Test Plan

Write a test strategy that answers two questions clearly: what deserves testing and how much testing is enough. Everything else is execution detail.

Step 1: Understand What's Being Tested

Before planning any tests, nail down what you're working with. Read the code or requirements. If the user hasn't provided enough context, ask.

Input	Question	Watch out for
Code / Feature	What does this do? What are the inputs and outputs?	Planning tests for code you haven't read
Critical paths	Which flows would break the product if they failed?	Treating all code paths as equally important
Dependencies	What external systems, APIs, or services does it touch?	Ignoring side effects and integrations
Edge cases	What unusual inputs or states could occur?	Only testing the happy path

If existing tests, specs, or prior conversations exist, pull from them. Don't invent context.

Step 2: Apply the Testing Pyramid

Distribute test effort deliberately. The pyramid exists because fast feedback loops matter more than exhaustive simulation.

Layer	Share	What to test	Characteristics
Unit tests	~70%	Pure functions, utilities, hooks, reducers, transformers	Fast, isolated, many. Test one thing per test. Mock nothing or mock only external I/O.
Integration tests	~20%	Component interactions, API routes, database queries, service boundaries	Test contracts between units. Verify that pieces work together correctly. Slower, fewer.
E2E tests	~10%	Critical user flows only: signup, checkout, core CRUD	Slow, flaky-prone, expensive. Reserve for flows where failure = revenue loss or user churn.

These ratios are targets, not rules. A utility library might be 95% unit tests. A UI-heavy app might lean harder on integration tests. Adjust with intent.

Step 3: Decide What to Test

Map each piece of functionality to the right test type. Use this decision table.

What you're testing	Test type	Why
Business logic, calculations, data transforms	Unit test	Fast feedback, easy to cover all branches
User-facing flows (form submit, navigation)	Integration or E2E	Need to verify real DOM and event behavior
Edge cases (null inputs, empty arrays, boundaries)	Unit test	Cheap to write, high defect-prevention value
Visual appearance and layout	Snapshot test (use sparingly)	Brittle; prefer visual regression tools for critical UI
API request/response contracts	Integration test	Verify serialization, status codes, error shapes
Error handling and failure modes	Unit + integration	Unit for thrown errors, integration for error boundaries and fallback UI
State management (stores, reducers)	Unit test	Pure logic, deterministic, fast to test
Third-party integrations	Integration test with mocks	Verify your code handles their API correctly

Step 4: Define Coverage Targets

Aim for meaningful coverage, not 100%. Chasing full coverage leads to brittle tests that test implementation details.

Coverage target	When it makes sense
90%+	Shared libraries, payment logic, auth flows — code where bugs have outsized impact
70-80%	Application code, feature modules — good balance of safety and velocity
50-70%	Rapidly prototyping, exploratory features — cover critical paths, skip the rest
Skip coverage metrics	One-off scripts, throwaway prototypes, generated code

Focus coverage on:

• Code with high cyclomatic complexity (many branches)
• Code that handles money, auth, or user data
• Code that's changed frequently (high churn = high risk)

Step 5: Choose Tools

Pick tools that match your stack. Don't over-engineer the test setup.

Purpose	Tool	Notes
Unit tests	Vitest	Fast, ESM-native, Jest-compatible API. Preferred for modern projects.
Component tests	Testing Library (@testing-library/react)	Test behavior, not implementation. Queries by role, text, label.
User interaction	@testing-library/user-event	Simulates real user events (click, type, tab). Prefer over fireEvent.
API mocking	msw (Mock Service Worker)	Intercepts at the network level. Works in tests and browser dev.
E2E tests	Playwright	Cross-browser, reliable, good DX. Prefer over Cypress for new projects.
Visual regression	Playwright screenshots or Chromatic	Use only for design-critical UI. Not a substitute for functional tests.

Output Format

Deliver the test plan as a prioritized list of test cases grouped by type.

# Test Plan: [Feature / Component Name]

## Unit Tests (priority order)
- [ ] [function/module] — [what behavior to verify]
- [ ] [function/module] — [edge case to cover]

## Integration Tests (priority order)
- [ ] [interaction/flow] — [what contract to verify]
- [ ] [API route] — [request/response shape to verify]

## E2E Tests (priority order)
- [ ] [critical user flow] — [what end-to-end behavior to verify]

## Coverage Notes
- Target: [X%] for [reason]
- Skip: [what's deliberately not tested and why]

## Open Questions
- [Anything unresolved about test approach]

Anti-Patterns to Avoid

Anti-pattern	Problem	Instead
Testing implementation details	Tests break on every refactor, provide no confidence	Test inputs/outputs and observable behavior
Mocking everything	Tests pass but nothing actually works together	Mock only external I/O; let units collaborate
Snapshot overuse	Giant snapshots nobody reviews, approved blindly	Use snapshots only for small, stable structures
Testing the framework	Verifying that React renders or that Vitest asserts	Test your logic, not the tool's behavior
100% coverage as a goal	Diminishing returns, brittle tests, wasted time	Cover critical paths thoroughly, accept gaps in trivial code
Copy-paste test cases	Hard to maintain, masks missing abstractions	Use test.each() or parameterized tests for repetitive cases