LlamaIndex and TruLens are two open-source tools that facilitate the construction and evaluation of applications using large language models (LLMs). LlamaIndex provides a framework for connecting data to LLMs and rapidly building diverse applications, while TruLens offers capabilities for evaluating, tracking, and refining these applications to enhance quality. The latest TruLens release introduces tracing for LlamaIndex applications, enabling developers to evaluate various components of the application stack, such as inputs, outputs, and LLM calls, with minimal code. By wrapping LlamaIndex applications with TruLens, developers can log query details and implement feedback functions to assess metrics like language match and relevance, which are crucial for evaluating the quality and accuracy of app responses. These evaluations can be visualized in the TruLens dashboard, allowing developers to identify and address potential issues, such as retrieval errors that lead to "hallucinations," and iterate on their applications for improved performance.