I think "harness" is a thing, the code/binary, and "agent" is a process, an instantiated run of that code/binary with a given LLM/env, etc.

So its like, "GTA 6" as the disc vs. the specific game you're in the middle of being chased by cops, harness vs agent.

In practice they're intertwined and it becomes hard not to use the terms somewhat interchangeably, but "you ask the agent how the harness works" vs. the other way around, clearly.