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 (``, ``, ``)
• Comprehensive error handling and validation

Installation

bun install

Setup

Option 1: Webhook Mode (Simple)

1. Create a Slack App and Webhook:

2. Configure MCP Client:

{
  "mcpServers": {
    "slack-webhook": {
      "command": "bun",
      "args": [
        "run", 
        "src/index.ts",
        "--webhook-url",
        "https://hooks.slack.com/services/YOUR/SLACK/WEBHOOK",
        "--channel",
        "#general"
      ],
      "cwd": "/path/to/slack-webhook-mcp-server"
    }
  }
}

Option 2: Bot Token Mode (Advanced)

1. Create a Slack App with Bot Token:

2. Get Channel IDs:

3. Configure MCP Client:

{
  "mcpServers": {
    "slack-bot": {
      "command": "bun",
      "args": [
        "run", 
        "src/index.ts",
        "--bot-token",
        "xoxb-YOUR-BOT-TOKEN",
        "--channel",
                  "CXXXXXXXXXX",
        "--username",
        "MCP Bot"
      ],
      "cwd": "/path/to/slack-webhook-mcp-server"
    }
  }
}

Usage

Command Line Arguments

Webhook Mode:

• `--webhook-url ` - Slack webhook URL (required)
• `--channel ` - Default channel name (optional, e.g., "#general")

Bot Mode:

• `--bot-token ` - Slack bot token (required, starts with `xoxb-`)
• `--channel ` - Default channel ID (required, e.g., "CXXXXXXXXXX")

Common Options:

• `--username ` - Default bot username (optional)
• `--icon-emoji ` - Default bot emoji (optional, e.g., ":robot_face:")
• `--help` - Show help message

Examples

Webhook Mode:

bun run src/index.ts \
  --webhook-url "https://hooks.slack.com/services/T00000000/B00000000/XXXXXXXXXXXXXXXXXXXXXXXX" \
  --channel "#general" \
  --username "Assistant"

Bot Mode:

bun run src/index.ts \
  --bot-token "xoxb-YOUR-BOT-TOKEN" \
  --channel "CXXXXXXXXXX" \
  --username "Assistant"

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:

Please post "Hello from MCP!" to the general channel

Mode Comparison

Slack-Specific Features

Mentions

• User mention: ``
• Channel notification: ``
• Here notification: ``

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

slack-webhook-mcp-server/
├── package.json          # Dependencies and scripts
├── src/
│   └── index.ts         # Main server implementation  
├── types/
│   └── slack.ts         # TypeScript types and schemas
└── README.md            # This file

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