What an 'Agentic Harness' Actually Is and Why It Matters for AI Agents

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The term 'agentic harness' is gaining traction in AI development circles, but its meaning is often misunderstood as simply referring to the IDE or app running an agent. Drawing on Simon Willison's definition, an AI agent is an LLM equipped with tools and running in a loop to achieve a goal. The harness refers to the underlying infrastructure that governs this loop, determining whether the agent continues working or considers its task complete. Crucially, the harness is separate from the user interface, meaning developers can swap front-end interfaces without altering the core agent logic. Some agents, in fact, require no user interface at all, making the harness the invisible but essential layer driving autonomous AI behavior.

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