[Bug] gmail_send: Non-ASCII characters in subject field result in mojibake (missing RFC 2047 encoding)

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?

Description

When using the built-in Google Workspace MCP server's gmail_send tool, email subjects containing non-ASCII characters (e.g., Chinese, Japanese, Korean) are displayed as mojibake/garbled text in the recipient's inbox.

The email body renders correctly — only the Subject header is affected.

Steps to Reproduce

1. Use mcp__google-workspace-{account}__gmail_send to send an email
2. Set the subject to a string containing non-ASCII characters, e.g.: Fwd: 東橫INN 首爾永登浦 預訂確認郵件
3. The recipient sees garbled subject: Fwd: ᠱᩫINN 顖çÂ^þ᰸登ᵦ ...

Expected Behavior

The subject should be properly encoded using RFC 2047 MIME encoded-word syntax (e.g., =?UTF-8?B?...?=) so that non-ASCII characters display correctly in all email clients.

Actual Behavior

Raw UTF-8 bytes are placed directly into the Subject header without MIME encoding. Recipients' email clients interpret these bytes as Latin-1/ISO-8859-1, producing mojibake.

Screenshot

<img width="1229" height="661" alt="Image" src="https://github.com/user-attachments/assets/299282f0-cccd-44d0-a4be-ff43bbfe552a" />

The email body (visible below the subject) displays Chinese characters correctly — confirming the issue is isolated to the Subject header encoding.

Environment

• Claude Code CLI (latest version)
• Built-in managed MCP server: google-workspace-*
• Tools affected: gmail_send, likely also gmail_reply
• OS: macOS

Suggested Fix

When constructing the raw email message for the Gmail API, the Subject header should use RFC 2047 encoded-word format for any non-ASCII content. Reference: https://github.com/googleapis/google-api-nodejs-client/issues/739

What Should Happen?

This MCP should create mails with title supporting Mandarin rendering.

Error Messages/Logs

Steps to Reproduce

1. In Claude Code CLI, use the built-in Google Workspace MCP integration's gmail_send tool
2. 2. Set the subject to a string containing non-ASCII characters, e.g.: Fwd: 東橫INN 首爾永登浦 預訂確認郵件
3. 3. Send the email to any recipient
4. 4. The recipient sees garbled subject like: Fwd: ᠱᩫINN 顖çÂ^þ...
5. 5. The email body displays non-ASCII characters correctly — only the Subject header is affected

Claude Model

None

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.1.37 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_