Braintrust is an infrastructure designed to support the building, scaling, and optimizing of AI evaluations, distinguishing itself from mere evaluation frameworks. It provides a robust system that transforms evaluations into practical tools for improving AI products by offering features like instrumentation, production data integration, reproducibility, real-time visualization, and scoring infrastructure. Braintrust's infrastructure allows for detailed tracing of test cases, capturing of metrics, and handling large-scale data, thanks to its specialized database, Brainstore. It emphasizes the importance of involving subject matter experts through accessible tools like playgrounds, enabling users to interact with evaluations without needing to write code. The infrastructure is versatile, working with any framework or none at all, and focuses on solving complex problems like scalability and user interface improvements, ultimately promoting the use of evaluations to enhance AI product development.