Slack MCP OAuth server silently drops from available MCPs when token expires

Summary

The Slack MCP server (provided by the slack@claude-plugins-official plugin) intermittently disappears from the list of available MCP tools during a session. The only reliable workaround is logging out and logging back in, which forces a fresh OAuth flow.

Expected behavior

When the Slack MCP OAuth token expires, Claude Code should either:

1. Auto-refresh the token silently (as documented), or
2. Prompt for re-authentication

Actual behavior

The Slack MCP server silently drops off the available MCP list. No error is shown, no re-auth prompt appears — the Slack tools just stop being listed. Logging out and back in resolves the issue until the token expires again.

Reproduction steps

1. Install the slack@claude-plugins-official plugin
2. Authenticate via OAuth when prompted
3. Use Slack MCP tools normally
4. After some time (hours/days), the Slack MCP tools disappear from the available tools list
5. Logout and login to Claude Code — Slack MCP reappears

Environment

• Claude Code version: 2.1.92 (updated to 2.1.109, not yet confirmed if resolved)
• OS: macOS 26.3.1 (arm64)
• Plugin: slack@claude-plugins-official v1.0.0
• MCP config: OAuth HTTP server at https://mcp.slack.com/mcp

Additional context

The Slack plugin's .mcp.json configures an OAuth-based HTTP MCP server. The token refresh mechanism appears to fail silently rather than triggering re-authentication. Other OAuth MCP plugins (Asana, Atlassian) may also be affected but I've only observed it consistently with Slack.