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LoopGuard separates what happened from what the application should do about it. A repeated call is evidence. Stopping the run is a policy decision. For the broader agent cycle, start with AI agents and loops. For the difference between a runtime decision and an evaluation, see tests, traces, and evals.

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Record what happened

The tracer stores ordered events. A tool call and its returned result are separate events, so three tool calls can produce six recorded events. An event describes one observation: which node acted, what it did, its normalized signature, and any attached details. For example, a search call might produce a tool:search(query=pricing) signature, followed by result:no results. Live callbacks record tool starts, tool results, and tool errors. They do not record every token, thought, or arbitrary graph node.

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Collect evidence

Detectors inspect the new event and the trace so far. A signal describes a finding, such as a repeated tool call, unchanged output, or a permanent failure. It includes a score and the evidence behind the finding. Consider these two runs:
Both repeat an action. The first returns new information; the second doesn’t. LoopGuard compares recorded output signatures and uses simple text heuristics. It does not prove whether the agent has achieved a business goal.

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Make a decision

A decision combines signals into an action, a risk score, and reasons. The default policy can produce:
REPLAN and PAUSE do not rewrite the plan or suspend execution. Your application must act on those recommendations. Observe-only mode also records STOP without raising an interrupt.
With the default policy, exact repetition alone produces a warning. Repetition or an action cycle combined with no-progress evidence can reach STOP. Repeated permanent failures, repeated closed handoff cycles, and exceeded budgets can also reach STOP. The monitor retains recent signals. When new evidence fires, the policy considers up to the last 12 signals. An event with no new signal gets a continue decision; an earlier STOP still makes the run’s interruption flag true.

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Enforce the decision

The callback handler raises LoopGuardInterrupt on STOP when interruption is enabled. The surrounding framework must let that exception propagate. The stream wrapper checks the monitor after a streamed update and stops consuming the stream at STOP. This does not undo completed tool calls or cancel external work that a tool has already started. Saved-trace analysis replays the entire export. It reports where STOP would have occurred without rerunning or interrupting the original agent.