When you go to the command line and type “Claude”, there is an LLM, and everything else is the harness

It’s the tool that calls the model, give it access to the local file system, calls the actual tools and commands for the model, etc, and provide the initial system prompt.

Basically a clever wrapper around the Anthropic / OpenAI / whatever provider api or local inference calls.

pi vs. claude code vs. codex These are all agent harnesses which run a model (in pi's case, any model) with a system prompt and their own default set of tools.