Worked on the fastify/fastify repository to enhance error handling by aligning TypeScript error definitions with actual runtime errors. This involved identifying and adding missing error codes to the errors.d.ts file, ensuring that type definitions accurately reflected the framework’s runtime behavior. To validate these updates, tests were implemented in JavaScript to confirm the correctness of the new type alignments. The work focused on improving type safety and reducing discrepancies between static type checks and runtime error handling, ultimately supporting a more stable developer experience. Core technologies used included TypeScript, JavaScript, and testing methodologies centered on type definition accuracy.