What Is an AI Agent Harness and How Does It Work?

An AI agent harness is a software environment that wraps around a large language model, supplying it with tools and context to complete multi-step tasks. It operates on a reason-act-observe loop, where the model decides an action, a tool executes it, and the model evaluates the result before continuing. This cycle repeats until the task is fully completed, combining model reasoning with external tool execution. A practical implementation involves defining available tools such as a calculator, file reader, or word counter, along with a system prompt guiding the model's behavior. The concept draws from the ReAct framework, a published research approach that formalizes the synergy between reasoning and acting in language models.