Building a custom annotation UI tailored to specific domain needs is both efficient and effective when using an existing data infrastructure like Langfuse, which provides clean APIs and TypeScript SDKs to streamline the process. Custom UIs address the unique requirements of non-technical reviewers, minimizing cognitive load and increasing productivity by eliminating unnecessary platform complexities and including useful features like keyboard shortcuts and domain-specific renderings. This approach is particularly advantageous when platform complexity hinders workflow efficiency and when reviewers require domain-specific data presentations. The process begins by identifying necessary API endpoints for queue management and score submissions, allowing for a focused development effort that avoids unnecessary complexity. Using tools like Claude and v0.dev, developers can rapidly prototype and deploy a functional UI, leveraging the existing infrastructure to handle backend operations, thereby concentrating on creating an intuitive and seamless user experience.