Google ADK Callbacks Can Enforce Agent Policy, Not Just Log Activity

Google's Agent Development Kit (ADK) provides callback interception points before and after agents, models, and tools execute — and developers are being urged to treat these as a policy enforcement layer rather than simple logging hooks. Each callback can return a value that halts or replaces normal execution, making them active participants in application behavior. A structured policy decision model — covering authorization, input normalization, budget checks, and evidence recording — can be wired into these callbacks to ensure actions like refunds are explicitly approved before proceeding. Callback registration order also matters, as earlier callbacks can modify inputs or short-circuit later ones, meaning authentication logic must not run after approval logic. Guardrail tests should verify that blocked operations never reach tool execution, not merely that the final response text indicates a block.