A complete Agent Harness methodology:
- The model is responsible for understanding, judgment, and planning;
- Skills are responsible for domain workflows;
- Tools connect to real systems;
- The UI is expressed through Tool Calls;
- The Harness enforces safety boundaries;
- Evals turn product requirements into automatically verifiable tests.
The core of this methodology is to split the Agent into two worlds:
- The model is responsible for handling open-ended, fuzzy problems that require judgment;
- The Harness is responsible for handling deterministic, sensitive problems that must be reliable.
In other words, the model is the “cognition and decision-making layer,” while the Harness is the “runtime and control layer.” Skills, Tools, UI, and Evals respectively solve the problems of knowledge, execution, interaction, and quality control.
What matters most here is not any single component, but the division of responsibilities.