Worked on stabilizing type definitions for Relay 21 in the facebook/relay repository, focusing on improving TypeScript correctness and developer experience. Addressed a critical bug by refactoring circular-dependent types in ReactRelayTypes.d.ts, converting type aliases to interfaces to resolve circular references that previously caused type-check failures during upgrades. This approach preserved the existing API shape, minimizing breaking changes while enhancing type resolution for downstream consumers. Validated the solution using minimal reproduction cases and type-check scripts, ensuring compatibility and preventing regressions. The work demonstrated strong skills in React, TypeScript, and type checking, enabling safer upgrades and improved type safety for users.