tmux teammate-mode non-functional in v2.1.76 (fish shell, macOS)

Description

tmux teammate-mode does not work in Claude Code v2.1.76. Teammates launch in split panes but fail to receive or process instructions from the lead session.

Environment

• Claude Code version: 2.1.76
• OS: macOS (Darwin 25.3.0)
• Shell: fish
• tmux: installed and running
• teammate-mode setting: "tmux"

Expected Behavior

When using "teammate-mode": "tmux", spawned teammate agents should:

1. Open in tmux split panes
2. Receive their initial instructions from the lead session
3. Execute tasks and communicate results back

Actual Behavior

Teammates launch in tmux split panes but appear disconnected from the lead's messaging system. They either:

• Sit idle at the prompt without processing any instructions
• Never receive their initial mailbox message

Possibly Related Issues

• #24771 — Teammates disconnected from messaging system
• #24108 — Teammates stuck at idle prompt
• #23527 — pane-base-index ≥ 1 causes silent failure
• #25375 — Shell compatibility issues

These issues suggest the problem has persisted across multiple versions. The partial fix in v2.1.33 ("Fixed agent teammate sessions in tmux to send and receive messages") does not appear to have fully resolved the underlying issue.

Steps to Reproduce

1. Set "teammate-mode": "tmux" in Claude Code settings
2. Run Claude Code inside a tmux session on macOS with fish shell
3. Trigger a task that spawns teammate agents
4. Observe that teammates launch but do not process instructions

Additional Context

• pane-base-index is at default (0)
• The issue may be related to fish shell compatibility, but the broader pattern across the related issues suggests a more fundamental messaging problem