I Built a Git Commit Message Generator with AI (Here's What I Learned)

I used to be that developer who commits with messages like "fixed bug" or "updated stuff" – and I hated myself for it. Every pull request required a frantic rewrite of commit history. So I decided to automate the process with AI. My goal: generate meaningful, conventional commit messages directly from my diff, without (too much) embarrassment.

Spoiler: it wasn't as simple as slapping a prompt in front of GPT. Here's the rollercoaster I went through, the dead ends, and the surprisingly elegant solution I eventually landed on.

The Real Problem (My Problem)

I was working on a medium-sized project with a dozen contributors. We enforced Conventional Commits (feat:, fix:, etc.), but I kept getting lazy. My brain simply didn't want to switch from code mode to prose mode after every diff. I needed a tool that could:

• Read the staged diff (additions/removals)
• Infer the type (feat, fix, refactor, etc.)
• Write a short, meaningful description
• Work offline? (Ideal but eventually scrapped)

I didn't want a full CI pipeline – I wanted a local script I could run before git commit.

What I Tried First (And Why It Sucked)

Attempt 1: API call with raw diff

git diff --cached | curl -X POST https://api.openai.com/v1/chat/completions -H "Authorization: Bearer $OPENAI_KEY" -d '{"model":"gpt-4", "messages": [{"role":"user", "content": "Generate a conventional commit message for this diff:"}]}'

Simple, right? The output was… verbose. It treated the diff like a novel and wrote a paragraph. Worse, it ignored the conventional prefix and just wrote random sentences. I tried adding more instructions, but then it would hallucinate features that weren't there. Total garbage.

Attempt 2: Prompt engineering with examples

I copied prompts from a known blog post – few-shot examples with proper Conventional Commits. Worked about 60% of the time. But for small changes (typo fix) it still generated refactor: instead of fix:. And the token cost was high because I included 10 examples every time.

Attempt 3: Local models (Llama 3)

I ran Ollama with Llama 3. It was slow (20 seconds per message) and often wrote commit messages in the style of a 19th century novel: "In this commit, improvements were made to the authentication module…" Absolutely unusable for a team.

What Eventually Worked: A Three-Step Pipeline

After two weeks of frustrating iterations, I settled on a hybrid technique that combines three parts:

1. Structured prompt with type classification first – ask the model to classify the change type before writing the message.
2. Context truncation – only send the first 200 lines of the diff (or even just file names and a summary).
3. Validation + retry – check the output against a regex, and if it fails, retry with a simplified prompt.

Here's the core of my implementation in Node.js (using the OpenAI SDK – but you can swap in any API that supports chat completions):

import { execSync } from 'child_process';
import OpenAI from 'openai';

const openai = new OpenAI({ apiKey: process.env.OPENAI_API_KEY });

function getDiff() {
  const output = execSync('git diff --cached --no-color', { encoding: 'utf-8' });
  // Truncate to first 250 lines to save tokens and keep focus
  return output.split('\n').slice(0, 250).join('\n');
}

function validateMessage(msg) {
  // Conventional Commit regex
  return /^(feat|fix|chore|docs|refactor|test|style|perf)\(?.*?\)?: .{1,72}$/.test(msg);
}

async function generateCommitMessage(diff) {
  const systemPrompt = `You are a git commit message generator.
First, classify the diff into one of these types: feat, fix, chore, docs, refactor, test, style, perf.
Then write a one-line commit message in the format: type(scope): description
Scope is optional. Keep the description under 50 characters.
Do not add extra commentary.`;

  for (let attempt = 0; attempt < 3; attempt++) {
    const response = await openai.chat.completions.create({
      model: 'gpt-4o-mini', // cheaper and faster
      messages: [
        { role: 'system', content: systemPrompt },
        { role: 'user', content: `Diff:\n\`\`\`diff\n${diff}\n\`\`\`` }
      ],
      max_tokens: 100,
      temperature: 0.2,
    });

    const message = response.choices[0].message.content.trim();
    if (validateMessage(message)) return message;
    console.warn(`Attempt ${attempt + 1} failed validation: ${message}`);
  }
  // Fallback: just concatenate type and first line of diff summary
  return `fix: ${diff.split('\n')[1]?.trim()?.slice(0, 50) || 'minor change'}`;
}

async function main() {
  const diff = getDiff();
  if (!diff) { console.log('No staged changes'); process.exit(0); }
  const message = await generateCommitMessage(diff);
  console.log('Suggested commit message:');
  console.log(message);
}

main().catch(console.error);

I run this as a Git alias:

git config --global alias.aimsg '!node ~/scripts/ai-commit.mjs'

Then git aismsg prints a suggestion. I copy/paste it into my actual commit. (Automated commit hooks are dangerous – I trust my brain more than an LLM for the final decision.)

What I Learned (The Hard Way)

• Token limits are your enemy. Diffs can be enormous. Truncation loses context but the model often still guesses the right type. If not, the fallback kicks in.
• Validation is non-negotiable. Without a regex check, you'll get poetic nonsense. Even with it, sometimes the model cheats by putting fix: in the wrong place.
• Temperature matters. 0.2 gives consistent, boring messages – exactly what I want. Higher temps produce creative but useless output.
• Local models aren't ready (for this). They're great for many tasks, but commit message generation requires adherence to strict formatting. GPT-4o-mini is cheap enough ($0.15 per million input tokens) that a single call costs less than a penny.

Trade-offs I Made

I sacrificed offline capability for reliability. If you're in a remote cabin without internet, my script won't help. You could swap in a local model, but expect lower accuracy.

I also deliberately didn't use the --amend or automatic commit. Why? Because AI makes mistakes. If I auto-commit a message like "fix: removed console.log" when I actually changed business logic, that's a lie in history. Manual review is cheap insurance.

When NOT to Use This

• Rebasing a long series of commits – generating messages for 10 commits one by one is tedious. Better to write them manually during interactive rebase.
• Highly sensitive codebases – sending diff to an external API may violate compliance. Use a local model or a self-hosted service.
• Trivial changes – if you're fixing a typo, just write fix: typo yourself. The overhead of running the script isn't worth it.

What I'd Do Differently Next Time

I'd explore a two-model approach: use a small, fast model (like GPT-4o-mini) for type classification and a cheaper summarization model (t5-small) for the description. That would cut costs further and allow more offline flexibility.

Also, I'd build a simple TUI (terminal UI) that shows the diff and the suggestion side-by-side, letting me edit before committing. But that's a weekend project I keep pushing off.

This approach works for me, but everyone's workflow is different. Have you tried automating commit messages? Did you end up with a similar pipeline, or do you swear by handwritten messages? I'd love to hear how you handle this (or why you think it's a bad idea in the first place).