[BUG] Custom statusLine command executed but output intermittently not rendered

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?

Custom statusLine command is executed by Claude Code (confirmed via debug logging to a temp file), but the output is intermittently not rendered in the TUI. Some sessions display the status line, others don't - with no clear pattern by terminal type or session state.

This is distinct from #29411 (resumed sessions only) and #32660 (script never executed). In this case the script is executed but the output is silently discarded by the renderer.

What Should Happen?

The custom status line should render consistently in all sessions when the command is configured and producing valid output.

Error Messages/Logs

No errors. The status line simply doesn't appear. Debug logging confirms the script is called:

19:35:57 pid=46355 ppid=86006
19:36:03 pid=46421 ppid=86006
19:36:05 pid=46538 ppid=86006

Script produces valid output when tested manually:

[Opus 4.6] myhostname | k8s:Nissan | ██░░░░░░░░ ~10% | mem:16/24G | 19:17:17

Steps to Reproduce

1. Configure a custom statusLine command in ~/.claude/settings.json:

``json
"statusLine": {
"type": "command",
"command": "/bin/bash ~/.claude/statusline-command.sh"
}
``

1. Open multiple Claude Code sessions (across tabs or terminals)
2. Some sessions will show the status line, others won't
3. No consistent pattern: happens in iTerm2, Termius; works in Antigravity
4. Both fresh and resumed sessions can be affected

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

Not sure of exact version - noticed recently

Claude Code Version

2.1.72 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

• Sandbox is disabled
• No statusLine override in settings.local.json or project-level settings
• The status line script runs in ~50ms (well within any timeout)
• iTerm2 shell integration was disabled as a test - did not fix the issue
• Multiple concurrent Claude Code sessions may be a factor, but the issue also occurs with fewer sessions
• Related issues: #29411, #32660, #32211