guard.py

#!/usr/bin/env python3
"""Block `git commit` until /simplify runs. Each commit attempt spends one /simplify.

The marker lives in the per-worktree Git directory because /simplify reviews
the worktree's index. Claude Code mints it through the Skill event; Codex mints
it through the explicit completion signal at the end of the skill. Other
runtimes remain unblocked because they have no supported completion path.
"""

import json
import os
import shlex
import subprocess
import sys
from pathlib import Path

data = json.load(sys.stdin)
event = data.get("hook_event_name", "")
tool_input = data.get("tool_input") or {}

COMPLETION_SIGNAL = "echo simplify-guard:complete"
SHELL_OPERATORS = {";", "&&", "||", "|", "(", ")"}
GIT_OPTIONS_WITH_VALUES = {"-C", "-c", "--config-env", "--git-dir", "--namespace", "--work-tree"}
GIT_OPTIONS_WITHOUT_SUBCOMMAND = {
    "-h",
    "--exec-path",
    "--help",
    "--html-path",
    "--info-path",
    "--man-path",
    "--version",
}


def marker(git=("git",)):
    git_dir = subprocess.run(
        [*git, "rev-parse", "--path-format=absolute", "--git-dir"],
        capture_output=True,
        cwd=data.get("cwd") or ".",
        text=True,
    ).stdout.strip()
    return Path(git_dir) / "simplify-guard.ok" if git_dir else None


def commit_prefix(command):
    lexer = shlex.shlex(command, posix=True, punctuation_chars=";&|()")
    lexer.whitespace_split = True
    tokens = list(lexer)
    for start, arg in enumerate(tokens):
        if Path(arg).name != "git":
            continue
        git = [arg]
        for token in tokens[start + 1 :]:
            if token in SHELL_OPERATORS:
                break
            if git[-1] in GIT_OPTIONS_WITH_VALUES:
                git.append(token)
            elif token == "commit":
                return git
            elif token in GIT_OPTIONS_WITHOUT_SUBCOMMAND or not token.startswith("-"):
                break
            else:
                git.append(token)


def main():
    is_claude = os.environ.get("CLAUDECODE") == "1"
    is_codex = bool(data.get("turn_id"))

    if event == "PostToolUse":
        if is_claude and tool_input.get("skill") == "simplify" and (m := marker()):
            m.touch()
        return

    if event != "PreToolUse" or not (is_claude or is_codex):
        return

    command = tool_input.get("command", "")
    if is_codex and command.strip() == COMPLETION_SIGNAL:
        if m := marker():
            m.touch()
        return

    if not (git := commit_prefix(command)) or not (m := marker(git)):
        return

    if m.exists():
        m.unlink()  # spend the token before Git runs; every attempt needs a fresh review
        return

    print(
        json.dumps(
            {
                "hookSpecificOutput": {
                    "hookEventName": "PreToolUse",
                    "permissionDecision": "deny",
                    "permissionDecisionReason": "simplify-guard: run /simplify on the staged diff first, then retry the commit.",
                }
            }
        )
    )


main()