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Slack MCP Configuration Guide

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Slack MCP Configuration Guide

This guide explains how to configure the Slack MCP server for use with Cursor IDE, specifically for the send-code-review-slack command.

Slack MCP Server Setup

This setup uses the custom Slack MCP server (ghcr.io/korotovsky/slack-mcp-server) which supports User OAuth Tokens (SLACK_MCP_XOXP_TOKEN). Messages will be posted as your authenticated user account, not as a bot.

Setup Overview

Workspace-Level Setup (One-Time, Done by Admin):

  • Step 1: Create a Slack App
  • Step 2: Configure User Token Scopes

User-Level Setup (Per Developer):

  • Step 3: Install App to Workspace
  • Step 4: Get Your User OAuth Token
  • Step 5: Environment Variables
  • Step 6: Restart Cursor

Step 1: Create a Slack App

Note: This is a one-time workspace setup step, typically done by a workspace admin. Once the app is created and configured, all users in the workspace can use it.

  1. Go to Slack API Apps
  2. Click "Create New App" → "From scratch"
  3. Name your app (e.g., "Opik Code Review")
  4. Select your workspace
  5. Click "Create App"

Step 2: Configure User Token Scopes

Note: This is a one-time workspace setup step, typically done by a workspace admin. Once the scopes are configured, all users in the workspace will have access to these scopes when they install the app.

CRITICAL: You must add scopes to "User Token Scopes" (NOT "Bot Token Scopes") for the User OAuth Token to appear.

  1. In your app settings, go to "OAuth & Permissions"
  2. Scroll down to the "Scopes" section
  3. Find "User Token Scopes" section (this is different from "Bot Token Scopes" which is above it)
    • You'll see the description: "Scopes that access user data and act on behalf of users that authorize them."
    • This confirms you're in the right section - these scopes allow the app to act as YOU, not as a bot
  4. Click "Add an OAuth Scope" under "User Token Scopes"
  5. Add the following scopes one by one:
    • chat:write - Send messages as your authenticated user account
    • channels:read - View basic information about public channels
    • users:read - Read user information (required by the MCP server for caching)
    • channels:history - View messages in public channels (required by the MCP server for channel caching)
  6. Click "Save Changes"

Note: The users:read and channels:history scopes are required by the ghcr.io/korotovsky/slack-mcp-server to properly cache and access channel information. Without these, you may see "missing_scope" errors in the logs, though basic message posting may still work.

Step 3: Install App to Workspace

Note: This is a per-user step. Each developer needs to install the app to their workspace to authorize it and get their own User OAuth Token.

  1. Scroll to the top of "OAuth & Permissions"
  2. Click "Install to Workspace" (or "Reinstall to Workspace" if you already installed it)
  3. Review permissions and click "Allow"
    • Note: The permission screen may say "Send messages as [App Name]" - this is just the app requesting permission. When you use the User OAuth Token, messages will be posted as your personal account, not as the app.

Step 4: Get Your User OAuth Token

After installing/reinstalling the app, you need to find your User OAuth Token:

  1. Stay on the "OAuth & Permissions" page (or refresh it)
  2. Scroll down to the "OAuth Tokens for Your Workspace" section
  3. Look for "User OAuth Token" (NOT "Bot User OAuth Token")
    • If you only see "Bot User OAuth Token", you need to:
      • Go back to Step 2 and make sure you added scopes to "User Token Scopes" (not "Bot Token Scopes")
      • Then come back here and click "Reinstall to Workspace" again
  4. The User OAuth Token should start with xoxp- (not xoxb-)
  5. Click "Show" or "Reveal" to see the full token
  6. Copy this token - this is what will post messages as your authenticated user account

Step 5: Environment Variables

  1. Create or edit .env.local in your project root and add your User OAuth Token:
bash
SLACK_MCP_XOXP_TOKEN=xoxp-your-user-oauth-token-here

Important Configuration Options:

  1. mcp-server --transport stdio: Required for the server to communicate with Cursor via the MCP protocol. Without these, the server will start an SSE server instead, which won't work with Cursor's MCP integration.

  2. SLACK_MCP_ADD_MESSAGE_TOOL: Required to enable the conversations_add_message tool (disabled by default for safety).

    • true or 1: Enable for all channels and DMs
    • Channel IDs (comma-separated): Enable only for specific channels (e.g., XXXXXXXX,YYYYYYYY)
    • !XXXXXXXX: Enable for all channels except the specified one
  • For the code review command: Use true to enable posting to #code-review
  1. envFile: Points to ${workspaceFolder}/.env.local where your SLACK_MCP_XOXP_TOKEN is stored securely.

