Custom statusLine command output renders vertically on Windows

Description

Custom statusLine command output renders each character on its own line instead of displaying horizontally on Windows.

Environment

• OS: Windows 11 Pro (10.0.26200)
• Terminal: PowerShell
• Claude Code version: Latest (native installer)

Steps to Reproduce

1. Add a custom status line to ~/.claude/settings.json:

"statusLine": {
  "type": "command",
  "command": "node \"C:/Users/jmj2z/.claude/statusline.js\""
}

1. With statusline.js:

let input = '';
process.stdin.on('data', chunk => input += chunk);
process.stdin.on('end', () => {
  try {
    const data = JSON.parse(input);
    const model = data.model?.display_name || data.model?.id || '?';
    const pct = Math.round(data.context_window?.used_percentage || 0);
    const cost = data.cost?.total_cost_usd;
    const costStr = cost != null ? ` | $${cost.toFixed(2)}` : '';
    process.stdout.write(`${model} | ctx: ${pct}%${costStr}`);
  } catch {
    process.stdout.write('--');
  }
});

1. Launch Claude Code

Expected Behavior

Status line displays horizontally at the bottom:

Opus 4.6 | ctx: 0% | $0.00

Actual Behavior

Each character renders on its own line:

O
p
u
s

4
.
6

|

c
t
x
:

0
%

|

$
0
.
0
0

Troubleshooting Done

• Confirmed the script outputs a single line (no \r\n in output)
• Tested with .replace(/[\r\n]/g, '') — same result
• Removing the custom statusLine config restores the default status line, which renders correctly
• Resizing the terminal window does not fix it
• Issue persists across new terminal windows and Claude Code restarts
• The built-in default status line works perfectly — only custom command status lines are affected

Likely Cause

The status line renderer appears to read pipe output character-by-character on Windows, treating each character as a separate line. This may be a Node.js pipe buffering difference on Windows vs Unix.