This depends greatly on the field of meaning you are looking for.In general language, an agent is someone who works for an agency - the agency is the group or the company, the agents are (some of) the people who work there.In linguistics, agency is the quality or characteristic held by an agent. that is, if you are an agent, you have agency.In other fields it has other meanings.It id often difficult to differentiate words in English, because it has such poor morphology. the best tip I can give you in this case is that the -y suffix suggests that this may be an abstract noun. I the same way literacy is the abstract quality of being able to read. Proficiency is the abstract quality of being proficient. Piracy is the abstract quality of being a pirate or the abstract noun that is a pirate's activity. Congruency is the abstract quality of being congruent, and so forth. However, this doesn't work all of the time, Facility is not an abstract noun, and the -y in that case is not a suffix.