[BUG] UserPromptSubmit hooks receive empty stdin on Windows — stdin piping broken

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?

UserPromptSubmit hooks receive empty stdin on Windows — stdin piping broken

Environment

• Claude Code version: 2.1.107
• OS: Windows 11 Enterprise (10.0.26100)
• Platform: Win32
• Node.js version: (used by hook system)

Summary

On Windows, UserPromptSubmit hook invocations receive empty stdin. This breaks any hook that depends on the hook input JSON passed via stdin by Claude Code.

Impact

Plugins and hooks that rely on stdin to receive session ID or other contextual data fail silently. Example:

• claude-mem plugin uses stdin to receive session_id in UserPromptSubmit hooks, which it needs to initialize sessions. With empty stdin, it falls back to {} and loses all session context, so no observations are ever recorded.
• Any custom hook or plugin using stdin for UserPromptSubmit will fail the same way

Root cause hypothesis

One of:

1. Claude Code on Windows doesn't pipe stdin to UserPromptSubmit hooks (regression or breaking change in hook contract)
2. The stdin relay through Node.js → cmd.exe → bun.cmd loses the piped data
3. Windows-specific issue with how stdin is inherited by child processes in the hook system

Investigation notes

• Other hook types may work (SessionStart, PostToolUse, Stop, SessionEnd) — unclear if they have the same issue
• The bun-runner.js script includes a workaround for Bun's EINVAL on pipes (#646), but it falls back to writing '{}' when no stdin is available, masking the original issue
• macOS/Linux users likely don't experience this (Unix pipes are more reliable)

Additional context

This affects the claude-mem plugin (v12.1.0), which is completely non-functional on Windows due to this stdin issue. The plugin worker is healthy and MCP tools respond, but no observations are ever collected because session initialization fails.

Relevant stdin handling code:

• bun-runner.js line 158-188: buffers stdin from Node → Bun, falls back to '{}'
• Affected hook: UserPromptSubmit hook invocation in Claude Code hook system

Workaround

Add the following to settings.json
"env": {
"CLAUDE_CODE_GIT_BASH_PATH": "C:\\Users\\YOURUSERNAME\\AppData\\Local\\Programs\\Git\\bin\\bash.exe"
}
Use "where bash" to locate your bash.exe

What Should Happen?

Expected behavior

UserPromptSubmit hooks should receive stdin containing JSON with at least:

• session_id or id (session identifier)
• cwd (current working directory)
• prompt (user message text, if present)

As per the documented hook input contract for Claude Code.

Error Messages/Logs

Steps to Reproduce

Steps to reproduce

1. Add a debug hook to ~/.claude/settings.json:

{
  "hooks": {
    "UserPromptSubmit": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "tee C:/temp/debug_hook_stdin.json",
            "timeout": 5
          }
        ]
      }
    ]
  }
}

1. Start a new Claude Code session
2. Send any prompt (e.g., "hello")
3. Check the captured file: cat C:/temp/debug_hook_stdin.json

Expected: File contains JSON with hook input (e.g., {"session_id": "...", "cwd": "...", "prompt": "..."})
Actual: File is empty (0 bytes)

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.107

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

_No response_