flaky-test-analyzer

name	flaky-test-analyzer
description	Use when tests pass locally but fail in CI, pass some runs and fail others, or someone suspects a test is unreliable but cannot reproduce the failure consistently. Diagnoses root causes and provides concrete fixes. Triggers on: "flaky test",
scenarios	["Our tests pass locally but randomly fail in CI — I need to fix these flaky tests","Help me diagnose why this test fails intermittently with a race condition","CI pipeline keeps red because of non-deterministic test failures","로컬에선 통과하는 테스트가 CI에서 랜덤으로 실패해","간헐적으로 실패하는 테스트 원인을 찾아줘"]
compatibility	{"recommended":["think-tool"],"optional":["sequential-thinking"],"remote_mcp_note":"think-tool이 있으면 간헐적 실패의 근본 원인을 더 체계적으로 추론합니다. Claude 설정 → MCP Servers에서 remote SSE 엔드포인트를 추가하세요."}

Flaky Test Analyzer

When to Use / When Not to Use

Use when:

• A test fails intermittently without code changes
• Tests pass locally but fail in CI
• Re-runs are being used as a workaround (treating symptoms, not the cause)

Do not use when:

• The test fails consistently — that is a bug, not flakiness
• You need to write new tests (use test-master or test-driven-development)

Process

1. Triage first — identify the failure category before attempting a fix
2. Reproduce in isolation — run only the failing test 20 times
3. Reproduce with ordering — run the failing test after each other test in the suite
4. Add logging — capture timestamps, thread names, and state snapshots at failure time
5. Check nondeterministic inputs — search for new Date(), Math.random(), UUID.randomUUID(), System.currentTimeMillis()
6. Check resource cleanup — verify @AfterEach teardown, unclosed streams, un-stubbed mocks

If sequential-thinking is available, use it to work through steps 1–5 in order — skipping triage before fixing is the primary failure mode.

Output Template

For each flaky test analysis, provide:

1. Failure category (from the triage table below)
2. Root cause diagnosis with evidence
3. Concrete fix (code snippet)
4. Prevention rule for the test suite
5. Recommended CI configuration to catch this class of flakiness earlier

What Claude Does / What You Do

Claude	You
Identifies failure category from symptom description	Provide the test code and failure log
Explains why the root cause causes intermittent failure	Run the test 20 times in isolation to confirm
Provides the specific fix pattern (waitFor, Clock injection, etc.)	Implement and verify the fix
Suggests @BeforeEach / @AfterEach cleanup patterns	Apply to the full test suite
Recommends CI flags (random ordering, etc.)	Configure CI pipeline

Triage: Find the Category First

Fails When	Category
Run repeatedly in isolation	Timing dependency or resource leak
Run after specific other tests	Ordering / shared state dependency
Run in parallel	Concurrency or shared resource conflict
Run on CI but not locally	Environment dependency (clock, timezone, path, env var)
Run with real external systems	External dependency (network, DB, third-party API)
Passes after a sleep/wait	Timing / async race condition

The 5 Flakiness Categories

1. Timing Dependencies — Asserts on async work before it completes. Fix: use waitFor/awaitility, event-driven signals, or inject a controllable Clock. Never use Thread.sleep.

2. Shared State Between Tests — Tests pollute database, static variables, in-memory caches, or file system. Fix: reset state in @BeforeEach/@AfterEach; use @Transactional rollback or explicit truncate; prefer instance injection over singletons.

3. External Dependencies — Tests call real HTTP APIs, databases, or message queues. Fix: mock at the boundary (WireMock, Mockito); use Testcontainers for integration tests needing a real DB.

4. Test Ordering Dependencies — Test B implicitly relies on state created by Test A. Fix: self-contained setup per test; run tests in random order (--randomly-seed=random).

5. Concurrency and Parallelism — Parallel tests share ports, files, or singletons. Fix: use port 0 (OS-assigned); inject resources so each test gets its own instance.

Fix Patterns Quick Reference

Root Cause	Fix
Async race condition	Use waitFor / awaitility; never Thread.sleep
System clock dependency	Inject Clock; use fixed clock in tests
Database pollution	@Transactional rollback or truncate in @BeforeEach
Static/singleton state	Reset in @BeforeEach/@AfterEach; prefer instance injection
Real HTTP/DB calls	Mock with WireMock, Mockito, or Testcontainers
Ordering dependency	Self-contained setup; run tests in random order
Port conflict	Use port 0; never hardcode test ports
Timezone sensitivity	Set TZ=UTC in CI; use ZonedDateTime not Date
Random data collisions	Use unique test data per run (UUID prefix)

For category-specific fix code examples, see references/flaky-fix-patterns.md.

Prevention

• Run tests in random order in CI by default
• Quarantine any test that flakes more than 1% of runs
• Never commit a "re-run on failure" workaround without a ticket to fix the root cause
• Fail the build on any @Disabled / @Ignore without a linked issue

Related Skills

• test-master — writing new tests with built-in flakiness prevention
• test-driven-development — TDD patterns that reduce flakiness by design
• chaos-engineer — when you want to intentionally test failure behavior