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brian austin
brian austin

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How I use Claude to write better commit messages (with a git hook)

#git #ai #productivity #devtools

How I use Claude to write better commit messages (with a git hook)

Every developer knows the pain: you've just fixed a gnarly bug at 11pm, and git is waiting for a commit message. You type fix stuff and move on.

I fixed this with a git hook that calls the Claude API to generate a meaningful commit message from my diff — automatically.

The problem with manual commit messages

Bad commit messages are a tax you pay in the future:

  • fix bug — which bug? Fixed how?
  • WIP — why is this in main?
  • asdfasdf — we've all been there

Good commit messages are like documentation that writes itself — if you have the discipline to write them at 11pm. Most of us don't.

The solution: a prepare-commit-msg git hook

Git has a hook called prepare-commit-msg that runs before your editor opens. It can pre-populate the commit message. This is perfect for AI generation.

Here's the hook I use:

#!/bin/bash
# .git/hooks/prepare-commit-msg

COMMIT_MSG_FILE=$1
COMMIT_SOURCE=$2

# Only auto-generate for normal commits (not merges, amends, etc)
if [ -n "$COMMIT_SOURCE" ]; then
  exit 0
fi

# Get the diff of staged changes
DIFF=$(git diff --cached --stat | head -20)
DIFF_CONTENT=$(git diff --cached | head -100)

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

# Call the API
RESPONSE=$(curl -s -X POST https://simplylouie.com/api/chat \
  -H "Content-Type: application/json" \
  -H "X-API-Key: $LOUIE_API_KEY" \
  -d "$(jq -n \
    --arg diff "$DIFF" \
    --arg content "$DIFF_CONTENT" \
    '{message: ("Write a concise git commit message (under 72 chars) for these changes. Use conventional commits format (feat/fix/refactor/docs/chore). Just the message, no explanation.\n\nStat:\n" + $diff + "\n\nDiff (first 100 lines):\n" + $content)}')") 

MESSAGE=$(echo $RESPONSE | jq -r '.response // empty')

if [ -n "$MESSAGE" ]; then
  echo "$MESSAGE" > "$COMMIT_MSG_FILE"
  echo "# AI-generated. Edit if needed." >> "$COMMIT_MSG_FILE"
fi
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Make it executable:

chmod +x .git/hooks/prepare-commit-msg
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Set your API key:

export LOUIE_API_KEY=your_key_here
# Add to ~/.bashrc or ~/.zshrc to make permanent
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Real examples from my workflow

Before (what I'd have typed at 11pm):

fix auth thing
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After (AI-generated from the same diff):

fix: resolve JWT expiry race condition in token refresh
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Another one:

before: update stuff
after:  refactor: extract UserRepository from UserService, add unit tests
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The hook sees exactly what changed. It generates messages that are actually useful 6 months later.

Making it a global git hook

If you want this in every repo, not just one:

mkdir -p ~/.git-hooks
cp .git/hooks/prepare-commit-msg ~/.git-hooks/
git config --global core.hooksPath ~/.git-hooks
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Now every repo on your machine gets AI commit messages automatically.

The cost

I use SimplyLouie's API — it's $2/month for unlimited access, which means this hook costs effectively $0 per commit. I run it probably 20-30 times a day.

If you're calling Anthropic directly, each commit message is maybe 500 input tokens + 50 output tokens. At $3/million input tokens, that's $0.0015 per commit. Still cheap, but it adds up if you're committing constantly.

Extending it

This same pattern works for:

PR descriptions — run the diff through the same API when you open a PR:

gh pr create --body "$(git diff main...HEAD | head -200 | curl -s -X POST https://simplylouie.com/api/chat -H 'Content-Type: application/json' -H "X-API-Key: $LOUIE_API_KEY" -d '{"message": "Write a PR description for this diff"}' | jq -r '.response')"
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Changelog generation — at release time:

git log v1.0..HEAD --oneline | curl -s -X POST https://simplylouie.com/api/chat \
  -H "Content-Type: application/json" \
  -H "X-API-Key: $LOUIE_API_KEY" \
  -d "{\"message\": \"Convert these commits to a user-friendly changelog:\\n$(git log v1.0..HEAD --oneline)\"}"
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Get the API key

You need a SimplyLouie API key to run this. Sign up at simplylouie.com — it's $2/month, the API is included.

Or adapt the hook to use any OpenAI-compatible API endpoint — the JSON format is standard.

What's your current commit message workflow? Drop it in the comments — I'm curious how many people are still typing fix stuff at 11pm.

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