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