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Start with the behavior, not the generated test
Before asking an assistant for tests, describe what must remain true after the code change. Use the acceptance criteria, relevant existing tests, and project conventions to give it context. A green test is useful only if it would detect a regression that matters to users or to another part of the system.
For example, rather than asking for tests for a new validation function, state the inputs it must accept, the invalid cases it must reject, and what the caller should observe. That gives you a basis for checking whether the proposed assertions protect the intended contract.
Use AI to draft tests, then inspect them
GitHub says Copilot can help generate unit and integration tests, and notes that complex scenarios may need more detailed prompts and strategies. Its guidance is a starting point for test drafting, not a reason to accept generated tests without review: GitHub’s guide to writing tests with Copilot.
Ask the assistant to propose cases before editing files, including boundary conditions and failure paths as well as the normal flow. For each suggested test, ask yourself:
- Which requirement or user-visible behavior does this test protect?
- Would its assertion fail if that behavior regressed?
- Does it check an outcome, or merely mirror the implementation’s current structure?
- Does it cover a meaningful edge case, such as an empty value, invalid input, or a failed dependency?
Remove tests that only confirm that a particular helper was called or that a component currently has a specific internal arrangement, unless that arrangement itself is part of the contract. Such checks can pass while the user-facing behavior is wrong, or fail after a harmless refactor.
Choose the test scope that matches the change
Different test types answer different questions. GitHub’s task guidance recommends unit tests for new functionality; its Copilot testing guide also covers unit and integration tests. For changes that affect a critical user journey, browser end-to-end coverage can check what a user actually experiences.
| Test scope | Best suited to | What it can tell you |
|---|---|---|
| Unit | Local logic with a clear input and output | Whether a function or small unit behaves correctly for the cases you specify |
| Integration | Behavior across component or service boundaries | Whether connected parts work together as expected |
| Browser end-to-end | Important user-visible flows | Whether a user can complete a real journey through the application |
Do not use a browser test for every small logic rule: it is broader than necessary for that job. Conversely, a unit test alone may not expose a problem at a component boundary or in the user flow. Add the scope that covers the risk introduced by the change.
Run a small relevant test selection first
Start with the smallest test selection that covers the changed behavior, then widen the run if the change or its risk calls for it. Visual Studio Code’s guide to testing code with AI recommends starting with a small selection for faster feedback.
- Identify the tests closest to the changed logic or interface.
- Run that focused selection and inspect failures rather than treating the pass/fail result as self-explanatory.
- Add integration tests if the change crosses a component or service boundary.
- Add or run end-to-end coverage when a critical user-visible flow is affected.
When a test fails, work out whether the implementation broke the expected behavior, the test still reflects an old expectation, or the test environment is unstable. Update an expectation only when the intended behavior has changed and that change is confirmed—not simply to make the run pass.
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Make browser tests resilient to refactors
Browser tests become fragile when they depend on incidental DOM structure or styling selectors that change during an unrelated refactor. Playwright recommends using locators resilient to DOM changes. Where possible, locate an element by its role, accessible name, visible text, or a test ID rather than by a CSS selector that reflects layout or class names: Playwright’s locator best practices.
Playwright’s test generator can record a flow and propose locators, prioritizing role, text, and test ID locators. Treat the generated script as a draft: check that each locator identifies the intended control and that the assertions express a product requirement, not just the page’s current appearance. See Playwright’s test generator documentation.
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Best Value
Use snapshots only when exact output is the contract
A snapshot compares a saved representation with the current output. That is appropriate when the serialized output itself must remain exact and reviewers can meaningfully inspect changes to it. It is less helpful when a broad rendering snapshot changes because of incidental markup or formatting while the user-facing behavior remains correct.
| Approach | Contract checked | Refactor sensitivity | Useful when |
|---|---|---|---|
| Behavior-oriented assertion | A stated outcome, such as a validation result or an available user action | Usually lower when the assertion targets a stable interface or outcome | The requirement matters more than the exact internal representation |
| Snapshot assertion | Equality with a saved serialized output or rendering | Can be high if the snapshot includes details that change without changing behavior | Exact serialized output is itself the contract and diffs receive meaningful review |
The problem is not that snapshots store expected values; it is using unclear or overly broad expectations that no one meaningfully reviews. Keep a snapshot focused, and make sure a changed snapshot is assessed against the actual contract rather than accepted automatically.
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