Regression testing
Every code change carries some risk of breaking something it wasn't meant to touch. A shared utility function gets tweaked for one feature and quietly changes behavior for three others. A dependency upgrade shifts how a library handles an edge case nobody was testing for. Regression testing exists to catch exactly that: it re-runs the tests that already passed against the old behavior, so a team finds out before a user does whether the new change broke something old.
What makes regression testing distinct from testing in general is the word "again." A new feature typically gets new test cases written specifically for it. Regression testing instead reruns a body of existing tests, unit, integration, or end-to-end, that were written for earlier work, purely to confirm nothing already working has quietly stopped working. That's straightforward for a team shipping monthly with a small test suite. It gets harder as release frequency climbs and the suite grows, because running every test on every change eventually takes longer than the release cycle can afford.
That tension is why regression testing has more than one shape. A full or "retest-all" pass reruns the entire suite and gives the most confidence, but it's also the slowest and most expensive to run constantly. Selective regression testing narrows the run to tests that cover the areas a specific change actually touches, trading some coverage for speed. Teams often land somewhere in between: a fast selective suite runs on every commit, and a full pass runs on a schedule or right before a release, so speed and thoroughness both get their turn.
Common approaches to regression testing
- Retest-all, rerunning the complete test suite for the most thorough but slowest and most resource-heavy check.
- Selective regression testing, running only the subset of tests that cover the code paths a change actually affects.
- Progressive regression testing, adding new test cases alongside existing ones when a change introduces new requirements rather than only fixing a defect.
- Corrective regression testing, used after refactoring or cleanup where no behavior is meant to change, to confirm none did.
- Automated regression testing, since manually rerunning hundreds of cases on every change isn't sustainable once a codebase reaches any real size.
When teams reach for it
- After a bug fix, to confirm the fix didn't introduce a new problem elsewhere in the same area.
- After adding a feature that touches shared code, such as a common component, API, or database schema.
- Before a release, as a final gate that existing functionality still behaves as expected.
- After upgrading a dependency, framework version, or third-party integration the application relies on.
Benefits and challenges
| Benefit | Challenge |
|---|---|
| Catches unintended side effects of a change before they reach users, rather than after a support ticket does. | A full suite grows heavier every release, and running all of it on every change eventually doesn't fit the pace a team wants to ship at. |
| Gives a team confidence to refactor or upgrade dependencies without re-verifying everything by hand. | Deciding what counts as "affected" for selective regression testing takes real analysis, and getting it wrong means a real regression slips through untested. |
| Automation makes rerunning a large suite realistic on every commit instead of only before a release. | An automated suite built on brittle scripts accumulates its own maintenance debt, and a broken test stops distinguishing a real regression from a stale assertion. |
Frequently asked questions
What's the difference between regression testing and retesting?
Retesting confirms that a specific defect was actually fixed, running the same steps that originally exposed the bug. Regression testing is broader: it checks that the fix, or any other change, didn't break something unrelated elsewhere in the application. Teams typically do both around the same change, for different reasons.
How often should regression tests run?
It depends on the suite's size and how fast a team ships. A fast, selective subset commonly runs on every commit or pull request, while a full regression pass often runs on a schedule or as a gate right before release, so teams get quick feedback without every change waiting on the slowest possible check.
Can regression testing be fully automated?
Much of it can, and most teams automate the bulk of a regression suite specifically because manual reruns don't scale with release frequency. Exploratory and highly visual checks still often involve a human, but the repetitive, well-defined checks that make up most regression suites are exactly what automation was built for.
How Klarent helps
Klarent's AI agents run regression testing at release velocity, using self-healing execution so a growing suite keeps pace with every commit instead of falling further behind as it accumulates.