[BUG] Slack MCP "Failed to connect" — cannot reconnect after re-authentication

Found 3 possible duplicate issues:

1. https://github.com/anthropics/claude-code/issues/36307
2. https://github.com/anthropics/claude-code/issues/10250
3. https://github.com/anthropics/claude-code/issues/30855

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🤖 Generated with Claude Code

Same issue here — additional findings

Experiencing the exact same problem. Slack MCP is the only one failing while Notion, Linear, Gmail, and Google Calendar MCP all work fine.

Key finding: v2.1.80 → v2.1.81 regression

Slack MCP worked on v2.1.80, but broke after upgrading to v2.1.81. This strongly suggests a regression introduced in v2.1.81.

The most likely culprit is the CIMD (SEP-991) support added in v2.1.81 for MCP OAuth with servers that don't support Dynamic Client Registration — which includes Slack's MCP server (mcp.slack.com).

My symptoms

| Item | Status |
|------|--------|
| Auth | ✔ authenticated |
| Status | ✘ failed |
| Error | App is not enabled for Slack MCP server access |
| Error URL | https://api.slack.com/apps/A08SF47R6P4/app-assistant → 404 |
| Claude chat in Slack | ✔ works fine |
| Notion/Linear/Gmail MCP | ✔ all working |

What I've tried (all ineffective)

• Claude Code restart
• Clear authentication → re-authenticate
• Remove Slack MCP → re-add with correct config (clientId: 1601185624273.8899143856786)
• Reproduced on 2 separate Macs (macOS, v2.1.81)

Environment

• Claude Code: v2.1.81
• OS: macOS (Darwin 25.3.0)
• Config: matches official Connect to Claude docs exactly

Analysis

The error "App is not enabled for Slack MCP server access" points to the app-level MCP access gate (post-OAuth), not the OAuth authentication itself. The v2.1.81 CIMD/SEP-991 change likely altered how the MCP client presents its identity to mcp.slack.com, causing Slack's server to reject the connection at the MCP enablement check.

Can confirm that I'm seeing the same thing and came to the same result as @nishijun - my logs and output in Claude code confirm the authentication is successful, but the server reconnection fails with that same "App is not enabled for access" message. I've also followed the same "remove it and reconnect it" steps to the same ineffective result.

Environment

• Claude Code: v2.1.81
• OS: macOS (Tahoe 26.3.1)

Same issue as well. It says here "Authorization with the MCP server failed. You can check your credentials and permissions."

Environment

• Claude Code: v2.1.81
• OS: macOS (Tahoe 26.2)

Same issue — OAuth redirect_uri mismatch

Authentication fails with:

redirect_uri did not match any configured URIs. Passed URI: http://localhost:3118/callback

Slack MCP shows as "Connected" but no tools are exposed. Other MCP servers (Figma, Atlassian, Firebase) all work fine.

Environment

• Claude Code: v2.1.81
• OS: macOS Tahoe 26.3.1 (25D2128)

Same issue -- seems to affect other similarly situated mcp tools as well.

Same here

<img width="1576" height="988" alt="Image" src="https://github.com/user-attachments/assets/c08093fb-2150-472b-9ab7-e62d576f3c59" />

Update: Slack MCP is working again 🎉

As of ~2026-03-24 06:50 UTC+8, Slack MCP is back and functioning normally. Search tools are returning results successfully.

No changes were made on my end — this appears to have been fixed server-side by Anthropic/Slack.

Environment (unchanged):

• Claude Code: v2.1.81
• OS: macOS (Darwin 25.3.0)

Can others confirm?