Harness Engineering: Why the System Around Your AI Agent Matters More Than the Model

Com In 2026, the AI engineering community discovered something counterintuitive: the model is the least important part of an AI agent. What actually determines whether an agent succeeds or fails in production is everything around the model - the tools it can access, the guardrails that keep it safe, the feedback loops that help it self-correct. This "everything around the model" now has a name: the harness. And the discipline of building it is called harness engineering. OpenAI's Codex team used harness engineering to ship over 1M lines of production code written entirely by AI agents.