Stax is a new developer tool designed to simplify the evaluation process for large language models (LLMs) by allowing users to create custom evaluation criteria and utilize both human and LLM-based autoraters. This tool aims to replace the inefficient "vibe testing" method with a structured approach that provides clear metrics for assessing the effectiveness of AI outputs. By leveraging Stax, developers can make more data-driven decisions and rigorously test their AI systems.