Use Locust when an agent needs to express user behavior as executable scenarios, run them at scale, and inspect latency or failure patterns before a release or capacity change. Invoke it instead of ad hoc curl loops or single-endpoint smoke checks when the real task is distributed scenario-based load testing. The scope boundary is specific and skill-shaped: model and run load scenarios against a target service, not general observability, generic backend development, or a plain load-testing product card.