Gmail MCP create_draft cannot thread into existing conversations (no threadId parameter)

Bug Description

The Gmail MCP create_draft tool does not support creating threaded replies. Every draft gets its own new threadId, making it impossible to reply to existing email threads.

Expected Behavior

create_draft should accept a threadId parameter (standard Gmail API functionality) so drafts can be created as replies within existing email threads.

Current Behavior

• create_draft only accepts: to, cc, bcc, subject, body, htmlBody
• No threadId parameter exists
• Every draft gets a new unique threadId regardless of subject prefix, recipient overlap, or forwarded-message headers
• Verified via list_drafts: each draft threadId matches its own id, not the original thread

Regression

This previously worked (confirmed working as recently as 2026-04-16). The MCP connector may have been updated and lost threadId pass-through.

Impact

• Cannot reply to emails programmatically
• Cannot forward emails with thread history/attachments
• Core email workflow broken

Environment

• Claude Code CLI on Windows 11
• Gmail MCP connector (claude.ai integration)
• Tested on both Opus 4.7 and Opus 4.6