[BUG] Agent teams: iTerm2 split-pane mode spawns in-process teammates despite correct configuration

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 using agent teams with teammateMode: "tmux" in iTerm2, teammates are spawned in-process instead of in split panes, even though all documented requirements are met.

Environment

• OS: macOS (Darwin 25.2.0)
• Terminal: iTerm2 (confirmed via TERM_PROGRAM=iTerm.app)
• it2 CLI: mkusaka/it2 v0.2.0 (installed at ~/.local/bin/it2)
• iTerm2 Python API: Enabled in Settings → General → Magic
• ITERM_SESSION_ID: Present and valid

Configuration

// settings.json
{
"env": {
"CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS": "1"
},
"teammateMode": "tmux"
}

Teammates report:

• TTY: not a tty (running as subprocess)
• TMUX: not set
• No split panes created

What Should Happen?

Teammates should appear in separate iTerm2 split panes, as documented at https://code.claude.com/docs/en/agent-teams#choose-a-display-mode

Error Messages/Logs

Steps to Reproduce

1. Enable agent teams via CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=1
2. Set teammateMode: "tmux" in settings.json
3. Install it2 CLI from https://github.com/mkusaka/it2 (as linked in docs)
4. Enable Python API in iTerm2 preferences
5. Start Claude Code in iTerm2
6. Create an agent team and spawn teammates

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.34 (Claude Code)

Platform

AWS Bedrock

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

_No response_