---
title: Prevention
description: Catch flaky and broken tests on pull requests before they reach your codebase.
---

Prevention monitors tests introduced or modified in pull requests. By
rerunning tests on PRs, it detects flaky behavior before code merges, keeping
your codebase reliable.

## How it works

When a pull request runs tests, Mergify reruns them to check for consistency.
Tests that produce different results on the same commit are flagged as flaky.
This happens transparently as part of your existing CI pipeline, with no changes
to your test code needed.

Tests caught as flaky on a PR are prevented from silently degrading your
test suite. You can review their health status before deciding to merge.

## What you can track

Prevention provides key metrics to help you understand test reliability
on pull requests:

### Caught flaky tests

The number of flaky tests detected during PR reruns. This is the core value
of Prevention: every caught test is a reliability problem that didn't make it
into your codebase.

### New tests

Tests being introduced on PRs, along with their health status. Each new test
is classified as healthy, flaky, or broken based on its rerun results. This
helps you spot unreliable tests before they're merged.

### CI budget spent

The total CI time spent on reruns. This metric helps teams understand the
cost of flaky test prevention and make informed trade-offs between
thoroughness and CI budget.

## Practical workflows

### Reviewing tests before merging

When a PR introduces or modifies tests, check the Prevention page to see
their health status. Tests with a flaky or broken status should be
investigated before merging.

### Filtering by pull request state

Use the pull request state filter to focus on specific PRs:

- **Open**: Tests on PRs still in review
- **Merged**: Tests on PRs that have already been merged
- **Closed**: Tests on PRs that were closed without merging

### Understanding confidence on new tests

New tests have limited run data, so their confidence level may be low. A low
confidence means the health status could change as more data is collected.
Consider waiting for more runs before drawing conclusions about a test's
reliability.

## Setup

Prevention requires test framework plugins that instrument test runs to track
flakiness on pull requests.

See the [test framework configuration](/ci-insights#test-framework-configuration)
for setup instructions specific to your framework (pytest-mergify,
rspec-mergify, etc.).