There is no dress code for FBI agents, apart from the requirement that they look neat and professional in their working attire. The agents are free to dress comfortably and in a manner that doesn't interfere with their ability to perform.
FBI personnel typically do not have similar assigned roles, with some making arrests and others working to gather evidence, process lab work or testify in court. Agents making a raid may wear raid jackets, or others gathering evidence may wear cargo pants. A universal dress code would be difficult to implement and maintain, given the different daily activities.