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Before a maintainer has to infer intent from your code, state the bugfix’s behavior delta: what happens now, what should happen instead, and why the change is warranted. Tie that explanation to a reproducible case, the patch, and checks you actually ran. This is a practical convention, not an acceptance formula; the repository’s current contribution guide and templates take precedence.
How do I describe expected vs. actual behavior in a bug report?
Describe what a user can observe, without treating your theory about the cause as established fact. Then define the expected result in terms that another person could verify.
- Problem: One sentence describing the symptom.
- Observed behavior: What the software does now, with a concrete input or steps.
- Expected behavior: What should happen instead, and why that result matters to the user.
For example: “When I save a configuration with an empty optional field, the application exits with an error. It should save the configuration and leave that field unset, because the field is documented as optional.” This separates the symptom and expected outcome from an unverified diagnosis such as “the parser is broken.”
How do I make a bug easy for maintainers to reproduce?
Give maintainers the smallest case that still shows the problem. A runnable minimal reproducer is useful when available; otherwise, provide exact steps and the relevant output. Record environment details that could affect the result rather than listing every detail about your machine.
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- Include the project and relevant dependency versions, plus the operating system or platform when relevant.
- State how the software was installed if that could matter, and whether the problem occurs on the current version or an older one you checked.
- Include an error message, stack trace, log, or trace when it helps explain the observed behavior.
- Remove credentials, personal data, and other sensitive information before sharing logs. Do not disclose a security vulnerability in a public tracker; follow the project’s security reporting policy.
Typelevel’s contribution guide recommends stating expected versus actual behavior and providing a runnable minimal reproducer; when that is not possible, it suggests steps, stack traces, or error messages. Typelevel contribution guide.
What should I include in a bugfix pull request?
Connect the user-visible bug to the change and give reviewers enough context to know what to examine. Apache Hop’s code review guidance says a behavior-changing pull request should explain the big picture so reviewers do not have to infer the change from the code. Apache Hop code review guide.
- Summarize the behavior gap. State what happens before the patch and what should happen after it.
- Explain the proposed change. Say which behavior changes for users and why the change is appropriate.
- Identify consequences worth reviewing. Note compatibility or edge-case implications you have considered. Do not claim there are none unless you checked.
- Show how you validated it. List the tests or other checks you actually ran and their results. If you did not run a check, do not imply that you did.
- Link relevant context. Reference the issue, design discussion, or maintainer approval when one exists.
Keep the patch focused and self-contained so reviewers can assess the behavior correction without disentangling unrelated changes. Typelevel’s guide puts it plainly: “Each pull request should contain a single self-contained change.” Typelevel contribution guide.
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A compact pull request description template
Use this as a starting point and replace the bracketed phrases with details that are true for your change:
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Before this change, calling
[operation]with[input]produces[observed result]. It should produce[expected result]because[user-visible reason]. This patch changes[behavior]. I reproduced the issue with[steps and version]and checked it with[tests actually run].
Should I open an issue before submitting an open-source bugfix?
Follow the target repository’s process; there is no universal rule that every bugfix needs an issue first. GitHub’s contributor guidance recommends checking a project’s own conventions, testing requirements, pull request process, development setup, and communication channels. GitHub: Contributing to open source.
| Situation | Practical next step |
|---|---|
| The guide or template requires an issue, proposal, or approval. | Use that process before submitting the patch. |
| The fix is tiny, its cause and intended behavior are clear, and the project permits direct pull requests. | A direct pull request may be appropriate. Modular’s guide explicitly allows a one- or two-line fix with an obvious cause and clear test to go directly to a pull request. |
| The change affects user-visible behavior, a public API, several components, or the intended outcome is uncertain. | Start a discussion or issue so maintainers can confirm direction before substantial implementation. |
| The bug is reproducible and expected behavior is clear, but the project’s preferred workflow is unclear. | Ask maintainers through the project’s stated communication channel before investing in a larger patch. |
Project advice differs: Typelevel asks contributors to begin with an issue or conversation, while Modular allows some small, obvious fixes to proceed directly. Check the current repository instructions rather than assuming either approach applies everywhere. Typelevel contribution guide; Modular repository guidance and templates.
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A reviewer can compare the stated outcome with the reproduction, proposed implementation, and validation. That makes the intended scope visible without asking the reviewer to guess what the patch is supposed to change. It does not guarantee acceptance, prove correctness, or establish that review will be faster; the explanation is context for review, not a substitute for sound code and project-specific checks.
