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testing-unit Unit testing patterns for isolated business logic tests — AAA pattern, parametrized tests (test.each, @pytest.mark.parametrize), fixture scoping (function/module/session), mocking with MSW/VCR at network level, and test data management with factories (FactoryBoy, faker-js). Use when writing unit tests, setting up mocks, structuring test data, optimizing test speed, choosing fixture scope, or reducing test boilerplate. Covers Vitest, Jest, pytest.
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name testing-unit license MIT compatibility Claude Code 2.1.220+. description Unit testing patterns for isolated business logic tests — AAA pattern, parametrized tests (test.each, @pytest.mark.parametrize), fixture scoping (function/module/session), mocking with MSW/VCR at network level, and test data management with factories (FactoryBoy, faker-js). Use when writing unit tests, setting up mocks, structuring test data, optimizing test speed, choosing fixture scope, or reducing test boilerplate. Covers Vitest, Jest, pytest. tags ["testing","unit","mocking","msw","vcr","fixtures","factories","vitest-4","aroundEach"] context fork agent test-generator version 2.1.0 author OrchestKit user-invocable false disable-model-invocation false complexity medium persuasion-type reference targets [{"library":"vitest","version":">=4.1.0"}] metadata {"category":"document-asset-creation"} allowed-tools ["Read","Glob","Grep","WebFetch","WebSearch"] path_patterns ["*.test.*","*.spec.*","**/vitest.config.*","**/jest.config.*"]
Unit Testing Patterns
Focused patterns for writing isolated, fast, maintainable unit tests. Covers test structure (AAA), parametrization, fixture management, HTTP mocking (MSW/VCR), and test data generation with factories.
Each category has individual rule files in rules/ loaded on-demand, plus reference material, checklists, and scaffolding scripts.
Core Principles (ALWAYS apply)
AAA structure : Every test MUST follow Arrange-Act-Assert. Use // Arrange, // Act, // Assert comments for clarity.Parametrize, don't duplicate : Use test.each (TypeScript) or @pytest.mark.parametrize (Python) when testing multiple inputs. Never copy-paste the same test body with different values.Fixture scoping matters : Use scope="function" (default) for mutable data. Use scope="module" or scope="session" ONLY for expensive read-only resources (DB engines, ML models). Mutable data with shared scope causes flaky tests.Speed target : Each unit test should run under 100ms . If it's slower, you're likely hitting I/O — mock it.Mock at the network level : Use MSW (TypeScript) or VCR.py (Python) to intercept HTTP at the network layer. Never mock fetch/axios/requests directly.
Quick Reference
Total: 8 rules across 3 categories, 4 references, 3 checklists, 1 example set, 3 scripts
Unit Test Structure
Core patterns for structuring isolated unit tests with clear phases and efficient execution.
AAA Pattern rules/unit-aaa-pattern.mdArrange-Act-Assert with isolation Fixture Scoping rules/unit-fixture-scoping.mdfunction/module/session scope selection Parametrized Tests rules/unit-parametrized.mdtest.each / @pytest.mark.parametrize
Reference: references/aaa-pattern.md — detailed AAA implementation with checklist
HTTP Mocking Network-level request interception for deterministic tests without hitting real APIs.
Rule File Key Pattern MSW 2.x rules/mocking-msw.mdNetwork-level mocking for frontend (TypeScript) VCR.py rules/mocking-vcr.mdRecord/replay HTTP cassettes (Python)
references/msw-2x-api.md — full MSW 2.x API (handlers, GraphQL, WebSocket, passthrough)references/stateful-testing.md — Hypothesis RuleBasedStateMachine for stateful tests
checklists/msw-setup-checklist.md — MSW installation, handler setup, test writingchecklists/vcr-checklist.md — VCR configuration, sensitive data filtering, CI setup Examples: examples/handler-patterns.md — CRUD, error simulation, auth flow, file upload handlers
Test Data Management Factories, fixtures, and seeding patterns for isolated, realistic test data.
