[BUG] "mcpServers" yaml frontmatter in Subagent.md don't work when launching with `--agent` cli command

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?

"mcpServers" yaml frontmatter in Subagent.md don't work when launching with --agent cli command

What Should Happen?

"mcpServers" yaml frontmatter in Subagent.md should work properly when launching with --agent cli command

Error Messages/Logs

Steps to Reproduce

1. Remove a mcp server from project config(.mcp.json)
2. Add this mcp server to subagent.md frontmatter like below

---
name: my-new-subagent
description: ***
tools: ***
model: inherit
color: yellow
mcpServers:
  - my-mcp-server:
       type: "stdio"
       command: "python"
       args: []
---

1. Lanch new session with cli args --agent my-new-subagent, and ask claude to do something with mcp tools.
2. It appears that claude doesn't get tools in mcp properly

Claude Model

None

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.55

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_