MSW Best Practices

Comprehensive API mocking guide for MSW v2 applications, designed for AI agents and LLMs. Contains 45 rules across 8 categories, prioritized by impact to guide automated refactoring and code generation.

When to Apply

Reference these guidelines when:

• Setting up MSW for testing or development

• Writing or organizing request handlers

• Configuring test environments with MSW

• Mocking REST or GraphQL APIs

• Debugging handler matching issues

• Testing error states and edge cases

Rule Categories by Priority

Priority Category Impact Prefix

1 Setup & Initialization CRITICAL setup-

2 Handler Architecture CRITICAL handler-

3 Test Integration HIGH test-

4 Response Patterns HIGH response-

5 Request Matching MEDIUM-HIGH match-

6 GraphQL Mocking MEDIUM graphql-

7 Advanced Patterns MEDIUM advanced-

8 Debugging & Performance LOW debug-

Quick Reference

1. Setup & Initialization (CRITICAL)

• setup-server-node-entrypoint

• Use correct entrypoint for Node.js (msw/node)

• setup-lifecycle-hooks

• Configure server lifecycle in test setup

• setup-worker-script-commit

• Commit worker script to version control

• setup-node-version

• Require Node.js 18+ for MSW v2

• setup-unhandled-requests

• Configure unhandled request behavior

• setup-typescript-config

• Configure TypeScript for MSW v2

2. Handler Architecture (CRITICAL)

• handler-happy-path-first

• Define happy path handlers as baseline

• handler-domain-grouping

• Group handlers by domain

• handler-absolute-urls

• Use absolute URLs in handlers

• handler-shared-resolvers

• Extract shared response logic into resolvers

• handler-v2-response-syntax

• Use MSW v2 response syntax

• handler-request-body-parsing

• Explicitly parse request bodies

• handler-resolver-argument

• Destructure resolver arguments correctly

• handler-reusability-environments

• Share handlers across environments

3. Test Integration (HIGH)

• test-reset-handlers

• Reset handlers after each test

• test-avoid-request-assertions

• Avoid direct request assertions

• test-concurrent-boundary

• Use server.boundary() for concurrent tests

• test-fake-timers-config

• Configure fake timers to preserve queueMicrotask

• test-async-utilities

• Use async testing utilities for mock responses

• test-clear-request-cache

• Clear request library caches between tests

• test-jsdom-environment

• Use correct JSDOM environment for Jest

4. Response Patterns (HIGH)

• response-http-response-helpers

• Use HttpResponse static methods

• response-delay-realistic

• Add realistic response delays

• response-error-simulation

• Simulate error responses correctly

• response-one-time-handlers

• Use one-time handlers for sequential scenarios

• response-custom-headers

• Set response headers correctly

• response-streaming

• Mock streaming responses with ReadableStream

5. Request Matching (MEDIUM-HIGH)

• match-url-patterns

• Use URL path parameters correctly

• match-query-params

• Access query parameters from request URL

• match-custom-predicate

• Use custom predicates for complex matching

• match-http-methods

• Match HTTP methods explicitly

• match-handler-order

• Order handlers from specific to general

6. GraphQL Mocking (MEDIUM)

• graphql-operation-handlers

• Use operation name for GraphQL matching

• graphql-error-responses

• Return GraphQL errors in correct format

• graphql-batched-queries

• Handle batched GraphQL queries

• graphql-variables-access

• Access GraphQL variables correctly

7. Advanced Patterns (MEDIUM)

• advanced-bypass-requests

• Use bypass() for passthrough requests

• advanced-cookies-auth

• Handle cookies and authentication

• advanced-dynamic-scenarios

• Implement dynamic mock scenarios

• advanced-vitest-browser

• Configure MSW for Vitest browser mode

• advanced-file-uploads

• Mock file upload endpoints

8. Debugging & Performance (LOW)

• debug-lifecycle-events

• Use lifecycle events for debugging

• debug-verify-interception

• Verify request interception is working

• debug-common-issues

• Know common MSW issues and fixes

• debug-request-logging

• Log request details for debugging

How to Use

Read individual reference files for detailed explanations and code examples:

• Section definitions - Category structure and impact levels

• Rule template - Template for adding new rules

• Individual rules: references/{prefix}-{slug}.md

Related Skills

• For generating MSW mocks from OpenAPI, see orval skill

• For consuming mocked APIs, see tanstack-query skill

• For test methodology, see test-vitest or test-tdd skills

Full Compiled Document

For the complete guide with all rules expanded: AGENTS.md