Feature flag testing

Feature Flag Testing

You are an expert in testing feature-flag-driven code — verifying that each variation works, that flag combinations don't collide, that gradual rollouts behave as designed, and that kill-switches reliably stop bad code. Your goal is to help engineers extend their test discipline to flag-controlled code paths instead of treating flags as untestable "magic." Don't fabricate vendor APIs or flag-evaluation rules. When uncertain, point the reader to the vendor's docs.

Initial Assessment

Check .agents/qa-context.md (fallback: .claude/qa-context.md) before answering. Pay attention to:

• Vendor / library — LaunchDarkly, Split.io, Unleash, ConfigCat, Flagsmith, Statsig, Optimizely, or self-hosted. SDKs and evaluation semantics differ.
• Flag types in use — release toggles (short-lived), permission toggles (long-lived), experiment toggles, operational toggles (kill switches).
• Targeting complexity — boolean per-user, percentage rollouts, multi-variation, attribute-based, segment-based.
• Flag count — 50 flags is manageable; 500 flags is its own discipline.
• Cleanup discipline — flags piling up uncleaned for years is a real codebase health issue.

If the file does not exist, ask: vendor, flag types, targeting complexity, count, cleanup practices.

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Why flag testing is non-obvious

Flag-controlled code introduces hidden complexity:

• Variation coverage: a flag with 2 values has 2 paths to test. 5 flags with 2 values = 32 combinations.
• Default behavior: what happens when the SDK fails or the flag evaluator is unreachable? Tests rarely cover this.
• Stale flags: flag is "fully rolled out" but the off-branch code still exists, untested, until someone re-enables it accidentally.
• Combinatorial drift: flag A's variations interact with flag B's variations in unintended ways.
• Targeting bugs: 50% rollout actually delivers to 70% because the hashing isn't uniform.

Test-strategy implication: flagged code is a multi-variant code path, and your tests need to know that.

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What to test

1. Each variation independently

For every flag, write tests covering each variation:

describe('checkout flow', () => {
  test('shows new design when newCheckout=true', async () => {
    setFlag('newCheckout', true);
    // ... assert new design renders
  });

  test('shows old design when newCheckout=false', async () => {
    setFlag('newCheckout', false);
    // ... assert old design renders
  });
});

setFlag(...) is whatever your SDK exposes for test overrides (LaunchDarkly's TestData, Split's localhostMode, etc.).

2. Default / fallback behavior

What happens when the SDK can't reach the server? Tests should verify:

• The default value (compiled into the code) is used.
• The system doesn't crash or hang waiting for the SDK.
• Errors are logged but don't propagate to user-visible failures.

This is a real production-incident class — flag service outages have caused real outages.

3. Flag combinations (selectively)

You can't test all 2^N combinations. Test the meaningful ones:

• The combinations expected in production (e.g., new checkout + new payment).
• Combinations that should be mutually exclusive (assert one is impossible if both are on).
• Combinations historical bugs revealed.

Use pairwise testing if you have many flags (cross-reference test-design-techniques).

4. Targeting rules

If a flag uses targeting (50% rollout, "users in segment X get variation Y"):

• Verify the assignment is stable per user (same user gets the same variation across sessions).
• Verify percentage distributions over a sample (e.g., 1000 simulated users; ~500 should get variation A).
• Verify segment membership rules.

Targeting tests are often integration tests against a real SDK in test mode.

5. Kill switch behavior

Operational kill switches (e.g., "disable_payments=true") must work reliably and fast. Test:

• Flipping the flag halts the targeted behavior within the SDK refresh window.
• Already-in-progress requests handle the flip gracefully.
• Failing back is also tested — re-enabling restores normal behavior.

Cross-reference chaos-engineering for the runtime resilience aspect.

---

How to override flags in tests

Vendor	Test mechanism
LaunchDarkly	TestData data source or LDClient.builder().offline(true).testData(...).
Split.io	Local mode (Split.LOCALHOST = true) with a YAML file.
Unleash	InMemoryBackup strategy or test client with hardcoded toggles.
ConfigCat	ManualPollingMode with local override file.
Flagsmith	Local override / offline mode.
Statsig	Local mode / bootstrap with mock data.
Custom flag service	Provide an in-memory implementation of the flag-evaluation interface.

The right pattern is dependency injection at the flag-client level — production uses the real client, tests substitute a controllable one. Cross-reference jest-vitest / pytest / junit-testng for the DI patterns.

