agento

The seam that is not there. Agi means to act, and agento is somebody who acts for you, so the word looks like ag- plus a suffix -ent- meaning the doer. It is not. Esperanto has no -ent- suffix. Not one. The letters are inside the borrowed root and stay there.

A whole shelf of words is built the same way. Latin present participles came into the international vocabulary as lumps, and Esperanto took them as lumps: torento a torrent, studento a student, kliento a client, momento a moment, akcidento an accident, talento a talent, prezidento a president, parlamento a parliament. Try to split any of them and you get a root that means nothing.

What Esperanto actually builds from agi. The real derived word for one who is acting is aganto, from the -ant- participle, and it means precisely that: the person doing the action right now, in a grammar sentence or a legal one. Agento is a job. The gap between the two is the whole point. Aganto is whoever happens to be acting; agento is somebody commissioned by another party to act on their behalf, which is why the word turns up in commerce (komerca agento), insurance (asekura agento) and espionage (sekreta agento).

The neighbours. A spiono is a spy and nothing else, so sekreta agento and spiono overlap without being synonyms: the agent has an employer, the spy has a target. Peranto is a go-between or broker, built honestly from peri, to mediate. Reprezentanto is a representative in the formal sense, the one who represents a body at a congress. An oficisto simply works in an office. Agentejo is the agency, with the ordinary place suffix doing ordinary work.