[FEATURE] gmail_create_draft: add threadId / inReplyTo support for threaded replies

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

The current gmail_create_draft tool has no way to attach a draft to an existing thread. There is no threadId, inReplyTo, or References parameter. This means every draft created via the Gmail MCP lands as a standalone email, completely disconnected from the original customer thread.

This makes the tool nearly unusable for any support or reply workflow, which is probably the most common real-world use case for drafting emails.

Proposed Solution

gmail_create_draft should accept an optional threadId parameter and automatically set In-Reply-To and References headers so the draft appears inline in the conversation thread, just like a normal reply draft in Gmail.

The Gmail API fully supports this. It just needs to be exposed in the tool.

Alternative Solutions

None. The raw Message-ID header needed to construct threading headers is not returned by get_thread either, so there is no way to work around this on the agent side.

Related issues: #32266, #28575

Priority

Critical - Blocking my work

Feature Category

MCP server integration

Use Case Example

An AI agent reads an incoming support email via get_thread, drafts a reply using gmail_create_draft, but the draft lands as a brand new email instead of a reply in the original thread. The customer and the agent both have to manually search for the draft. With threadId support, the draft would appear directly under the customer's email, ready to send.

Additional Context

_No response_