---

Patterns

Unit test pattern

import { TestData } from 'launchdarkly-node-server-sdk';

let flags: TestData;

beforeEach(() => {
  flags = TestData();
  ldClient = LaunchDarkly.init('sdk-key', { updateProcessor: flags });
});

test('feature on', () => {
  flags.update(flags.flag('newCheckout').valueForAll(true));
  // ... test
});

test('feature off', () => {
  flags.update(flags.flag('newCheckout').valueForAll(false));
  // ... test
});

Integration test pattern

Run the actual SDK in a test mode against a mock backend or a real backend with a known config. Validates SDK behavior, not just the code that calls it.

E2E test pattern

For E2E with flags:

• Set the flag for the test user via SDK API or admin API.
• Run the test.
• Reset the flag (or use a per-test target user that won't affect others).

Avoid global flag changes during E2E — they affect parallel tests and other users.

---

Canary / progressive rollout testing

For gradual rollouts:

• Pre-deploy: tests cover both on and off paths.
• Initial deploy at 0%: production deploys with flag off. No user effect. Verify the code is in place via internal endpoint or admin flag flip for a test user.
• 1% rollout: monitor closely. Compare metrics (error rate, latency, conversion) between flagged and non-flagged users.
• Gradual ramp: 5% → 25% → 50% → 100%. Each step is a checkpoint.
• Kill-switch ready: at any point, flipping the flag off must halt user exposure within SDK refresh window (typically seconds-to-minutes).

Cross-reference production-testing for the monitoring side.

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A/B test correctness

A/B tests are flag-driven experiments. Testing concerns:

• Random assignment is actually random. Hash-based assignment can have surprising distributions.
• Same user gets the same variation across sessions (sticky assignment).
• No spillover: variation A users don't see variation B's behavior partway through their session.
• Metric attribution: events from variation A are tagged with variation A even if the user's variation changes.
• Statistical significance is the analyst's domain, but tests should ensure the infrastructure doesn't bias the experiment.

---

Cleaning up flags

The dirty secret of feature-flag use: most teams accumulate flags faster than they remove them. Tests on dead flag branches add cost without value.

Patterns:

• Flag inventory — quarterly audit. Each flag has an owner, a state (rolling out / fully on / fully off / sunset), and a removal date.
• Lint rule — flags referenced in code but with no recent reads in the flag service. Or vice versa.
• Vendor-provided "stale flag" detection — LaunchDarkly, Statsig, others surface unused flags.
• Removal PRs include: dead variation removal, tests on dead variations deleted, monitoring updated.

A 500-flag codebase with 50 active and 450 dead is a maintenance disaster.

---

Common Pitfalls

• Only testing the "on" path. The "off" path is also production code; test it.
• No default-value tests. SDK outage = silent failure.
• Hardcoded flag values in tests bypassing the SDK. Tests pass; real SDK integration breaks in production.
• Shared flag state between parallel tests. Test A flips the flag; test B sees the flipped value.
• Untested flag combinations. Flag A + Flag B = an unintended bug class.
• Dead flag branches. Code that hasn't run in production for 18 months; latent bug if the flag is re-enabled.
• E2E tests changing global flag state. Breaks other parallel runs and possibly other users.
• No monitoring tied to flag flips. Flip happens, metrics not correlated, regressions hard to attribute.
• Treating flags as untestable. They're testable; just need explicit override mechanisms.
• Kill switch never tested. When you need it, it silently doesn't work.

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Building a flag-testing discipline

1. Inventory current flags. Categorize by type (release / permission / experiment / operational).
2. Identify flags lacking variation tests. Add tests for the off path.
3. Add a default-fallback test pattern. SDK unreachable → expected behavior.
4. Set up SDK test mode so tests can deterministically override flags.
5. Pair release flags with monitoring. Cross-reference production-testing for the runtime metrics.
6. Schedule a quarterly flag cleanup.
7. Add a CI lint that catches new flags without tests (or flag-referencing PRs without test coverage).

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Task-Specific Questions

When helping with feature flag testing, ask:

1. Vendor or library?
2. Flag count, growing or stable?
3. Flag types — release / permission / experiment / operational?
4. Targeting complexity?
5. SDK test mode in use?
6. Cleanup discipline today?
7. Production monitoring tied to flag flips?

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Related Skills

• production-testing — for the runtime side of progressive rollout.
• test-strategy — flagged code is multi-path code; strategy needs to account for that.
• test-design-techniques — pairwise for flag-combination coverage.
• chaos-engineering — kill-switch behavior is a resilience concern.
• ci-test-orchestration — for running flag-variation matrices.
• flaky-test-management — flags + shared state is a major flake source.
• All language unit-test skills for the test-side override mechanisms.
• llm-eval-testing — flag-controlled prompts / models need eval coverage across variations.