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Stop Writing Commit Messages — Use AI Instead

#git #ai #productivity #tutorial

Your git log looks like this:

fix bug
update stuff
wip
asdf
more changes
final fix
final fix 2
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Every developer has been there. Writing good commit messages takes effort you'd rather spend coding. But bad commit messages make debugging, reverting, and onboarding painful.

The fix: let AI write your commit messages from your actual diff.

The prepare-commit-msg Hook

Git has a hook called prepare-commit-msg that runs before the commit message editor opens. We'll use it to auto-generate messages.

Create .git/hooks/prepare-commit-msg:

#!/bin/bash

# Only run for regular commits (not merges, amends, etc.)
if [ "$2" != "" ]; then
  exit 0
fi

COMMIT_MSG_FILE=$1

# Get staged diff
DIFF=$(git diff --cached --stat)
DIFF_CONTENT=$(git diff --cached | head -c 5000)

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

# Generate commit message
MESSAGE=$(curl -s https://api.anthropic.com/v1/messages \
  -H "content-type: application/json" \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -d "{
    \"model\": \"claude-sonnet-4-20250514\",
    \"max_tokens\": 100,
    \"messages\": [{
      \"role\": \"user\",
      \"content\": \"Write a conventional commit message for this diff. Format: type(scope): description. One line, max 72 chars. Types: feat, fix, refactor, docs, test, chore. No quotes around the message.\\n\\nFiles:\\n$DIFF\\n\\nDiff:\\n$DIFF_CONTENT\"
    }]
  }" | jq -r '.content[0].text')

# Write to commit message file
echo "$MESSAGE" > "$COMMIT_MSG_FILE"
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Make it executable:

chmod +x .git/hooks/prepare-commit-msg
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What You Get

Now when you run git commit, the hook reads your staged changes and generates a message:

git add src/auth/login.ts src/auth/middleware.ts
git commit
# Auto-generated message: feat(auth): add JWT login with token refresh
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git add src/utils/format.ts
git commit
# Auto-generated message: fix(utils): handle null values in date formatter
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git add tests/auth.test.ts
git commit
# Auto-generated message: test(auth): add integration tests for login flow
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The message appears in your editor. Keep it, edit it, or rewrite it. You're always in control.

Team-Wide Setup with a Shared Hook

Git hooks live in .git/hooks/ which isn't committed. To share with your team, use a hooks directory:

mkdir .githooks
cp .git/hooks/prepare-commit-msg .githooks/
git config core.hooksPath .githooks
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Commit .githooks/ to your repo. Add setup instructions to your README:

git config core.hooksPath .githooks
export ANTHROPIC_API_KEY=your-key-here
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A Faster Alternative: Local Models

Don't want to hit an API on every commit? Use a local model:

# Using ollama
MESSAGE=$(echo "$DIFF_CONTENT" | ollama run codellama \
  "Write a conventional commit message for this diff. One line, max 72 chars.")
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Local models are faster (no network latency) and free. The quality is slightly lower but still better than "fixed stuff."

Parsing Commit Messages for Changelogs

Once your commit messages follow a consistent format, you can automatically generate changelogs. Extract structured data from your git log using StructureAI:

COMMITS=$(git log --oneline -20)

curl -X POST https://api-service-wine.vercel.app/api/extract \
  -H "Content-Type: application/json" \
  -H "X-API-Key: YOUR_KEY" \
  -d "{
    \"text\": \"$COMMITS\",
    \"schema\": \"custom\",
    \"custom_fields\": [\"features\", \"bug_fixes\", \"breaking_changes\", \"version_bump\"]
  }"
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Returns:

{
  "features": ["JWT login with token refresh", "user profile page"],
  "bug_fixes": ["null values in date formatter", "race condition in auth middleware"],
  "breaking_changes": [],
  "version_bump": "minor"
}
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Feed that into your CHANGELOG.md generator and you have fully automated releases.

Conventional Commits Cheat Sheet

The AI uses these types by default:

Type When to use
feat New feature
fix Bug fix
refactor Code change that doesn't fix a bug or add a feature
docs Documentation only
test Adding or fixing tests
chore Build, CI, dependencies
perf Performance improvement

Cost

Claude Sonnet costs about $0.003 per 1K input tokens. A typical diff is 200-500 tokens. That's roughly $0.001 per commit. At 20 commits per day, that's $0.02/day or $0.60/month.

You spend more on coffee.

Setup in 2 Minutes

  1. Create the hook file (copy the script above)
  2. chmod +x .git/hooks/prepare-commit-msg
  3. Set ANTHROPIC_API_KEY in your shell profile
  4. Run git commit and watch the magic

Your git log will finally tell a coherent story. Future you will be grateful.

Built by Avatrix LLC. Turn messy text into clean JSON with StructureAI.

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