What Is a Regression Test?
A regression test checks that existing behavior continues to work correctly after code changes, ensuring a new feature or fix did not break something that previously functioned. Regression suites accumulate over time as bugs are fixed and tests are added to prevent their return. They are the safety net that makes frequent change possible.
Why it matters
Every change risks unintended side effects elsewhere. A strong regression suite catches those side effects automatically, which is what gives teams the confidence to deploy often. Because these suites grow large, parallelizing and sharding them keeps CI fast.
Related concepts
- Often the largest, slowest part of a test suite
- Sharding and parallelism keep it fast
- A fixed bug usually adds a new regression test
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