TruLens is an open-source tool designed to evaluate and track large language model (LLM) applications. It offers a variety of feedback functions to programmatically assess the quality of inputs, outputs, and intermediate results. By providing detailed insights into LLM app performance, TruLens helps developers improve relevance, reduce toxicity, and enhance overall application effectiveness. TruLens seamlessly integrates into the development workflow, supporting rapid iteration and continuous improvement of LLM applications.