Noticing when an agent is going round in circles, calling the same tool with the same arguments and getting the same result. It is a common failure and it burns money quickly, so a simple check that breaks the cycle is worth having before anything more sophisticated.
Technical view
Detect by hashing recent action and argument pairs and flagging repeats, by measuring similarity across consecutive states, or by tracking absence of progress against a task-specific measure. Responses include injecting a message noting the repetition, forcing a different action, escalating to a stronger model, or terminating. Root causes are usually a tool returning an unhelpful error the model cannot act on, or a goal the available tools cannot achieve.
Related
Part of AI Terminology Index, section: Agents and Tools.