Slack Webhook MCP Server

A Model Context Protocol (MCP) server for posting messages to Slack via webhooks or bot tokens.

Features

• Two modes of operation: Webhooks or Bot API

• Post messages to Slack channels

• Support for channel overrides

• Configurable bot username and emoji

• Slack-specific mention support (<!channel>, <!here>, <@USERID>)

• Comprehensive error handling and validation

Installation

Setup

Option 1: Webhook Mode (Simple)

1. Create a Slack App and Webhook:

   • Go to Slack API Apps

   • Create a new app or select an existing one

   • Navigate to "Incoming Webhooks"

   • Enable incoming webhooks

   • Add a new webhook to your workspace

   • Copy the webhook URL (format: https://hooks.slack.com/services/T.../B.../...)

2. Configure MCP Client:

Option 2: Bot Token Mode (Advanced)

1. Create a Slack App with Bot Token:

   • Go to Slack API Apps

   • Create a new app or select an existing one

   • Go to "OAuth & Permissions"

   • Add bot token scopes: chat:write, chat:write.public

   • Install app to workspace

   • Copy the "Bot User OAuth Token" (starts with xoxb-)

2. Get Channel IDs:

   • Right-click on channel in Slack → "View channel details" → Copy channel ID

   • Or use Slack API to list channels

3. Configure MCP Client:

Usage

Command Line Arguments

Webhook Mode:

• --webhook-url <url> - Slack webhook URL (required)

• --channel <channel> - Default channel name (optional, e.g., "#general")

Bot Mode:

• --bot-token <token> - Slack bot token (required, starts with xoxb-)

• --channel <channel> - Default channel ID (required, e.g., "CXXXXXXXXXX")

Common Options:

• --username <username> - Default bot username (optional)

• --icon-emoji <emoji> - Default bot emoji (optional, e.g., ":robot_face:")

• --help - Show help message

Examples

Webhook Mode:

Bot Mode:

Available Tools

post_slack_message

Post a message to your configured Slack integration.

Parameters:

• message (required): The message content to send (max 4000 characters)

• channel (optional): Channel to post to (overrides default)

  • Webhook mode: Channel name (e.g., "#random")

  • Bot mode: Channel ID (e.g., "CXXXXXXXXXX")

• username (optional): Override the default username for this message

• icon_emoji (optional): Override the default emoji for this message

Example usage in MCP client:

Mode Comparison

Slack-Specific Features

Mentions

• User mention: <@U1234567890>

• Channel notification: <!channel>

• Here notification: <!here>

Channel References

• Webhook mode: Use #channel-name format

• Bot mode: Use channel IDs like CXXXXXXXXXX

Development

Scripts

• bun run start - Start the server

• bun run dev - Start with file watching

• bun run build - Build for production

Project Structure

Error Handling

The server includes comprehensive error handling for:

• Invalid webhook URLs and bot tokens

• Network connectivity issues

• Slack API errors (different handling for webhook vs bot)

• Message length validation (4000 char limit)

• Channel ID validation (bot mode)

• Malformed requests

License

MIT