An AI agent works in a loop. It reasons, calls tools, reads the results and keeps going until it decides it is done. Evaluating one means checking more than the final message. This post covers what an eval is, how graders work, how to decide what to test, how to run the tests and look inside a run, how different kinds of agents are evaluated, and how to keep evals useful over time. Table of Contents What an eval is An eval is a test for an AI system. An input goes in, and gra