Important:

  • Replace xoxp-your-user-oauth-token-here in .env.local with your User OAuth Token from Step 4 (starts with xoxp-, NOT xoxb-)
  • The token is loaded from .env.local via the envFile configuration, keeping it out of mcp.json

Note: This configuration uses the custom Slack MCP server that supports User OAuth Tokens. Messages will be posted as your authenticated user account, not as a bot.

Requirements:

  • Docker must be installed and running on your system
  • The Docker image ghcr.io/korotovsky/slack-mcp-server:latest will be pulled automatically on first use
  • .env.local should be added to .gitignore to prevent committing your token

Step 6: Restart Cursor

  1. Close and reopen Cursor IDE
  2. The Slack MCP server should now be available with message posting enabled
  3. You can verify by checking Cursor Settings > Features > MCP
  4. Test by running: cursor send-code-review-slack

Security Notes

  • Never commit tokens to Git:
    • Add .env.local to .gitignore to prevent committing your token
    • The mcp.json file references the token via environment variable, but it may contain secrets, check it before committing
    • Token storage: The SLACK_MCP_XOXP_TOKEN is stored in .env.local (which should be in .gitignore), not directly in mcp.json

Troubleshooting

MCP Server Not Appearing

  1. Check that mcp.json is in the correct location (.cursor/mcp.json or ~/.cursor/mcp.json)
  2. Verify JSON syntax is valid (use a JSON validator)
  3. Restart Cursor completely (quit and reopen, not just reload window)
  4. Check Cursor Settings > Features > MCP for error messages
  5. Verify Docker is installed and running: docker --version and docker ps

Docker Command Errors

If you see errors like "Usage: docker [OPTIONS] COMMAND" in the MCP logs:

  1. Verify Docker is running:

    bash
    docker ps

    If this fails, start Docker Desktop or Docker daemon

  2. Test the Docker command manually:

    bash
    docker run -i --rm -e SLACK_MCP_XOXP_TOKEN=xoxp-your-token -e LOG_LEVEL=error ghcr.io/korotovsky/slack-mcp-server:latest

    This should start the MCP server. If it fails, check the error message.

  3. Pull the Docker image:

    bash
    docker pull ghcr.io/korotovsky/slack-mcp-server:latest

  4. Check mcp.json structure: Ensure the args array is properly formatted with each argument as a separate string element

Authentication Errors

  1. Verify your user token is correct (should start with xoxp-, NOT xoxb-)
    • If you see xoxb- in the logs, you're using a bot token instead of a user token
    • Go back to Step 4 and get the User OAuth Token (not Bot User OAuth Token)
  2. Ensure the app has been installed to your workspace
  3. Verify user token scopes include all required scopes in User Token Scopes:
    • chat:write (required for posting messages)
    • channels:read (required for channel access)
    • users:read (required by MCP server for caching)
    • channels:history (recommended for full functionality)
  4. Make sure you're using SLACK_MCP_XOXP_TOKEN in your Docker args (not SLACK_BOT_TOKEN)
  5. Check your actual mcp.json file - the token in the Docker args should be xoxp-...
  6. Restart Cursor completely after updating the token in mcp.json
  7. Verify Docker is running and can pull the image

Missing Scope Errors

If you see "missing_scope" errors in the MCP logs:

  1. Check which scope is missing - the error message will indicate the specific scope
  2. Add the missing scope to your Slack app's User Token Scopes:
    • Go to your Slack app's "OAuth & Permissions" page
    • Scroll to "User Token Scopes"
    • Click "Add an OAuth Scope" and add the missing scope
    • Common missing scopes: users:read, channels:history
  3. Reinstall the app to your workspace after adding scopes
  4. Update the token in mcp.json if a new token was generated
  5. Restart Cursor to reload the MCP configuration

Note: Some "missing_scope" errors may be warnings that don't prevent basic functionality (like posting messages), but adding all recommended scopes ensures full MCP server functionality.

Permission Errors

  1. Ensure the user token has chat:write scope in User Token Scopes
  2. Verify you have access to the #code-review channel
  3. Check that you're a member of the channel
  4. If using Docker, ensure Docker has network access to reach Slack API

Additional Resources