Worked on core stability and documentation accuracy across module-federation/core and vuejs/core repositories. Addressed a documentation issue in module-federation/core by correcting an MDX configuration example, ensuring developers avoid common misconfigurations when using module federation. In vuejs/core, resolved a bug in the defineCustomElement API by implementing a shallow copy strategy to prevent unintended mutation of the options object, thereby improving API reliability. Added regression tests to lock in the new behavior and maintain immutability. Utilized JavaScript, TypeScript, and testing best practices throughout, with a focus on enhancing developer experience and reducing support overhead through precise documentation and robust code.