en.wikipedia.org/wiki/Rat
a rational agent is an agent that has clear preferences, models uncertainty via expected values, and always chooses to perform the action with the optimal expected outcome for itself from among all feasible actions.
Apparently, a knowledge-based agent includes a knowledge base and an inference system.
en.wikipedia.org/wiki/Inf
The inference engine applied logical rules to the knowledge base and deduced new knowledge.
IMHO, this would make a knowledge-based agent also rational…. ;^)