Overview
A language model by itself accepts text and returns text. An agentic system emerges when that model is placed inside a harness that lets it observe an environment, select tools, take actions, receive results, retain useful state, and continue through a multi-step loop.
The harness is everything around the model that is not the model itself. It can include system instructions, project instructions, tools, hooks, memory, files, permission rules, context-compaction behavior, turn limits, token limits, execution environments, and mechanisms for observing or verifying outcomes. The model supplies general intelligence. The harness focuses that intelligence on a particular environment and determines what the model can actually do there.