Windows: synchronous command hooks wait for persistent descendants

Summary

On Windows, a synchronous command hook can delay its enclosing Claude Code event until a persistent descendant exits, even though the configured hook process has already exited successfully.

This is a bounded delay, not the worktree-transition result-loss symptom reported in #90049.

Environment

• Claude Code 2.1.263
• Windows 11 Pro 10.0.26100
• Interactive CLI, except where noted

Minimal reproduction

Create C:\temp\child.ps1:

Start-Sleep -Seconds 90

Create an empty C:\temp\in.txt, then create C:\temp\hook.ps1:

$null = [Console]::In.ReadToEnd()
Start-Process powershell.exe -ArgumentList '-NoProfile', '-ExecutionPolicy', 'Bypass', '-File', 'C:\temp\child.ps1' `
  -RedirectStandardInput 'C:\temp\in.txt' `
  -RedirectStandardOutput 'C:\temp\out.txt' `
  -RedirectStandardError 'C:\temp\err.txt'
exit 0

Use this project .claude/settings.json:

{
  "hooks": {
    "PostToolUse": [{
      "matcher": "PowerShell",
      "hooks": [{
        "type": "command",
        "command": "powershell.exe",
        "args": ["-NoProfile", "-ExecutionPolicy", "Bypass", "-File", "C:\\temp\\hook.ps1"],
        "timeout": 120
      }]
    }]
  }
}

In a fresh interactive Claude Code session in a Git repository, run a harmless PowerShell tool command such as:

Get-Date -Format o

Expected

The tool result is delivered when hook.ps1 exits.

Actual

The main tool completes, and hook.ps1 exits immediately, but Claude Code delays the tool result until child.ps1 exits about 90 seconds later. In one controlled run:

[Stall] tool_dispatch_end tool=PowerShell toolUseId=call_JDUVNrA2A9kA9qxZ48LWiZz9 outcome=ok durationMs=925
Slow PostToolUse hooks: 91931ms for PowerShell (1 hooks)

The child has stdin, stdout, and stderr redirected to files.

Additional controls

The same parent/child pattern was reproduced across synchronous command hooks:

| Event | Observed continuation point |
| --- | --- |
| PreToolUse | The tool dispatch began only after the 30-second child exited. |
| PostToolUse | The tool result was delayed until the child exited. |
| PostToolUseFailure | The following model request began only after the child exited. |
| UserPromptSubmit | The model request began only after the child exited. |
| SessionStart | The first model turn began only after the child exited. |
| Stop | The turn ended only after the child exited. |
| SessionEnd | The hook completed only after the child exited in a print-mode control. |
| Notification:idle_prompt | The hook completed only after the child exited. |

In valid controls, the configured hook parent exited with code 0 in roughly 50–100 ms and the descendant lasted 30–90 seconds.

Configuring the hook with async: true avoids this synchronous wait in the tested PostToolUse control.

Scope

This does not establish the internal mechanism. In particular, it does not prove explicit process-tree tracking. It establishes that, on Windows, these synchronous command-hook event paths do not complete while a persistent descendant remains alive.