An eval suite is a test-set + automated grading rig for measuring LLM output quality on a defined task. The closest equivalent in traditional software engineering is the test pyramid -- minus the determinism. Evals quantify how a model performs on the work the business actually needs done.
A well-built eval suite holds: a fixed set of input prompts, the expected output shape, a grading function (LLM-as-judge, exact-match, scored rubric, or human review on a sample), and a regression dashboard that compares the candidate model against the production baseline.
Enterprises that move LLMs into production without an eval suite are flying blind. The model behaves differently across versions, prompts drift, and business outcomes shift quietly. An eval suite reviewed before every model-version upgrade is the gate that turns "we tried it and it felt better" into a defensible production decision.