claude.ai Slack MCP connector stuck 'unauthorized' in CLI despite successful OAuth

Summary

The \claude.ai Slack\ MCP connector repeatedly reports \"still unauthorized\" via \/mcp\ in the CLI, even after completing the browser OAuth flow successfully and confirming the connector is connected on claude.ai's own Connectors settings page (green checkmark).

Steps to reproduce

1. Have a CLI session where \/mcp\ shows \claude.ai Slack\ as "needs authentication".
2. Run \/mcp\, select \claude.ai Slack\, complete the browser OAuth flow. Browser shows "Your connector was connected. You can now use it with Claude."
3. Run \/mcp\ again in the CLI — still reports: "Tried reconnecting, but claude.ai Slack is still unauthorized. Make sure the browser sign-in completed, then try again from /mcp."
4. Fully quit and relaunch Claude Code, run \/mcp\ again — same unauthorized error.
5. Go to claude.ai → Settings → Connectors, remove the Slack connector entirely, re-add it, re-authenticate (browser flow succeeds again) — CLI still reports unauthorized on \/mcp\.

Throughout all of this, claude.ai's own Connectors page shows Slack as connected (checkmark) the entire time.

Expected

Once the OAuth flow completes successfully (confirmed by claude.ai's own UI), \/mcp\ in the CLI should pick up the valid connector state on the next reconnect attempt.

Actual

CLI keeps reporting unauthorized indefinitely, surviving a full process restart and a full connector remove/re-add cycle.

Environment

• Claude Code CLI (terminal), macOS
• Connector: "claude.ai Slack" (claudeai-proxy at mcp.slack.com)

Suspected cause (unverified)

Possible sync gap between the account-level connector auth state (claude.ai web) and the CLI's local MCP OAuth token/session cache — the CLI may be caching a stale unauthorized state rather than re-checking after a reconnect.