Rule File Key Pattern Data Factories rules/data-factories.mdFactoryBoy / @faker-js builders Data Fixtures rules/data-fixtures.mdJSON fixtures with composition Seeding & Cleanup rules/data-seeding-cleanup.mdAutomated DB seeding and teardown
Reference: references/factory-patterns.md — advanced factory patterns (Sequence, SubFactory, Traits)
Checklist: checklists/test-data-checklist.md — data generation, cleanup, isolation verification
Quick Start
TypeScript (Vitest + MSW) import { describe, test, expect, beforeAll, afterEach, afterAll } from 'vitest' ;
import { http, HttpResponse } from 'msw' ;
import { setupServer } from 'msw/node' ;
import { calculateDiscount } from './pricing' ;
describe ('calculateDiscount' , () => {
test.each ([
[100 , 0 ],
[150 , 15 ],
[200 , 20 ],
])('for order $%i returns $%i discount' , (total, expected ) => {
const order = { total };
const discount = calculateDiscount (order);
expect (discount).toBe (expected);
});
});
const server = setupServer (
http.get ('/api/users/:id' , ({ params } ) => {
return HttpResponse .json ({ id : params.id , name : 'Test User' });
})
);
beforeAll (() => server.listen ({ onUnhandledRequest : 'error' }));
afterEach (() => server.resetHandlers ());
afterAll (() => server.close ());
test ('fetches user from API' , async () => {
const response = await fetch ('/api/users/123' );
const data = await response.json ();
expect (data.name ).toBe ('Test User' );
});
Python (pytest + FactoryBoy) import pytest
from factory import Factory, Faker, SubFactory
class UserFactory (Factory ):
class Meta :
model = dict
email = Faker('email' )
name = Faker('name' )
class TestUserService :
@pytest.mark.parametrize("role,can_edit" , [
("admin" , True "viewer" , False
def test_edit_permission (self, role, can_edit ):
user = UserFactory(role=role)
result = user_can_edit(user)
assert result == can_edit
Vitest 4.1 Features
aroundEach / aroundAll (preferred for DB transactions) Wraps each test in setup/teardown — cleaner than separate beforeEach/afterEach for transactions:
test.aroundEach (async (runTest, { db }) => {
await db.transaction (runTest)
})
test ('insert user' , async ({ db }) => {
await db.insert ({ name : 'Alice' })
})
aroundAll wraps entire suites the same way.
mockThrow / mockThrowOnce Replaces the verbose mockImplementation(() => { throw err }) pattern:
const fn = vi.fn ()
fn.mockThrow (new Error ('connection lost' ))
fn.mockThrowOnce (new Error ('timeout' ))
vi.defineHelper (clean stack traces) Custom assertion helpers that point errors to the call site, not the helper internals:
const assertPair = vi.defineHelper ((a, b ) => {
expect (a).toEqual (b)
})
Test Tags Filter tests by tags in CLI — useful for CI fast paths:
test : {
tags : {
unit : { timeout : 5000 },
flaky : { retry : 3 },
}
}
vitest --tags-filter="unit and !flaky"
vitest --tags-filter="(unit or integration) and !slow"
Agent Reporter Minimal output (failures only) — use in AI agent / CI contexts:
Key Decisions Decision Recommendation Test framework (TS) Vitest 4.1+ (modern, fast, aroundEach, test tags) or Jest (mature ecosystem) Test framework (Python) pytest with plugins (parametrize, asyncio, cov) HTTP mocking (TS) MSW 2.x at network level, never mock fetch/axios directly HTTP mocking (Python) VCR.py with cassettes, filter sensitive data Test data Factories (FactoryBoy/faker-js) over hardcoded fixtures Fixture scope scope="function" for mutable (default). module/session ONLY for expensive immutable resourcesExecution time Under 100ms per unit test — if slower, mock external calls Coverage target 90%+ business logic, 100% critical paths
Common Mistakes
Testing implementation details instead of public behavior (brittle tests)Mocking fetch/axios directly instead of using MSW at network level (incomplete coverage)Shared mutable state between tests via module-scoped fixtures (flaky tests)Hard-coded test data with duplicate IDs (test conflicts in parallel runs)No cleanup after database seeding (state leaks between tests)Over-mocking — testing your mocks instead of your code (false confidence)Verbose throw mocking — mockImplementation(() => { throw err }) instead of mockThrow(err) (Vitest 4.1+)
Scripts Script File Purpose Create Test Case scripts/create-test-case.mdScaffold test file with auto-detected framework Create Test Fixture scripts/create-test-fixture.mdScaffold pytest fixture with context detection Create MSW Handler scripts/create-msw-handler.mdScaffold MSW handler for an API endpoint
