During September 2025, Sagjin Kim contributed to the tc39/ecma262 repository by addressing both specification accuracy and code correctness. He fixed an edge case in JavaScript’s String.prototype.lastIndexOf, ensuring that when the search string is longer than the target, the method reliably returns -1, aligning with ECMAScript standards. Alongside this, he enhanced documentation and API clarity for class instantiation and built-in objects, clarifying constructor argument types and updating internal representations to include private methods and fields. Working primarily with JavaScript and HTML, Sagjin demonstrated strong technical writing and specification skills, delivering maintainable improvements with careful attention to normative behavior.