'Agent' refers to a person, while an agency a company or organization. However, you wouldn't call just anyone an 'agent'. You might call someone who works for security organizations like the United State's CIA or FBI 'agents', but you wouldn't call an engineer an 'agent'.
If you've watched The Matrix (1999), one of the antagonists was called "Agent Smith", partly because the description of "Agent" gives the feeling that Smith worked for a large, mysterious agency.
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Has difficulty understanding even short answers in this language.
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