Harness refers to the tooling that allows you to interact with an LLM. Web chat interface is a harness, CLI coding tools (claude code, codex, opencode, pi, etc.) are harnesses, agent systems (openclaw, hermes, etc.) are harnesses. They present different capabilities to the underlying LLM. Codex, for example, is more likely to write code if you say "I want to build an app" than ChatGPT on web will.