[BUG] Agent teams tmux split-pane fails with custom tmux prefix key (C-Space instead of default C-b)

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?

Agent teams with teammateMode: "tmux" fail to split tmux panes when using a custom prefix key (C-Space instead of the default C-b). Teammates either don't spawn in separate panes or silently fall back to in-process mode.

I suspect Claude Code internally relies on the default C-b prefix when sending tmux key sequences, which breaks for any user with a custom prefix binding. Direct tmux CLI commands (tmux split-window, tmux send-keys) don't require the prefix key, so the fix would be to avoid prefix-dependent key sequences entirely.

What Should Happen?

Teammates should spawn in separate tmux split panes regardless of the user's configured prefix key. Claude Code should use direct tmux CLI commands which are prefix-independent, or query the current prefix via tmux show-options -g prefix before sending key sequences.

Error Messages/Logs

No error messages displayed. The fallback to in-process mode is silent — no warning or log indicating that tmux split failed or that the prefix key was an issue.

Steps to Reproduce

1. Configure tmux with a custom prefix key in ~/.tmux.conf:

unbind C-b
set -g prefix C-Space
bind C-Space send-prefix

1. Reload tmux config: tmux source-file ~/.tmux.conf
2. Start a tmux session: tmux new -s claude
3. Ensure settings.json contains:

   {
     "env": { "CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS": "1" },
     "teammateMode": "tmux"
   }

1. Launch claude
2. Ask Claude to create an agent team and spawn teammates
3. Observe: panes don't split, teammates run in-process

Optional verification:

1. Reset prefix to default: tmux set -g prefix C-b
2. Repeat steps 5-6
3. Observe: split panes work correctly (if confirmed, this isolates the prefix as root cause)

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.41

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

_No response_