Worked on core improvements to the nestjs/nest repository, focusing on enhancing reliability, performance, and developer productivity. Developed comprehensive unit test coverage for key public APIs, including DiscoveryService and ClassSerializerInterceptor, using TypeScript and Node.js to validate public methods and edge cases. Refactored the module registry from an array to a Set, which improved lookup performance and simplified dependency management. Addressed regression risks by adding targeted tests for scenarios such as empty modules and undefined instances. Improved documentation and the test feedback loop, supporting safer future changes and faster onboarding for developers working with the API and full stack features.