actual/scripts/agent-hooks/git-guard.sh

#!/bin/sh
# Shared agent guard for shell/Bash commands (Claude PreToolUse[Bash],
# Codex PreToolUse[^Bash$], Cursor beforeShellExecution via adapter).
#
# Deterministically enforces the project's git-safety and workspace rules that
# used to live as prose in AGENTS.md / .github/agents/pr-and-commit-rules.md.
# Reads the command from `.tool_input.command` on stdin. Exit code 2 + stderr
# blocks the call and feeds the reason back to the agent.
#
# Best-effort by design: it catches the honest mistakes agents actually make
# (plain push-to-main, a forgotten [AI] commit prefix, --no-verify, yarn in a
# workspace). It does NOT try to defend against deliberate evasion via unusual
# shell forms (git global options before the subcommand, `git commit -F`/`-C`/
# `--amend`, etc.) — CI and branch protection are the real gates for what lands.

block() {
  echo "$1" >&2
  exit 2
}

# Fail closed: a malformed payload (jq parse failure) blocks rather than allows.
cmd=$(jq -r '.tool_input.command // empty' 2>/dev/null) ||
  block "Blocked: could not parse the hook payload (.tool_input.command)."
[ -z "$cmd" ] && exit 0

# Enforce: always run yarn from the repo root, never in a child workspace.
# Catch cd/pushd into a package dir, or `yarn --cwd packages/...`.
case "$cmd" in
  *"cd packages/"* | *"cd ./packages/"* | *"cd packages "* | *"cd ./packages "* | \
    *"pushd packages/"* | *"pushd ./packages/"* | *"pushd packages "* | *"pushd ./packages "* | \
    *"yarn --cwd packages/"* | *"yarn --cwd ./packages/"* | *"yarn --cwd packages "* | *"yarn --cwd ./packages "* | \
    *"yarn --cwd=packages/"* | *"yarn --cwd=./packages/"* | *"yarn --cwd=packages "* | *"yarn --cwd=./packages "*)
    case "$cmd" in
      *yarn*)
        block "Blocked: run yarn from the repo root, not a child workspace (AGENTS.md). Use 'yarn workspace <name> <cmd>' from the root instead." ;;
    esac ;;
esac

# Everything below only applies to actual git invocations. Match `git` as a
# whole token (not a substring) so commands that merely mention git — e.g.
# `rg "git config" AGENTS.md` — aren't falsely blocked.
is_git=0
set -f
for tok in $cmd; do
  [ "$tok" = "git" ] && {
    is_git=1
    break
  }
done
set +f
[ "$is_git" -eq 1 ] || exit 0

# Never skip git hooks.
case "$cmd" in
  *--no-verify* | *--no-gpg-sign*)
    block "Blocked: skipping git hooks (--no-verify/--no-gpg-sign) is forbidden." ;;
esac

# Never change git config (reads are fine).
case "$cmd" in
  *"git config"*)
    case "$cmd" in
      *--get* | *--list* | *" -l "* | *" -l") ;;
      *) block "Blocked: 'git config' changes are forbidden." ;;
    esac ;;
esac

# Never force-push or push to main/master.
case "$cmd" in
  *"git push"*)
    case "$cmd" in
      *--force* | *" -f "* | *" -f")
        block "Blocked: force push only on explicit user request. Use --force-with-lease and confirm with the user first." ;;
    esac
    # Match main/master only as a whole ref/token (padding avoids matching
    # branches like "maintenance" or "main-feature"). Also catch explicit
    # refspecs like `HEAD:refs/heads/main` / `origin refs/heads/master`.
    case " $cmd " in
      *" main "* | *" master "* | *":main "* | *":master "* | \
        *" refs/heads/main "* | *" refs/heads/master "* | \
        *":refs/heads/main "* | *":refs/heads/master "*)
        block "Blocked: never push to main/master. Push the feature branch instead." ;;
    esac ;;
esac

# Commit messages must start with [AI].
case "$cmd" in
  *"git commit"*)
    # The [AI] rule applies to the commit subject, which git takes from the
    # FIRST -m/--message flag (later -m flags become body paragraphs, e.g. a
    # Co-Authored-By trailer). ${var#pattern} strips the shortest matching
    # prefix, i.e. finds the first occurrence; when both flag spellings are
    # present, the longer remainder is the one whose flag came first.
    rest=''
    case "$cmd" in
      *" --message"*)
        rest=${cmd#*" --message"}
        rest=${rest#=} ;;
    esac
    case "$cmd" in
      *" -m"*)
        m_rest=${cmd#*" -m"}
        [ ${#m_rest} -gt ${#rest} ] && rest=$m_rest ;;
    esac
    # First line of the remainder tells us the message form. An empty result
    # means no inline message (editor/--amend/-F) — left alone.
    first=$(printf '%s\n' "$rest" | sed -n '1s/^[[:space:]]*//p')
    msg=''
    case "$first" in
      *'$('*'<<'*)
        # Heredoc via command substitution: -m "$(cat <<'EOF' ... EOF)".
        # The subject is the first non-blank line of the heredoc body (git's
        # default cleanup strips leading blank lines).
        msg=$(printf '%s\n' "$rest" | sed -n '2,$p' |
          sed -n '/[^[:space:]]/{s/^[[:space:]]*//;p;q;}') ;;
    esac
    # Inline string: strip one leading quote, truncate at the next quote
    # (good enough for a prefix check). Also the fallback when the heredoc
    # pattern matched but no body line followed — e.g. a single-line <<<
    # here-string — so a statically unknowable message fails closed instead
    # of slipping past as empty.
    [ -n "$msg" ] || msg=$(printf '%s\n' "$first" | sed "s/^['\"]//; s/['\"].*//")
    case "$msg" in
      "" | "[AI]"*) ;;
      *) block "Blocked: commit messages must start with '[AI]'. Got: $msg" ;;
    esac ;;
esac

exit 0