The best AI prompt for a commit message asks the model to explain why a change was made, not just what changed — and it works best when you feed it your actual diff, not a vague description of the work. “Fixed bug” and “updated files” are useless six months later; a good commit message (and a good pull request description) is a record future-you, or a teammate, will actually rely on when something breaks.
Why Do Most Commit Messages Fail to Be Useful Later?
A commit message written quickly at the end of a task tends to describe the mechanical change (“update auth.py”) because that’s the easiest thing to state — but the diff itself already shows what changed. What a diff can’t show is why: what bug this fixed, what tradeoff was made, or what would break if this were reverted. That’s the information worth spending a prompt on, because it’s the part a future reader can’t reconstruct just by reading the code.
What’s a Reliable Prompt for Writing a Commit Message From a Diff?
Paste the actual diff and prompt: “Write a commit message for this diff. The first line should be a concise summary under 50 characters in imperative mood (e.g. ‘Fix’, not ‘Fixed’). Below that, add a short paragraph explaining why this change was necessary and any tradeoffs made — don’t just restate what the diff shows.” The instruction to explain “why, not what” is the single highest-leverage line in this prompt, since it’s the part a model will skip by default if you don’t ask for it directly.
Should You Use the Conventional Commits Format?
If your team already has a convention, tell the model to follow it exactly rather than defaulting to its own style. Many teams use the Conventional Commits specification, which structures a message as a type (like feat, fix, or refactor), an optional scope, and a short description — for example, fix(auth): handle expired refresh tokens. If you use this format, include the spec’s rules directly in your prompt or point the model to a short example, since the exact punctuation and casing matter for tools that parse commit history automatically (for generating changelogs, for instance).
How Is a Pull Request Description Different From a Commit Message?
A commit message documents one change for the permanent project history; a pull request description is written for a reviewer who’s about to spend time evaluating your work right now. A useful prompt for a PR description asks for more context than a commit message needs: “Write a pull request description for this set of changes. Include: a one-sentence summary, the problem this solves, how you tested it, and anything a reviewer should pay special attention to.” That last item — flagging the risky or uncertain part of the change — is what actually speeds up review, since it tells a reviewer where to focus instead of reading every line with equal attention.
Can AI Generate These From the Diff Alone, With No Extra Context?
It can generate something plausible-sounding, but a diff alone doesn’t contain the business reason for a change — only you know why a bug mattered or why a particular approach was chosen over another. Always add one or two sentences of your own context (the ticket number, the bug report, the reason for the approach) alongside the diff. This is the same principle behind writing good AI code review prompts: the model does better work when it has the intent behind a change, not just the code itself.
Frequently Asked Questions
Should I let AI write my commit messages automatically on every commit?
For small, obvious changes, an AI-drafted message you skim and confirm is fine. For anything non-trivial, review and edit the draft — a model can misread the intent of a diff, especially when a change touches multiple unrelated things at once, which is itself a sign the commit should probably be split up.
What’s the ideal length for a commit message body?
Keep the first line under about 50 characters and the body to a short paragraph or a few bullet points — long enough to explain the reasoning, short enough that someone scanning git log doesn’t skip it. If an explanation needs more than a few sentences, that’s often a sign the change itself should be broken into smaller commits.
Do AI-generated commit messages work well with automated changelogs?
Yes, as long as you enforce a consistent format like Conventional Commits in your prompt, since changelog generators typically parse the commit type and scope from the first line. An AI model asked to follow that exact format consistently can actually improve changelog quality compared to commit messages written ad hoc under time pressure.
For the full specification, see the official Conventional Commits documentation. For more on using AI throughout your development workflow, see our complete guide to prompts for programming.
