What is an agent harness?
An agent harness is the surrounding engineering scaffolding built around the core model-and-tool-calling loop.
“Harness is sort of the surrounding support that we add onto that core model and tool-calling loop.”
LangChain explains that an agent harness is the out-of-the-box engineering support wrapped around the basic model-plus-tools loop, handling complexity across all stages of the loop. It's a useful conceptual framing for agent builders but is educational rather than a major industry-moving announcement.