What are flaky tests and how do you deal with them?

Question

IT Interview Practice · Accepted Answer

**Flaky tests** are tests that **pass or fail inconsistently** without code changes — sometimes passing, sometimes failing on the same code. They're a serious problem because they erode trust in the test suite. Understanding their causes and fixes is important.

## What flaky tests are and why they're harmful

```text
A FLAKY test gives INCONSISTENT results (pass sometimes, fail other times) on the SAME code:
  → harmful: ERODES TRUST — people start ignoring failures ("oh, it's just flaky") →
    real failures get missed too
  → waste time on false alarms / re-runs; break CI; reduce confidence in the whole suite
→ Flaky tests are worse than no test if they make people distrust all tests.
```

## Common causes

```text
✗ TIMING / async → race conditions; not waiting properly for async operations (a top cause
  in UI/E2E tests); arbitrary sleeps
✗ ORDER DEPENDENCE → tests depending on each other / shared mutable state
✗ EXTERNAL dependencies → real network/services (network blips, rate limits, downtime)
✗ NON-DETERMINISM → time/dates, randomness, timezone, locale
✗ Test ENVIRONMENT → leftover state, uncleaned data, concurrency/parallelism issues
```

## Fixing and managing flaky tests

```text
✓ Fix the ROOT CAUSE: wait for conditions (not sleeps); make tests INDEPENDENT and clean
  up state; MOCK external dependencies; control time/randomness (inject them)
✓ Ensure proper isolation (no shared state, no order dependence)
✓ Don't just RETRY blindly (it hides the problem) — investigate and fix
✓ QUARANTINE persistently flaky tests (isolate so they don't block) WHILE fixing them
✓ Treat flakiness seriously — track and address it (it degrades the whole suite)
```

## Why it matters

Understanding flaky tests and how to deal with them is important because **flaky tests are a serious, common problem that undermines the value of an entire test suite**, so it's valuable practical knowledge.

A flaky test (passing or failing inconsistently on the same code) is particularly harmful because it **erodes trust**: when tests fail unpredictably, people start ignoring failures ("it's just flaky"), which means **real failures get missed too** — so flakiness degrades confidence in the whole suite, potentially making it worse than useless.

Understanding this harm motivates taking flakiness seriously.

Understanding the **common causes** — **timing/async issues** (race conditions, not waiting properly for async operations, arbitrary sleeps — a top cause especially in UI/E2E tests), **order dependence** (tests depending on each other or shared state), **external dependencies** (real network/services with their blips and downtime), **non-determinism** (time, randomness, timezone, locale), and **environment issues** (leftover state, concurrency) — is necessary to diagnose flakiness.

Understanding how to **fix and manage** them — fixing root causes (waiting for conditions instead of sleeps, making tests independent and clean, mocking external dependencies, controlling time and randomness), ensuring isolation, **not blindly retrying** (which hides problems), quarantining persistently flaky tests while fixing them, and treating flakiness as a serious issue to track and address — reflects the right approach.

Since flaky tests are a common, serious problem that undermines trust in and the value of a test suite (a real issue in many real-world projects), and since understanding their causes and proper handling (fixing root causes, isolation, not just retrying) is necessary to maintain a trustworthy test suite, understanding flaky tests is valuable, practically-important knowledge — addressing a significant real-world testing problem, important for keeping a test suite reliable and trusted, and reflecting the practical maturity to handle one of testing's most frustrating and damaging issues.

What flaky tests are and why they're harmful

Common causes

Fixing and managing flaky tests

Why it matters

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