Two kinds of automation get sold to advertisers, and they solve different problems. Rule engines run conditions on a schedule. Agents hold a conversation and act on your instruction. Knowing which one you actually want saves a lot of disappointment.
What a rule engine is good at
A rule is a promise with no judgement in it: if this number crosses that line for this long, do this thing. It never gets tired, never forgets, and never surprises you. For guardrails, that is exactly right.
What a rule cannot do is notice that the reason CPL doubled is a creative that stopped working last Thursday, or that two ad sets are now bidding against each other. Rules do not reason about causes; they react to thresholds.
What an agent is good at
An agent turns a question into a sequence of tool calls, then explains the answer in words. "Which creative brings the cheapest leads, and what changed since last month" is a research task, not a threshold. So is "make a new ad set like the one that works, but for a wider audience".
What an agent cannot do, at least not honestly, is guarantee it will be right. That is why the useful design is not "agent instead of rules" but "agent plus rules": the rule is the floor you never fall through, the agent is the pair of hands that does the work in between.
The failure modes are different
- Rules fail silently in the wrong direction. A threshold set once in March keeps firing in August against a budget that tripled.
- Agents fail loudly and confidently. A wrong number in a fluent paragraph reads exactly like a right one.
Both failure modes have the same antidote: make the change visible before it happens. In Liaison, an agent's action is a card you read and confirm, and a rule's action is an entry in Relay with the number that triggered it.
How to combine them
Put rules on the money: a CPL ceiling per campaign, a window that matches your lead volume. Use the agent for everything that needs a look: weekly reviews, creative rotation, duplicating what works, answering the question your client asked in an email. Neither one replaces the person deciding what the account is for.