Statusline: context_window.used_percentage always 0 on native Windows (Git Bash)

Environment

• Claude Code version: 2.1.138
• OS: Windows 10 (MINGW64 / Git Bash / MSYS2)
• Bash version: 5.3.9
• Node.js: v25.0.0

Description

When using a custom statusline command on native Windows (Git Bash / MSYS2), the JSON data passed via stdin has incorrect context_window values:

• used_percentage is always 0
• remaining_percentage is always 100
• total_input_tokens / total_output_tokens reflect only the most recent API call, not cumulative session values

The same script on WSL (same Claude Code version) works correctly — used_percentage reflects the real cumulative window occupancy.

Steps to Reproduce

1. On native Windows with Git Bash, configure a statusline command in ~/.claude/settings.json:

``json
"statusLine": {
"type": "command",
"command": "path/to/statusline-command.sh"
}
``

1. Add debug logging to the script to dump the raw JSON from stdin
2. Have a multi-turn conversation (accumulating significant cost, e.g. $0.50+)
3. Inspect the logged JSON

Actual Behavior

After multiple turns with cost.total_cost_usd reaching ~$1.00:

"context_window": {
    "total_input_tokens": 354,
    "total_output_tokens": 136,
    "context_window_size": 200000,
    "used_percentage": 0,
    "remaining_percentage": 100
}

Note: cost.total_cost_usd in the same JSON payload is correctly cumulative ($1.00+), confirming the session has significant token usage.

Expected Behavior

used_percentage should reflect the actual context window occupancy (as it does on WSL/Linux), and total_input_tokens/total_output_tokens should be cumulative session values.

Additional Context

• The statusline command is called at a normal frequency (~14s intervals) — not a call frequency issue
• The script executes correctly (confirmed via process listing and debug logs)
• Only the data passed to the script is incorrect on Windows
• cost.total_cost_usd in the same JSON payload is accurate and cumulative, suggesting the issue is specific to context_window field population on Windows
• WSL on the same machine with the same script and same Claude Code version works correctly