[BUG] Slack MCP server rejects connection: "App is not enabled for Slack MCP server access" (Anthropic app A08SF47R6P4 needs MCP toggle)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Slack MCP server connection fails with Status: ✗ failed even though Auth: ✓ authenticated. The OAuth token is valid (not expired), but Slack's MCP server actively rejects the connection.

Root Cause (verified via direct API call)

I tested the MCP handshake directly with curl using the stored OAuth token:

curl -X POST \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $SLACK_TOKEN" \
  -d '{"jsonrpc":"2.0","method":"initialize","id":1,"params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"0.1.0"}}}' \
  https://mcp.slack.com/mcp

Response:

{
  "jsonrpc": "2.0",
  "id": null,
  "error": {
    "code": -32600,
    "message": "App is not enabled for Slack MCP server access. Please enable it here: https://api.slack.com/apps/A08SF47R6P4/app-assistant"
  }
}

Analysis

1. App ID A08SF47R6P4 is the Anthropic-owned Slack app that Claude Code uses for OAuth. End users cannot modify this app's settings.

1. Slack's MCP server documentation (docs.slack.dev) states that apps must explicitly enable MCP via Features > Agents & AI Apps > Model Context Protocol toggle. This requirement was introduced with the Slack MCP server launch on Feb 17, 2026.

1. This is a regression — the integration was working as recently as last week (around March 16-17, 2026). This suggests Slack recently began enforcing the MCP enablement check on apps that were previously grandfathered in.

1. The fix is on Anthropic's side — someone with access to the Slack app A08SF47R6P4 needs to enable the "Model Context Protocol" toggle in the app settings at https://api.slack.com/apps/A08SF47R6P4/app-assistant.

Key Evidence

| Check | Result |
|---|---|
| Server reachable | ✅ mcp.slack.com responds (HTTP 401 without token = expected) |
| OAuth token valid | ✅ expiresAt is in the future, refresh token present |
| Auth status in /mcp | ✅ Auth: ✓ authenticated |
| MCP handshake | ❌ -32600: App is not enabled for Slack MCP server access |
| Clear auth + re-auth | ❌ Same result — token is not the problem |
| Other MCP servers | ✅ Figma, Linear, Confluence, Datadog, Sentry all work |

What Should Happen?

Slack MCP should connect successfully and expose tools (slack_read_channel, slack_search_public_and_private, etc.) as it did until last week.

Steps to Reproduce

1. Configure Slack MCP in .mcp.json with the standard Anthropic client ID
2. Authenticate via /mcp → Slack → Re-authenticate (OAuth succeeds)
3. Observe Status: ✗ failed despite Auth: ✓ authenticated
4. Run the curl command above with the stored token to confirm the exact error

Related Issues

• #37625 — Same symptom (Slack MCP failed), reported today. Shows missing_token but likely the same underlying cause (the server rejects before processing the token).
• #35995 — Slack MCP partial failure (message tools broken, profile tools work). Reported March 18.
• #30564 — Claude Code can't access Slack MCP due to DCR incompatibility.

Claude Model

Opus

Is this a regression?

Yes — was working until approximately March 16-17, 2026.

Last Working Version

Claude Code 2.1.79 (approximately, last week)

Claude Code Version

2.1.81

Platform

Claude Max (claude.ai OAuth)

Operating System

macOS (Darwin 25.3.0)

Terminal/Shell

VS Code integrated terminal (zsh)

Additional Information

The .mcp.json configuration:

{
  "slack": {
    "type": "http",
    "url": "https://mcp.slack.com/mcp",
    "oauth": {
      "clientId": "1601185624273.8899143856786",
      "callbackPort": 3118
    }
  }
}

This likely affects all Claude Code users connecting to Slack MCP via the bundled Anthropic app credentials. The fix requires Anthropic to enable the MCP toggle on their Slack app — no action from end users can resolve this.