prepare-commit-msg

#!/usr/bin/env bash
# prepare-commit-msg hook: generate a commit message using OpenCode AI
#
# Powered by: opencode run (https://opencode.ai/docs/cli)
#
# Triggered for every commit EXCEPT:
#   - merge commits  (source == "merge")
#   - squash commits (source == "squash")
#   - explicit -m/--message commits (source == "message")
#
# Usage:
#   git commit            # AI message is generated and opened in editor
#   git commit --no-edit  # AI message is used as-is
#
# Requirements:
#   - opencode must be in PATH and authenticated
#   - git must be available

COMMIT_MSG_FILE="$1"
COMMIT_SOURCE="$2"   # "message", "template", "merge", "squash", or empty

# Skip generation for merge, squash, or when the user explicitly supplied -m
case "$COMMIT_SOURCE" in
  merge|squash|message)
    exit 0
    ;;
esac

# Require opencode
if ! command -v opencode &>/dev/null; then
  echo "prepare-commit-msg: 'opencode' not found in PATH, skipping AI message generation." >&2
  exit 0
fi

# Collect context -------------------------------------------------------

# Staged diff (what is actually being committed)
STAGED_DIFF=$(git diff --cached --stat 2>/dev/null)
STAGED_DIFF_FULL=$(git diff --cached 2>/dev/null | head -c 8000)   # cap to avoid huge prompts

# Last commit message for style reference
LAST_COMMIT=$(git log -1 --pretty=format:"%s%n%n%b" 2>/dev/null)

# Build the prompt -------------------------------------------------------
PROMPT="You are a git commit message generator.

Your job: write a concise, conventional git commit message for the staged changes shown below.

Rules:
- Use the Conventional Commits format: <type>(<optional scope>): <short description>
  Types: feat, fix, docs, style, refactor, test, chore, ci, build, perf
- Subject line: imperative mood, ≤72 chars, no period at end
- If useful, add a blank line then a short body (why / what changed) – keep it factual
- Do NOT add any explanation, preamble, or markdown fencing – output ONLY the commit message

Last commit (use as style reference, do NOT repeat it):
${LAST_COMMIT}

Staged file changes (stat):
${STAGED_DIFF}

Staged diff (truncated to 8 KB):
${STAGED_DIFF_FULL}

Output only the commit message now:"

# Run OpenCode -----------------------------------------------------------
AI_MESSAGE=$(opencode run "$PROMPT" 2>/dev/null)
EXIT_CODE=$?

if [ $EXIT_CODE -ne 0 ] || [ -z "$AI_MESSAGE" ]; then
  echo "prepare-commit-msg: OpenCode did not return a message (exit $EXIT_CODE), skipping." >&2
  exit 0
fi

# Strip potential markdown code fences that some models add
AI_MESSAGE=$(printf '%s' "$AI_MESSAGE" | sed '/^```/d')

# Write the AI-generated message, followed by the original file content
# (which typically contains commented-out git status / diff hints)
ORIGINAL_CONTENT=$(cat "$COMMIT_MSG_FILE")
{
  printf '%s\n' "$AI_MESSAGE"
  if [ -n "$ORIGINAL_CONTENT" ]; then
    printf '\n'
    printf '%s\n' "$ORIGINAL_CONTENT"
  fi
} > "$COMMIT_MSG_FILE"