Teammate mode fails to deliver instructions when tmux pane-base-index is non-zero

Description

When using teammate/team mode with tmux split panes, Claude Code spawns new panes and launches Claude Code instances in them, but the initial instructions are never delivered to the teammates. They sit idle at the welcome screen.

The root cause appears to be that Claude Code assumes 0-based pane indexing when sending keys/instructions to teammate panes, but tmux allows users to configure pane-base-index to start at 1 (or any other value). When the indices don't match, the instructions are sent to the wrong target (or a non-existent pane).

Environment

• Claude Code v2.1.32
• macOS (Darwin 25.2.0)
• tmux 3.x (installed via Homebrew)

Relevant tmux config

set -g base-index 1
set -g pane-base-index 1
set-window-option -g pane-base-index 1

These are extremely common settings — they're included in tmux-sensible and recommended by most tmux setup guides.

Reproduction steps

1. Add set -g pane-base-index 1 to your tmux config (or use the full snippet above).
2. Start a tmux session and launch Claude Code.
3. Spawn a team with teammates:

``
Use the Teammate tool to create a team, then spawn teammates using the Task tool with team_name and name parameters.
``

1. Observe that new tmux panes are created and Claude Code instances launch in them.
2. The teammate instances sit at the welcome screen ("Try ...") and never receive their instructions.

Expected behavior

Teammates should receive their initial instructions and begin working on their assigned tasks.

Actual behavior

Teammates launch but remain idle at the Claude Code welcome prompt. No instructions are delivered. Running tmux capture-pane on the teammate panes confirms they never received any input.

Diagnosis

Running tmux list-panes -a -F '#{pane_index} #{pane_id}' shows panes indexed 1, 2, 3 — but Claude Code appears to target pane indices assuming 0-based numbering. The fix would be to query the actual pane index (e.g., via tmux list-panes -F '#{pane_index}' or use pane IDs like %0, %1 which are always stable) rather than assuming 0-based indexing.

Workaround

Temporarily set pane-base-index 0 before spawning teammates:

tmux set pane-base-index 0

(Not yet confirmed — leaving this as a suggested workaround based on the diagnosis.)