[BUG] "Gmail MCP label_thread returns 403 - gmail.modify scope blocked by Google"

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?

The Gmail MCP connector (gmail.mcp.claude.com) fails on all operations
requiring the gmail.modify scope (label_thread, label_message,
unlabel_thread, unlabel_message). Every call returns 403. When attempting
to reconnect the connector with broader permissions, Google displays
"Esta aplicación está bloqueada" (This app has been blocked), preventing
authorization entirely. The gmail.mcp.claude.com OAuth app is not verified
by Google for sensitive scopes.

What Should Happen?

label_thread, label_message and related tools should successfully apply
and remove labels on Gmail threads. The OAuth app should be verified by
Google for the gmail.modify scope, or unsupported tools should be clearly
marked as non-functional in the connector UI.

Error Messages/Logs

MCP server "Gmail" requires re-authorization (token expired)
Streamable HTTP error: Server returned 403 after trying upscoping

Steps to Reproduce

1. Connect Gmail MCP connector from Claude Code (web or desktop)
2. Call label_thread with a valid threadId and labelId
3. Observe 403 error returned
4. Attempt to reconnect Gmail connector and grant broader Google permissions
5. Google blocks the OAuth flow with "Esta aplicación está bloqueada"
6. label_thread remains non-functional regardless of reconnection attempts

Claude Model

Opus

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

v2.1.119

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

<img width="797" height="345" alt="Image" src="https://github.com/user-attachments/assets/68a28cf1-30fb-423d-850d-a5807183cb81" />