An agent with a Blood: Gas partition coefficient of 12.0 and an oil: gas partition coefficient of 0.5 is described as:

A Blood: Gas partition coefficient indicates how soluble an agent is in blood compared to gas; a higher number suggests that the agent is more soluble in blood, which typically means it will have a slower onset of action. In this case, a coefficient of 12.0 signifies that the agent is highly soluble in blood, indicating that it will accumulate slowly in the bloodstream and, thus, lead to a slower effect when administered.

The oil: gas partition coefficient relates to the lipid solubility of the agent. An oil: gas partition coefficient of 0.5 is relatively low, suggesting that the agent is not very potent. An agent that has low lipid solubility generally means that it is less effective at producing anesthesia, as it does not support the strong interaction required for potency.

Considering these two factors together, the agent's high Blood: Gas partition coefficient indicates that it will accumulate slowly in the bloodstream, and the low oil: gas partition coefficient suggests a lack of potency. Therefore, the most accurate description of the agent is that it is lacking potency and accumulating slowly.