DEV Community

Enjoy Kumawat
Enjoy Kumawat

Posted on

I Fixed the "AI Commit Messages" Problem in 20 Lines of Python

#git #python #ai #devtools

You've probably seen that trending post — "I Asked AI to Write My Commit Messages and It Was Embarrassing."

Same. But instead of accepting embarrassing output, I fixed it.

Here's the thing: the problem isn't AI writing commit messages. The problem is how you ask it. One clear system prompt + the actual diff = surprisingly good results.

The Setup

No new packages. No API key. If you have Claude Code, you're already set.

#!/usr/bin/env python3
import subprocess

SYSTEM = (
    "You are a git commit message generator. "
    "Output ONLY the commit message — no explanation, no markdown, no quotes. "
    "Follow Conventional Commits: type(scope): subject. "
    "Types: feat, fix, docs, style, refactor, test, chore. "
    "Subject: imperative, lowercase, max 72 chars."
)

diff = subprocess.check_output(["git", "diff", "--staged"], text=True)
if not diff.strip():
    print("Nothing staged. Run `git add` first.")
    raise SystemExit(1)

msg = subprocess.check_output(
    ["claude", "-p", SYSTEM + "\n\n" + diff],
    text=True,
).strip()
print(msg)
Enter fullscreen mode Exit fullscreen mode

That's it. 20 lines. Uses the claude CLI under the hood — no API key, no config, just your existing Claude Code OAuth session.

Why It Works

The system prompt does the heavy lifting. Three constraints:

  1. Output ONLY the commit message — no preamble, no explanation
  2. Follow Conventional Commits — feat, fix, chore, etc.
  3. max 72 chars — keeps it readable in git log

The diff is the context. You're not asking "write a commit message". You're asking "given these exact changes, what happened?" That's a much more answerable question.

Usage 

# No setup needed if you have Claude Code. Just:
git add .
python /path/to/git_commit.py
# → feat(server): add AI commit message generator via Claude CLI
Enter fullscreen mode Exit fullscreen mode

Or wire it into a git alias:

git config --global alias.ai '!python /path/to/git_commit.py'
# git ai
Enter fullscreen mode Exit fullscreen mode

The Results

Before:

update stuff
fix bug
WIP
added the thing
Enter fullscreen mode Exit fullscreen mode

After:

feat(api): add generate_commit_message tool to MCP server
fix(auth): handle expired token on refresh
refactor(db): extract query builder into separate module
Enter fullscreen mode Exit fullscreen mode

As an MCP Tool Too

I also wrapped it as an MCP tool so Claude Code can call it directly from any conversation:

@mcp.tool()
def generate_commit_message(diff: str) -> str:
    """Generate a Conventional Commits message from a git diff string."""
    full = SYSTEM + "\n\n" + diff
    return subprocess.check_output(["claude", "-p", full], text=True).strip()
Enter fullscreen mode Exit fullscreen mode

Full project: github.com/enjoy-kumawat/my-git-manager

20 lines. No new dependencies. No API key. Conventional Commits every time.

The embarrassing part was waiting this long to build it.

Top comments (1)

Collapse
 
alexshev profile image
Alex Shev •

Commit messages improve a lot when the input is the actual diff plus a clear style constraint. The next step I like is separating "what changed" from "why it changed"; the diff can explain the first, but the human often owns the second.