Position: Agents Should Invoke External Tools ONLY When Epistemically Necessary

This position paper discusess Theory of Agent (ToA), a framework that redefines large language model agents as decision-makers who must choose between internal reasoning and external tool use. The authors argue that agents should only invoke external tools when epistemically necessary, meaning the task cannot be reliably solved using the model's existing internal knowledge and logic. This perspective addresses common failures like overthinking and overacting, which occur when an agent's internal solvability estimates are poorly calibrated. By treating reasoning and acting as co-equal methods for reducing uncertainty, the framework highlights that unnecessary delegation to tools can stagnate the growth...