tl;dr: An agent harness is software that wraps an AI model to create an agent, typically providing four things: a system prompt, a set of callable tools, an "agentic loop" letting the model iteratively decide actions, and a translation layer for swapping between different models. Unlike the models themselves, users can own, modify, and run harnesses locally—preserving agency, choice, and portability across providers. The author pitches Pi, their open-source harness, as a neutral alternative to lab-controlled apps like Claude Code.