first cause vs causal agent

first cause and causal agent both are nouns.

Word	Noun	Adjective	Verb	Adverb
first cause	Yes	No	No	No
causal agent	Yes	No	No	No

As nouns, causal agent is a hypernym of first cause; that is, causal agent is a word with a broader meaning than first cause:

first cause: an agent that is the cause of all things but does not itself have a cause
causal agent: any entity that produces an effect or is responsible for events or results

Other hypernyms of first cause include causal agency, cause.

first cause (noun)	causal agent (noun)
an agent that is the cause of all things but does not itself have a cause	any entity that produces an effect or is responsible for events or results