PR description from commit messages — tradeoffs

As engineers, we're always looking for ways to automate repetitive tasks and improve efficiency. One area that often feels like a chore is writing pull request (PR) descriptions. They're crucial for context, code review, and future reference, but crafting a good one takes time and effort. It's natural, then, to look at the rich history embedded in our version control system and wonder: can we just generate PR descriptions from commit messages?

It's a tempting idea, offering the promise of consistency and reduced manual effort. But like many shortcuts, it comes with a set of tradeoffs that are important to understand.

The Promise: Efficiency and Consistency

The allure of generating PR descriptions from commit messages is clear:

  • Automation: Eliminate the manual typing. If the information is already in the commits, why re-type it?
  • Consistency: Encourage a uniform style for PR descriptions if commit messages adhere to a standard.
  • Context Preservation: Ensure that every detail from every commit is carried forward into the PR's narrative.
  • Reduced Friction: Lower the barrier to creating PRs, especially for smaller changes.

The basic premise is that by aggregating the subject lines and bodies of the commits within a PR, you can construct a comprehensive description. Tools like git log can easily provide this data, making it straightforward to script a basic solution.

How it Works (Conceptually)

At its simplest, generating a PR description from commit messages involves:

  1. Identifying all commits within your branch that are not yet merged into the target branch (e.g., main).
  2. Extracting the subject line and body of each of these commits.
  3. Concatenating them, perhaps with some formatting, into a single block of text.

For instance, a simple shell script might run git log --pretty=format:"%s%n%b%n---%n" origin/main..HEAD to get a raw dump of commit messages since the last merge point. This output could then be used as the basis for a PR description.

The Upside: When it Shines

Generating PR descriptions directly from commit messages works remarkably well in specific scenarios:

  • Small, focused PRs with a single logical change: If your PR consists of just one or two commits that are themselves well-written and encapsulate the entire change, this approach is effective. For example, a PR to fix a typo might have a single commit: fix(docs): Correct typo in README.md. This commit message is the PR description.

  • Strict Commit Message Guidelines: Teams that rigorously enforce commit message conventions, such as Conventional Commits, can benefit. These conventions provide structure (e.g., type(scope): subject, body) that can be parsed and presented cleanly.

    Consider a monorepo where every change is small and atomic, and developers are disciplined:

    feat(auth): Add JWT token validation for incoming requests fix(ui): Correct button alignment on mobile for login screen perf(db): Optimize user lookup query with new index

    In this ideal world, concatenating these messages provides a decent overview. Each commit is a self-contained unit, and their combination tells a clear story of the changes introduced in the PR. The subject lines form a good summary, and the bodies provide necessary detail.

  • Rapid Iteration, Fresh Context: When developers are working quickly on a feature, and the context for each commit is fresh in their minds, a simple aggregation might suffice for internal team reviews.

The Downside: Where it Falls Apart

While the promise is alluring, the reality of everyday development often introduces significant friction for this approach.

The Messy Reality of Multi-Commit PRs

Most PRs aren't a pristine sequence of perfectly crafted, atomic commits. The typical development workflow involves:

  1. Initial implementation: git commit -m "feat: initial user profile page"
  2. Addressing linting issues: git commit -m "fix: lint errors"
  3. Refactoring: git commit -m "refactor: move styles to separate file"
  4. Debugging: git commit -m "fix: profile image not loading"
  5. Adding more functionality: git commit -m "feat: add user bio field"
  6. "WIP" commits, "oops" commits, "revert" commits, "test" commits.

If you simply concatenate these commit messages, your PR description becomes a noisy, confusing chronological log of how the feature was built, rather than a clear explanation of what the feature is and why it exists.

Imagine a PR description that reads:

feat: initial user profile page
fix: lint errors
refactor: move styles to separate file
fix: profile image not loading
feat: add user bio field

This doesn't tell a reviewer: * What the overall goal of the PR is. * How to test the new profile page. * What potential risks or side effects there are. * Any design decisions or alternatives considered.

The narrative gets lost. Intermediate commits, which are crucial for the development process, are rarely suitable for a PR-level summary.

Lack of PR-Specific Context

Commit messages are inherently focused on the individual change they represent. A PR, however, represents a collection of changes that together achieve a larger goal. Critical information for a PR description is often missing from commit messages:

  • Overall Summary: A concise high-level overview of the entire PR's purpose. Individual commit messages are too granular.
  • Test Plan: How should the reviewer test this PR? What steps should they take? What specific scenarios should they validate? This is almost never found in commit messages.
  • Risk Analysis: What are the potential impacts of this change? Are there performance concerns, security implications, or breaking changes? These are higher-level concerns that span the entire PR.
  • Deployment Considerations: Are there specific steps needed for deployment? Database migrations? Feature flags?
  • Design Decisions & Alternatives: Often, significant changes involve choices between different approaches. These discussions happen in tickets or design docs, not typically within individual commit messages.

Enforcement Overhead

To make this approach viable, your team needs extremely strict commit message hygiene. This means:

  • Atomic Commits: Every commit must be a perfectly self-contained, logical change.
  • Meaningful Messages: Every message must be descriptive and relevant to the PR's final state.
  • Rebasing and Squashing: Developers must consistently rebase and squash their commits before opening a PR, often into a single, perfectly crafted commit message that then serves as the PR description. This adds a cognitive load and can be a source of friction, especially for less experienced team members.

Noise vs. Signal

The biggest problem is the signal-to-noise ratio. Commit messages often contain a lot of "noise" – the intermediate steps of development – and obscure the "signal" – the ultimate purpose and impact of the PR. A good PR description distills the essence; simple aggregation rarely achieves this.

Mitigating the Downsides (Manually)

Developers have traditionally tried to mitigate these downsides through manual processes:

  • Squash and Rebase: The most common approach is to squash all intermediate commits into a single, clean commit before opening a PR. This forces the developer to write one good commit message that effectively acts as the PR description. While effective, it's manual, can be difficult for junior developers, and requires discipline