool: validates args with schema, resolves channel ID, checks env-enabled channels, posts message via Slack chat.postMessage, handles errors.

  export async function conversationsAddMessage(args: any): Promise<ToolResponse> {
    try {
      // Safety check - require explicit enable
      const enabledChannels = process.env.SLACK_MCP_ADD_MESSAGE_TOOL;
      if (!enabledChannels) {
        return {
          success: false,
          error: 'Message posting is disabled by default for safety. Set SLACK_MCP_ADD_MESSAGE_TOOL environment variable to enable.'
        };
      }
  
      const validated = ConversationsAddMessageSchema.parse(args);
      const client = new WebClient(validated.accessToken);
  
      console.log('Posting message to channel:', validated.channel_id);
  
      // Resolve channel name/username to ID if needed
      const channelId = await resolveChannelId(client, validated.channel_id);
  
      // Check if channel is in enabled list (if not '*')
      if (enabledChannels !== '*') {
        const allowedChannels = enabledChannels.split(',').map(c => c.trim());
        if (!allowedChannels.includes(channelId) && !allowedChannels.includes(validated.channel_id)) {
          return {
            success: false,
            error: `Message posting not enabled for channel: ${validated.channel_id}`
          };
        }
      }
  
      // Post message
      const result = await client.chat.postMessage({
        channel: channelId,
        text: validated.payload,
        thread_ts: validated.thread_ts,
        mrkdwn: validated.content_type === 'text/markdown'
      });
  
      return {
        success: true,
        data: {
          ok: result.ok,
          channel: result.channel,
          ts: result.ts,
          message: result.message
        }
      };
    } catch (error: any) {
      if (error.name === 'ZodError') {
        return { success: false, error: `Validation error: ${error.errors.map((e: any) => e.message).join(', ')}` };
      }
      return handleSlackError(error);
    }
  }

• src/schemas.ts:29-39 (schema)

  Zod input schema for conversations_add_message tool, defining parameters: accessToken, channel_id, optional thread_ts, payload, content_type.

   * Schema for conversations_add_message tool
   * Posts messages to channels, threads, or DMs
   * Note: Disabled by default for safety - requires configuration to enable
   */
  export const ConversationsAddMessageSchema = z.object({
    accessToken: z.string().describe("Slack OAuth token (xoxp-... or xoxb-...) with chat:write scope"),
    channel_id: z.string().describe("Target channel ID, name (#general), or DM (@username)"),
    thread_ts: z.string().optional().describe("Thread timestamp; omit to post to channel directly"),
    payload: z.string().describe("Message content to post"),
    content_type: z.enum(['text/markdown', 'text/plain']).optional().describe("Content format (default: text/markdown)")
  });

• src/index.ts:106-108 (registration)

  Tool registration in MCP server's listTools response: specifies name, description, and converts Zod schema to MCP inputSchema.

  name: 'conversations_add_message',
  description: 'Posts a message to a channel, thread, or DM. Supports markdown and plain text. NOTE: Disabled by default for safety - set SLACK_MCP_ADD_MESSAGE_TOOL environment variable to enable.',
  inputSchema: zodToMCPSchema(ConversationsAddMessageSchema)

• src/index.ts:137-139 (registration)

  Dispatch logic in MCP callTool handler: routes 'conversations_add_message' calls to the conversationsAddMessage function.

  case 'conversations_add_message':
    result = await conversationsAddMessage(args);
    break;