A callback is a function, URL endpoint, or handler that is registered to be invoked automatically when a particular asynchronous event completes — such as a tool call returning a result, a long-running agent finishing its task, or an external service responding. Callbacks are fundamental to event-driven agentic architectures where the orchestrator dispatches work and reacts to completions rather than polling. They enable efficient resource use and natural integration with webhooks, message queues, and serverless functions.