Rules first. If the task follows steps you can write down, rule-based automation does it more cheaply, more predictably and with less to go wrong. Reaching for an agent to run a fixed process is paying for judgement you do not need. An agent earns its place where the work needs reading and deciding, an email that could be one of six requests, a document that has to be checked against your data, a case where the right next step depends on what was found. That is judgement, and rules handle it badly.
A person keeps the parts where being wrong is expensive or irreversible. The design we recommend is an agent doing the routine majority end to end, with a clear line where it stops and hands over, and the escalation rate watched as a health measure, not treated as a failure. Most real deployments are all three: rules for the predictable, an agent for the judgement, a person for the consequential. The value comes from putting each in the right place rather than from choosing one. Getting this wrong is common: Gartner predicts over 40% of agentic AI projects will be cancelled by the end of 2027 (Gartner, 2025).