In plain language
The model does the thinking; the harness is everything that turns that thinking into real, controlled actions.
Example
A coding agent’s harness reads project files into context, lets the model run tests, asks permission before editing files, and repeats until the tests pass.
Related terms
AI Agent · Tool Use (Function Calling) · Context Window · Guardrails
Common misconception
Agent quality is not only about the model; the same model can perform very differently depending on the harness around it.
Implementation guidance
Give the harness clear tool definitions, sensible permission prompts, context management for long tasks, logging, and limits on cost and run time.