the model alone is not the agent. The harness plus the model is the agent.

An agentic harness is the orchestration and control layer wrapped around a base language model that transforms it from a stateless text predictor into an agent capable of taking actions, calling tools, maintaining state across steps, and executing multi-step tasks. The model provides raw capability; the harness provides the structure that turns that capability into coordinated behavior. Different harnesses wrapping the same model produce materially different agent behavior, which is why harness design is considered a discipline in its own right.

What a harness typically contains
A system prompt defining the agent's role and boundaries

A tool schema and invocation loop (function calling, API access, code execution)

A memory layer, short-term through the context window and often long-term through an external store

Orchestration logic for multi-step or multi-agent flows

Verification or reflection steps between actions

Error handling, retries, and termination conditions

Input and output format enforcement

Examples from the field
ReAct (Yao et al., 2022): a harness pattern that interleaves reasoning traces and action calls in a loop, letting the model decide when to think and when to act.

Claude Computer Use: a harness that wraps a language model with screenshot capture, mouse and keyboard simulation, and a perception and action loop for controlling a desktop.

OpenAI Assistants runtime: a managed harness around the OpenAI models that handles thread persistence, file retrieval, code interpreter sessions, and function calling.

Devin (Cognition): a tightly engineered harness combining a planning module, a browser, a code editor, and a shell, all driven by an underlying model.

LangGraph: a graph-based harness where nodes are model calls or tools and edges encode the control flow, letting the developer define the agent's reasoning topology explicitly.

The defining property across all of them: the model alone is not the agent. The harness plus the model is the agent.

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