testing-best-practices

Test layering policy

Unit tests

• Data-driven: parameterized tables covering happy path, boundary, error, and edge cases.
• Property-based: fuzz invariants that must hold across all inputs (e.g., idempotency, sort stability, roundtrip serialization).
• Derive cases from the module's public API surface: input types/constraints, output shape, error modes, invariants.

Integration / contract tests

• API envelope: request/response shape, status codes, content types, pagination.
• Error contract: error codes, error shapes, rate limiting, retries.
• Auth and scoping: token validation, role-based access, tenant isolation.
• Eventual consistency: verify convergence within bounded time; poll rather than sleep.
• Reuse auth state across tests where possible; avoid redundant login flows.

E2E tests

• No mocks; exercise real services, databases, and APIs.
• Happy-path workflows only; save edge cases for lower layers.
• State-tolerant: never assume a clean slate; tolerate and work with prior state.
• Idempotent: safe to run repeatedly without cleanup between runs.
• Flow-oriented: validate real data paths end-to-end rather than isolated assertions.

Hard rules

• Never invent signatures, source locations, or line numbers. Only reference what you have read from the codebase.
• No fabricated fixtures. Derive test data from actual schemas, types, or seed data in the repo.
• No test-only hacks in product code. No

  if (process.env.TEST)

  branches, no test-specific exports, no test backdoors.
• E2E must not rely on clean slate. Tests must tolerate pre-existing data, prior test runs, and shared environments.

Execution guidance

Preflight checks (before e2e)

1. Verify the target environment is reachable (health endpoint, ping).
2. Confirm required services are running (database, API, auth provider).
3. Validate test user / credentials exist and are functional.
4. Check for leftover state that could cause false failures; log it, do not fail on it.

Deterministic fixtures

• Use seeded randomness for generated data (seeded faker, deterministic UUIDs).
• Fixtures should be self-contained; avoid cross-test fixture dependencies.
• Prefer factory functions over shared mutable fixture objects.

Async handling

• Poll with bounded timeout and backoff; never use fixed

  sleep

  /

  waitForTimeout

  .
• Set explicit timeout per operation; fail fast with a descriptive message on timeout.
• Bound retry attempts (e.g., max 3 retries with exponential backoff).
• Use framework-native waiting (Playwright

  expect

  , async assertions) over manual loops.

Flake handling

• Single infrastructure retry per test run; if it fails twice, it is not flake.
• On retry failure, collect diagnostics: screenshots, network logs, service health, timestamps.
• Classify the failure (flaky / outdated / bug) before attempting a fix.
• Never add arbitrary delays or retry loops as a flake "fix."

API surface discovery

• Read the module source to enumerate exports/public functions.
• Confirm scope from the user request and inspected code context; if ambiguous, state assumptions and proceed conservatively.
• For each function: input types/constraints, output shape, error modes, invariants.
• Probe for state dependencies and ordering constraints between functions.

Output format

ID | Category | Name | Input | Expected

{CATEGORY}-{NN}

CI guidance

Fast PR smoke lane

• Unit tests + linting + type-check on every PR.
• Subset of integration tests covering critical contracts.
• Target: under 5 minutes.

Nightly full lane

Workflow

1. Spec or code defines the module behavior (types, constraints, API surface).
2. Agent (with this skill) produces test strategy, matrix, and implementation plan.
3. test-writer agent translates the plan to runnable code in the target language's idiom.
4. Developer implements to pass the tests.
5. If implementation reveals missing cases, propose them first; append to spec only when explicitly requested.