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

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 delays only for timing behavior under test
• 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](references/_sections.md) - Category structure and impact levels
• Individual rules: references/{prefix}-{slug}.md

Related Skills

• For consuming mocked APIs, see tanstack-query skill
• For test framework patterns, see vitest; for methodology, see tdd

Full Compiled Document

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