Gmail MCP connector returns empty error after full re-auth cycle

Description

The standalone Gmail MCP connector consistently returns "Tool returned error result" with no error message body on every call. This is a silent failure — no details about what went wrong are surfaced to the user.

Steps to Reproduce

1. Gmail connector was previously working fine
2. Started returning empty errors on all tool calls (gmail_get_profile, gmail_search_messages, etc.)
3. Attempted the following fixes — none resolved the issue:

• Disconnected and reconnected the connector in Claude settings
• Fully uninstalled and reinstalled the connector
• Revoked "Claude for Gmail" access from Google Account permissions (myaccount.google.com/permissions)
• Deleted the connector entirely and re-added it as a new connector
• Re-authorized through the full Google OAuth consent screen

Expected Behavior

After revoking access from Google's side and adding a fresh connector, the OAuth flow should establish a new valid token and tool calls should succeed.

Actual Behavior

• Every tool call returns: Tool returned error result with an empty error body
• The connector ID (315616fb-0a9d-415c-a9be-bcc5338b1440) appears to persist even after delete + re-add, suggesting the backend is reusing a stale connector instance
• The GWS (Google Workspace) connector's Gmail tools work fine against the same Google account, confirming the issue is specific to the standalone Gmail connector infrastructure, not Google-side auth

Environment

• Claude Code CLI (macOS, Darwin 24.6.0)
• Connector type: Anthropic cloud-managed Gmail connector
• Connector ID: 315616fb-0a9d-415c-a9be-bcc5338b1440

Workaround

Using the GWS connector's Gmail tools (mcp__gws__gmail_users_*) as a fallback — these work correctly with valid auth.