MCP subprocess env vars with semicolons in value are silently dropped

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?

When Claude Code spawns an MCP server subprocess, environment
variable values containing semicolons (;) are silently dropped —
the key never reaches the subprocess environment at all.

What Should Happen?

Expected behavior:

All env vars defined in settings.json are passed to the subprocess
unchanged, regardless of their value content.

Actual behavior:

Env vars whose values contain semicolons are silently omitted from
the subprocess environment.

Error Messages/Logs

Steps to Reproduce

1. Add an env var with a semicolon-containing value to

settings.json:
{
"mcpServers": {
"my-server": {
"command": "...",
"env": {
"MY_COOKIE": "foo=bar; baz=qux; session=abc123"
}
}
}
}

1. In the MCP server process, print os.environ.get("MY_COOKIE") →

returns None

1. Other env vars without semicolons (e.g. JIRA_URL) are passed

correctly

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.14.5

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

_No response_