[BUG] Plugin SessionStart hook fails on Windows (hook script never executes)

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?

Description

The superpowers@claude-plugins-official plugin's SessionStart hook consistently fails on Windows, displaying error messages to users on every session start and /clear command.

What Should Happen?

Expected Behavior

The SessionStart hook should execute successfully and inject the superpowers skill context without displaying any error messages.

Actual Behavior

Error messages are displayed on every session start and /clear command. The hook script never executes, despite being properly configured and working when run manually.

Error Messages/Logs

**Error messages:**
- `⎿  SessionStart:startup hook error` (on session start)
- `⎿  SessionStart:clear hook error` (when running `/clear`)

Steps to Reproduce

Steps to Reproduce

1. Ensure superpowers plugin is enabled in ~/.claude/settings.json:

``json
{
"enabledPlugins": {
"superpowers@claude-plugins-official": true
}
}
``

1. Start a new Claude Code session
2. Observe SessionStart:startup hook error message
3. Run /clear command
4. Observe SessionStart:clear hook error message

Claude Model

None

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.22

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Other

Additional Information

Environment

• OS: Windows 11 with Git Bash (MSYS)
• Claude Code Version: 2.1.22
• Bash Version: GNU bash 5.2.26(1)-release (x86_64-pc-msys)
• Plugin Version: superpowers 4.1.1
• Git for Windows: Installed at C:\Program Files\Git\bin\bash.exe

Investigation & Root Cause

<details>
<summary><strong>Debug Log Analysis</strong></summary>

From Claude Code debug logs (~/.claude/debug/*.txt):

2026-01-28T13:31:13.197Z [DEBUG] Getting matching hook commands for SessionStart with query: startup
2026-01-28T13:31:13.197Z [DEBUG] Found 2 hook matchers in settings
2026-01-28T13:31:13.197Z [DEBUG] Matched 1 unique hooks for query "startup" (2 before deduplication)
2026-01-28T13:31:24.595Z [DEBUG] Hook output does not start with {, treating as plain text

Key observation: The hook matcher successfully finds the hook, but ~11 seconds later Claude Code reports: "Hook output does not start with {, treating as plain text"

This suggests Claude Code is receiving output from the hook invocation, but it's not the expected JSON (likely an error message).

</details>

<details>
<summary><strong>Hook Script Never Executes</strong></summary>

We instrumented the hook script to write to a log file immediately upon invocation:

#!/usr/bin/env bash
echo "HOOK_START: $(date)" >> "/tmp/hook-trace.log" 2>&1
# ... rest of script

Result: When Claude Code attempts to run the hook, the log file receives NO entries, proving the script never executes.

However, running the script manually works perfectly:

$ bash ~/.claude/plugins/cache/claude-plugins-official/superpowers/4.1.1/hooks/session-start.sh
{
  "hookSpecificOutput": {
    "hookEventName": "SessionStart",
    "additionalContext": "<EXTREMELY_IMPORTANT>..."
  }
}
# Exit code: 0

</details>

<details>
<summary><strong>Hook Configuration</strong></summary>

The hook is properly configured in:
~/.claude/plugins/cache/claude-plugins-official/superpowers/4.1.1/hooks/hooks.json

{
  "hooks": {
    "SessionStart": [
      {
        "matcher": "startup|resume|clear|compact",
        "hooks": [
          {
            "type": "command",
            "command": "${CLAUDE_PLUGIN_ROOT}/hooks/session-start.sh"
          }
        ]
      }
    ]
  }
}

The script itself:

• Has proper shebang: #!/usr/bin/env bash
• Uses set -euo pipefail for error handling
• Uses pure bash constructs (no external dependencies beyond cat)
• Produces valid JSON output when run manually
• Follows documented format from the plugin's docs/windows/polyglot-hooks.md

</details>

Hypothesis

Claude Code 2.1.x is supposed to auto-detect .sh files in hook commands and prepend bash on Windows (per the superpowers plugin documentation). However, one of these issues appears to be occurring:

1. Path Resolution Failure: ${CLAUDE_PLUGIN_ROOT} may not be expanded correctly during hook invocation
2. Shell Invocation Issue: The automatic bash prepending may be failing or malformed
3. Silent Command Failure: The constructed command may be failing before reaching the script
4. Path Format Issue: Mixed Windows/Unix path formats may be causing command execution to fail

Impact

• Poor user experience with persistent error messages
• Superpowers skills not automatically loaded (though still manually accessible via Skill tool)
• Affects all Windows users of the superpowers plugin

Workaround

Disable the superpowers plugin to eliminate the error:

{
  "enabledPlugins": {
    "superpowers@claude-plugins-official": false
  }
}

Suggested Fix

The Claude Code hook execution logic needs to be enhanced to:

1. Log the exact command being executed - Include in debug logs:

• The constructed command line
• Working directory
• Environment variables (especially CLAUDE_PLUGIN_ROOT)
• Full stdout/stderr from the hook process
• Exit code

1. Verify path variable expansion - Ensure ${CLAUDE_PLUGIN_ROOT} is properly expanded on Windows before executing

1. Handle Windows path formats - Consider if paths need normalization (backslash vs forward slash)

1. Better error reporting - When hooks fail, show users the actual error message instead of just "hook error"

Related

The superpowers plugin includes Windows-specific documentation at docs/windows/polyglot-hooks.md that references:

• anthropics/claude-code#9758 - .sh scripts open in editor on Windows
• anthropics/claude-code#3417 - Hooks don't work on Windows
• anthropics/claude-code#6023 - CLAUDE_PROJECT_DIR not found

This issue may be related to those previous Windows hook execution problems.

Additional Context

<details>
<summary><strong>Superpowers Plugin Hook Documentation Notes</strong></summary>

From superpowers/4.1.1/docs/windows/polyglot-hooks.md:

Claude Code 2.1.x changed the Windows execution model for hooks: Before (2.0.x): Hooks ran with shell:true, using the system default shell. This required polyglot wrapper scripts. After (2.1.x): Claude Code now auto-detects .sh files in hook commands and prepends "bash " on Windows.

The hooks.json was updated to reference the .sh file directly (not a .cmd wrapper) for Claude Code 2.1.x compatibility, but this appears to not be working as intended on Windows.

</details>