Requisite variety
A controller needs at least as many responses as the situation has surprises.
W. Ross Ashby, An Introduction to Cybernetics
1956
Ashby's Law, from cybernetics: only variety can absorb variety. To regulate a system, the regulator must produce at least as many distinct responses as the system can produce distinct states. Whatever is left over has to be absorbed somewhere or it shows up as failure.
This is the formal reason a human stays in the loop. An agent can own a loop only where its variety matches the loop's; the residual escalates to a person. When people say a system needs human oversight, this is the mathematics they are gesturing at.
Which decisions may an agent make alone, and who is able to stop it?
For each workflow, what is the human to agent ratio, and who signs the quality baseline?
What synchronizes the work when there is no room and half the team is not a person?