The harness is the agent. LLM's can be asked to output things in JSON for example. The LLM then literally asks for things like "execute this cmd" or search/replace this string. The LLM outputs text, but in a deterministic format that can be parsed. The harness calls the LLM, exposes tools, executes tools the LLM asks for, gates tool use based on security controls. It's the runtime that the agent uses to do work.
The LLM is the core model, but the harness has the prompts/tool definitions, guidance/recovery/correction code. The harness itself is the agent, because same model may perform vastly differently on different harnesses. Agent is the system working as a whole, harness+llm.
LLM is just the thing generating text. It can generate a book, an article, a paper or a conversation. It's the raw "autocomplete" mechanism. You give it input, it generates output.
The agent/harness is the sotware that leverage this "dumb" autocompletion engine to do useful things by sending the good input to the model and doing useful things with the output.
LLM is the digital brain. Agent is a software robot with "sensors" (tooling to read data from files/web) and "actuators" (tooling to perform changes in the environment) that is based on the digital brain.
think of it like the words "car" and "engine", one goes into the other and both have their impact on overall capacity, feel, performance etc. each in their own way. The llm is an engine, the harness is the rest of the car.
