# What is Regression?

> A bug where something that used to work stops working after a change.

Canonical: https://www.decimal.app/glossary/regression · Last reviewed: 2026-09-23

A regression is a defect introduced by a code, config, or dependency change that breaks previously working behavior.

## Why it matters

Regressions show up in support before they show up anywhere else. Connecting a ticket spike to the deploy that caused it is the fastest way to a fix.

## Best practices

1. Correlate with deploys. Check what changed right before tickets started.
2. Add a test for each one. So it can't come back quietly.
3. Flag them in triage. Regressions usually deserve higher priority.

## How Decimal helps

Decimal reads recent commits and deploys during investigation and can point to the change that likely introduced a regression.

## Related terms

- [Root Cause Analysis (RCA)](https://www.decimal.app/glossary/root-cause-analysis): Finding the underlying reason a problem happened, not just its symptoms.
- [Hotfix](https://www.decimal.app/glossary/hotfix): An urgent fix shipped outside the normal release cycle.
- [Feature Flag](https://www.decimal.app/glossary/feature-flag): A switch that turns code paths on or off per customer without a deploy.
- [Bug Report](https://www.decimal.app/glossary/bug-report): A write-up of unexpected behavior with enough detail for an engineer to reproduce it.

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