# Cross-Platform Polyglot Hooks for Claude Code

Claude Code plugins need hooks that work on Windows, macOS, and Linux. This document describes the single generic dispatcher pattern used in `hooks/run-hook.cmd`.

> **Authoritative source:** `hooks/run-hook.cmd` is the canonical implementation. When this document and the code diverge, trust the code.

## The Problem

Claude Code runs hook commands through the system's default shell:
- **Windows**: CMD.exe
- **macOS/Linux**: bash or sh

This creates several challenges:

1. **Script execution**: Windows CMD can't execute `.sh` files directly
2. **Path format**: Windows uses backslashes (`C:\path`), Unix uses forward slashes (`/path`)
3. **Environment variables**: `$VAR` syntax doesn't work in CMD
4. **`.sh` auto-prepend**: Claude Code on Windows automatically prepends `bash` to any command that contains `.sh` in its path — this interferes with the dispatcher if scripts have extensions

## The Solution: Extensionless Scripts + Single Generic Dispatcher

The repo uses one generic `run-hook.cmd` dispatcher for all hooks. Hook scripts are **extensionless** (`session-start`, not `session-start.sh`). This is deliberate: it prevents Claude Code's Windows auto-detection from prepending `bash` to the dispatcher command and breaking it.

### File Structure

```
hooks/
├── hooks.json          # Points to run-hook.cmd with extensionless script name
├── run-hook.cmd        # Cross-platform dispatcher (the polyglot wrapper)
└── session-start       # Actual hook logic — extensionless bash script
```

### hooks.json

```json
{
  "hooks": {
    "SessionStart": [
      {
        "matcher": "startup|clear|compact",
        "hooks": [
          {
            "type": "command",
            "command": "\"${CLAUDE_PLUGIN_ROOT}/hooks/run-hook.cmd\" session-start",
            "async": false
          }
        ]
      }
    ]
  }
}
```

The path is quoted because `${CLAUDE_PLUGIN_ROOT}` may contain spaces.

## How `run-hook.cmd` Works

`run-hook.cmd` is a polyglot script — valid syntax in both CMD and bash:

```cmd
: << 'CMDBLOCK'
@echo off
REM Cross-platform polyglot wrapper for hook scripts.
REM On Windows: cmd.exe runs the batch portion, which finds and calls bash.
REM On Unix: the shell interprets this as a script (: is a no-op in bash).
REM
REM Hook scripts use extensionless filenames (e.g. "session-start" not
REM "session-start.sh") so Claude Code's Windows auto-detection -- which
REM prepends "bash" to any command containing .sh -- doesn't interfere.
REM
REM Usage: run-hook.cmd <script-name> [args...]

if "%~1"=="" (
    echo run-hook.cmd: missing script name >&2
    exit /b 1
)

set "HOOK_DIR=%~dp0"

REM Try Git for Windows bash in standard locations
if exist "C:\Program Files\Git\bin\bash.exe" (
    "C:\Program Files\Git\bin\bash.exe" "%HOOK_DIR%%~1" %2 %3 %4 %5 %6 %7 %8 %9
    exit /b %ERRORLEVEL%
)
if exist "C:\Program Files (x86)\Git\bin\bash.exe" (
    "C:\Program Files (x86)\Git\bin\bash.exe" "%HOOK_DIR%%~1" %2 %3 %4 %5 %6 %7 %8 %9
    exit /b %ERRORLEVEL%
)

REM Try bash on PATH (e.g. user-installed Git Bash, MSYS2, Cygwin)
where bash >nul 2>nul
if %ERRORLEVEL% equ 0 (
    bash "%HOOK_DIR%%~1" %2 %3 %4 %5 %6 %7 %8 %9
    exit /b %ERRORLEVEL%
)

REM No bash found - exit silently rather than error
REM (plugin still works, just without SessionStart context injection)
exit /b 0
CMDBLOCK

# Unix: run the named script directly
SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
SCRIPT_NAME="$1"
shift
exec bash "${SCRIPT_DIR}/${SCRIPT_NAME}" "$@"
```

### How it works on Windows (CMD.exe)

1. `: << 'CMDBLOCK'` — CMD sees `:` as a label (no-op) and ignores `<< 'CMDBLOCK'`
2. The batch section validates the script name, resolves `HOOK_DIR` from the dispatcher's own location, then tries bash in three places:
   - `C:\Program Files\Git\bin\bash.exe`
   - `C:\Program Files (x86)\Git\bin\bash.exe`
   - `bash` on `PATH` (MSYS2, Cygwin, or a non-default Git install)
3. If no bash is found, the dispatcher exits `0` silently — the plugin continues working, it just skips the hook
4. `exit /b` stops CMD before it reaches the Unix section

### How it works on Unix (bash/sh)

1. `: << 'CMDBLOCK'` — `:` is a no-op; `<< 'CMDBLOCK'` opens a heredoc
2. The entire CMD batch block is consumed by the heredoc (ignored)
3. After `CMDBLOCK`, bash resolves the script directory and `exec`s the named extensionless script directly

### Key design decisions

| Decision | Why |
|----------|-----|
| Extensionless scripts | Prevents Claude Code's Windows `.sh`-auto-prepend from interfering with the dispatcher command |
| No `-l` (login shell) | Not needed; hook scripts should be self-contained and not depend on login-shell PATH setup |
| No `cygpath` | Bash receives the Windows path directly and handles it correctly; `cygpath` was needed by the old `-c "..."` invocation pattern, not by direct exec |
| Silent exit on no-bash | Avoids breaking the plugin for users who don't have Git for Windows; hook context injection is skipped gracefully |

## Writing Cross-Platform Hook Scripts

Your hook logic goes in the extensionless script file. A few portable patterns:

### Do
- Use pure bash builtins when possible
- Use `$(command)` instead of backticks
- Quote all variable expansions: `"$VAR"`

### Avoid
- Relying on PATH-dependent tools without fallbacks (the hook runs without `-l`, so login-shell PATH is not set)
- Giving scripts a `.sh` extension — this triggers Claude Code's Windows auto-prepend

### Example: JSON escaping without external tools

```bash
escape_for_json() {
    local input="$1"
    local output=""
    local i char
    for (( i=0; i<${#input}; i++ )); do
        char="${input:$i:1}"
        case "$char" in
            $'\\') output+='\\' ;;
            '"') output+='\"' ;;
            $'\n') output+='\n' ;;
            $'\r') output+='\r' ;;
            $'\t') output+='\t' ;;
            *) output+="$char" ;;
        esac
    done
    printf '%s' "$output"
}
```

## Troubleshooting

### "bash is not recognized"

CMD couldn't find bash in any of the three locations the dispatcher tries. The dispatcher exits silently (0) rather than erroring, so the hook is skipped. Install Git for Windows at the standard path or ensure `bash` is on `PATH`.

### Hook runs on Unix but does nothing on Windows

Check that the script filename is **extensionless** in `hooks.json`. A command like `run-hook.cmd session-start.sh` can trigger Claude Code's `.sh` auto-detection and bypass the intended CMD dispatcher path, or just try to run a non-existent `session-start.sh` script.

### Hook doesn't fire at all

Verify the `matcher` in `hooks.json` matches the event type your harness emits. Claude Code uses `startup|clear|compact`; Codex uses `startup|resume|clear`. Check `hooks-codex.json` for the Codex variant.

## Related Issues

- [anthropics/claude-code#9758](https://github.com/anthropics/claude-code/issues/9758) — `.sh` scripts open in editor on Windows
- [anthropics/claude-code#3417](https://github.com/anthropics/claude-code/issues/3417) — Hooks don't work on Windows