[BUG] `roots/list` is incorrect on Windows

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?

On the first (hard-coded) call from the server to the client for roots/list on Windows, the output is:

file://d:\\dev\\flutter\\tmp

What Should Happen?

But the correct output would be:

file:///d:/dev/flutter/tmp

Coming from:

• https://github.com/dart-lang/ai/issues/499

Error Messages/Logs

No logs but this makes some servers confused when processing requests.

Steps to Reproduce

1. Create a default Flutter project with flutter create tmp.
2. Add the MCP server for Dart:

{
	"type": "stdio",
	"command": "dart",
	"args": [
		"run",
		"dart_mcp_server@"
	]
}

1. Connect

Claude Model

Not sure / Multiple models

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

Claude for VSCode 2.1.175

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Other

Additional Information

_No response_