All new tests must be written using the `runServerless` utility instead of legacy testing approaches. This utility provides a more reliable way to test functionality by replicating full command runs, ensuring that cross-dependent machinery works as expected in production. The `runServerless` approach offers several advantages: - Tests the complete integration rather than isolated components - Reduces maintenance burden during refactors - Provides consistent testing patterns across the codebase - Eliminates the need for complex mocking setups When writing new tests, use `runServerless` with appropriate fixtures and configuration: ```javascript it('should handle function configuration correctly', async () => { const { cfTemplate, awsNaming } = await runServerless({ fixture: 'function', command: 'package', configExt: { functions: { basic: { handler: 'index.handler', memorySize: 512, }, }, }, }); const functionResource = cfTemplate.Resources[awsNaming.getLambdaLogicalId('basic')]; expect(functionResource.Properties.MemorySize).to.equal(512); }); ``` Do not expand or modify existing legacy tests - they should remain intact or be completely replaced. For test efficiency, consider consolidating multiple test scenarios into a single `runServerless` call when testing related functionality, as these runs are expensive and can prolong test execution time.