prepare-commit-msg

#!/usr/bin/env bash
# .git/hooks/prepare-commit-msg
# Pre-fills the commit message with a Claude-generated conventional commit.

set -euo pipefail

COMMIT_MSG_FILE="$1"
COMMIT_SOURCE="${2:-}"

case "$COMMIT_SOURCE" in
  message|template|merge|squash|commit)
    exit 0
    ;;
esac

if [[ "${SKIP_CLAUDE:-0}" == "1" ]]; then
  exit 0
fi

# Skip silently if claude isn't installed on this machine.
if ! command -v claude >/dev/null 2>&1; then
  exit 0
fi

if git diff --cached --quiet; then
  exit 0
fi

LINES=$(git diff --cached --numstat | awk '{s+=$1+$2} END {print s+0}')
MAX_LINES=${CLAUDE_MSG_MAX_LINES:-2000}
if (( LINES > MAX_LINES )); then
  exit 0
fi

DIFF=$(git diff --cached)
FILES=$(git diff --cached --name-only)
RECENT=$(git log -n 10 --pretty=format:'%s' 2>/dev/null || true)

# Same reproducibility flags as the pre-commit hook.
SETTINGS_JSON='{"disableAllHooks": true}'

read -r -d '' PROMPT <<'EOF' || true
Write a Conventional Commits message for the staged diff below.

Format:
  <type>(<optional scope>)<!>: <subject>

  <optional body explaining WHY, wrapped at 72 chars>

  <optional "BREAKING CHANGE: <description>" footer>

Rules:
  - type is one of: feat, fix, docs, style, refactor, perf, test, build, ci, chore, revert, clean
  - subject is imperative mood, lowercase, no trailing period, <= 72 chars
  - Use a scope only if there's an obvious one from the changed paths
  - Add a body ONLY if the "why" isn't obvious from the subject; otherwise omit
  - If the diff is purely mechanical (formatting, renames, version bumps), keep it to one line
  - Match the style of the recent commits shown below where reasonable
  - Mark a breaking change by appending `!` after the type/scope AND adding
    a `BREAKING CHANGE: <description>` footer (both are required together).
    Flag a breaking change when the diff:
      * alters or removes inputs accepted from parameter files (e.g. renames,
        removes, or changes the type/meaning of a parameter; changes default
        values in a non-backward-compatible way)
      * alters or removes command-line options/arguments
      * changes the structure, schema, or format of output files in a way
        that existing consumers would need to be updated for
    Additive, backward-compatible changes (new optional parameters, new
    optional output fields) are NOT breaking.

Output ONLY the commit message. No code fences, no preamble, no explanation.
EOF

INPUT=$(cat <<EOF
$PROMPT

---RECENT COMMITS---
$RECENT

---FILES CHANGED---
$FILES

---DIFF---
$DIFF
EOF
)

MSG=$(printf '%s' "$INPUT" \
  | claude -p \
      --settings "$SETTINGS_JSON" \
      --strict-mcp-config \
      --mcp-config '{"mcpServers": {}}' \
      --allowedTools "" \
      --max-turns 2 \
      --output-format json 2>/tmp/claude-msg.err) || {
  exit 0
}

GENERATED=$(echo "$MSG" | jq -r '.result // empty')

if [[ -z "$GENERATED" ]]; then
  exit 0
fi

EXISTING=$(cat "$COMMIT_MSG_FILE")
{
  printf '%s\n\n' "$GENERATED"
  printf '# --- claude suggested above; edit freely or delete to write your own ---\n'
  printf '%s' "$EXISTING"
} > "$COMMIT_MSG_FILE"