[BUG] Spawned team agents on macOS report "Spawned successfully" but never read mailbox; processes idle indefinitely

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report
• [x] I am using the latest version of Claude Code

What's Wrong?

After TeamCreate followed by Agent (with team_name, name, run_in_background: true), the parent reports "Spawned successfully" and the child agent process does start (visible in ps), but the agent never consumes its mailbox. Messages in ~/.claude/teams/<team>/inboxes/<name>.json stay read: false indefinitely. The team-lead never receives a reply.

Reproduced consistently across:

• Both Opus 4.7 and Haiku 4.5
• Both single-agent and two-agent spawns (proponent + opponent)
• Fresh TeamCreate after manual cleanup of stale team/inbox dirs
• Idle process lifetimes observed up to 12+ minutes with zero inbox reads

TeamDelete does not terminate child processes; shutdown_request messages also go unread (since the inbox poll never runs). Manual pkill -f 'agent-id.*<team>' is the only reliable cleanup.

What Should Happen?

A spawned background teammate should poll its inbox and either consume the initial prompt sent at spawn time or respond to subsequent SendMessage calls. The team-lead should receive responses (or at least see read: true on the inbox file).

Error Messages/Logs

No error output. Failure is silent. Inbox file content (truncated):

\\\
[
{
"from": "team-lead",
"text": "...prompt...",
"timestamp": "2026-04-27T06:08:22.512Z",
"read": false
}
]
\\\

\ps\ showed long-lived child processes such as:
\\\
/Users/<user>/.local/share/claude/versions/2.1.119 --agent-id proponent@love-dialectic --agent-name proponent --team-name love-dialectic --agent-color blue --parent-session-id <uuid> --agent-type general-purpose --model claude-opus-4-7
\\\

Steps to Reproduce

1. Enable agent teams (\CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=1\) and start Claude Code on macOS.
2. From the parent session, call \TeamCreate\ with a fresh team name.
3. Call \Agent\ with \team_name\, a \name\, \run_in_background: true\, and a short prompt instructing the agent to immediately \SendMessage\ back to \team-lead\.
4. Wait several minutes. Inspect \~/.claude/teams/<team>/inboxes/<name>.json\.
5. Observe: \read: false\, no SendMessage to team-lead, child process still alive in \ps\.

This differs from #42391 (Linux tmux 255-byte split): on macOS the launch command runs and the child process does start; it just never polls its mailbox.

Claude Model

Opus

Is this a regression?

I don't know

Claude Code Version

2.1.119 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

• Darwin 24.6.0
• A large number of MCP servers are loaded (chrome-devtools, playwright, jetbrains, claude-in-chrome, discord, context7, plus several skill plugins) — possibly relevant to bootstrap, but the symptom is consistent across attempts.
• Related (different mechanism, same area:agents): #42391.