Functions in Braintrust are flexible AI engineering primitives that allow developers to build and scale AI-powered products with ease. They enable the creation of atomic, reusable building blocks for executing AI-related logic, including prompts, tools, and custom scorers. Functions can be created in code or the UI, linked together, and invoked through APIs. This enables the development of agentic applications quickly and efficiently, while maintaining security and scalability. The functions are open and embrace standards, allowing them to work with various models and frameworks. They represent a significant shift in the software engineering lifecycle, bringing together evaluation, logging, datasets, prompts, and the playground into one powerful workflow.