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Slack’s official MCP server does not support automatic OAuth client registration. To connect it in OpenWork, ask a Slack admin to create or approve a Slack app for MCP, then add the app’s OAuth client credentials in OpenWork. You do not need the old Slack bot tokens (xoxb-... or xapp-...) for this flow.

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Before you start

You need:
  • A Slack workspace where you can install or request approval for Slack apps.
  • A Slack app that is published in the Slack Marketplace or marked as an internal app.
  • A Slack OAuth client ID and client secret from that app.
  • The Slack MCP server URL: https://mcp.slack.com/mcp.
Keep the client secret out of chats and source control. Treat it like any other workspace secret.

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Create or approve the Slack app

In Slack, an admin should create or approve an app for OpenWork’s MCP access.
  1. Open Slack API apps and create a new app, or open the existing internal app you want to use.
  2. Go to OAuth & Permissions.
  3. Add this redirect URL: http://127.0.0.1:19876/mcp/oauth/callback.
  4. Add the User Token Scopes for the Slack MCP tools your team wants OpenWork to use.
  5. Save the app and install or approve it for the workspace.
  6. Go to Basic Information > App Credentials and copy the Client ID and Client Secret.
Slack MCP uses user token scopes. Choose the smallest set that matches what your team wants agents to do. For a useful read-first setup, start with: 
search:read.public search:read.private search:read.mpim search:read.im search:read.files search:read.users channels:history groups:history mpim:history im:history users:read users:read.email channels:read groups:read mpim:read
Add write scopes only if you want agents to take action in Slack: 
chat:write reactions:write channels:write groups:write im:write mpim:write canvases:read canvases:write
Use this table to pick scopes by capability:
Slack MCP capabilityUser token scopes
Search public and private messages/channelssearch:read.public, search:read.private, search:read.mpim, search:read.im
Search filessearch:read.files
Read filesfiles:read
Search emojiemoji:read
Search userssearch:read.users
Send messageschat:write
Read channels and threadschannels:history, groups:history, mpim:history, im:history
Create conversations/channelschannels:write, groups:write, im:write, mpim:write
Add reactionsreactions:write
Read and write canvasescanvases:read, canvases:write
Read user profiles and emailusers:read, users:read.email
List channel memberschannels:read, groups:read, mpim:read

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Open Extensions

In OpenWork, open your workspace and go to Settings > Extensions. Click Add Custom App.
OpenWork Extensions page with Add Custom App button

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Add the Slack MCP server

In the Add Custom App dialog:
  1. Set App name to slack.
  2. Keep Type set to Remote (URL).
  3. Set Server URL to https://mcp.slack.com/mcp.
  4. Open Advanced OAuth.
  5. Paste the Slack OAuth client ID.
  6. Paste the Slack OAuth client secret.
  7. Paste the same user token scopes you added in Slack, separated by spaces.
  8. Click Add App.
Add Custom App dialog with Advanced OAuth fields for Slack MCP
OpenWork writes the MCP entry with the OAuth client credentials, similar to this: 
{
  "mcp": {
    "slack": {
      "type": "remote",
      "enabled": true,
      "url": "https://mcp.slack.com/mcp",
      "oauth": {
        "clientId": "your-slack-client-id",
        "clientSecret": "your-slack-client-secret"
      }
    }
  }
}
If you entered scopes, OpenWork also saves them as oauth.scope. This is the scope list Slack sees during OAuth consent.

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Sign in to Slack

After the MCP app is added, reload the engine if OpenWork asks you to. Then start Slack MCP sign-in from the Slack app card or the MCP auth prompt. Slack opens in your browser and shows the scopes you configured. Approve the Slack app for the workspace you want OpenWork to use, then return to OpenWork when authorization finishes.

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Troubleshooting

If you see an error like Incompatible auth server: does not support dynamic client registration, the Slack MCP entry is missing a pre-registered OAuth client ID and client secret. Reopen the Slack MCP setup, add those values under Advanced OAuth, reload the engine, and try sign-in again. If Slack says the redirect URL is invalid, add http://127.0.0.1:19876/mcp/oauth/callback to the Slack app’s OAuth & Permissions redirect URLs. If Slack says a scope is invalid or unavailable, remove that scope from both the Slack app and OpenWork’s OAuth scopes field, then retry. Slack workspace policies and plan settings can limit which scopes an app may request. If your Slack workspace blocks app installation, ask a Slack admin to approve the app first. OpenWork cannot bypass Slack workspace approval policies.