The antonyms for the phrase "are agent for" could include "are not agent for," "are against," "are opposed to," or "are unrelated to." These antonyms suggest a lack of involvement or representation in a particular situation. For example, if a company is not an agent for a product, they would not be responsible for its sales or marketing. If someone is against a particular policy, they would not serve as an agent for its implementation. Similarly, if an individual is not involved with a particular organization or cause, they would be unrelated to it and not act as an agent for its purposes.