The agentic loop
A basic chat completion is strictly reactive: it accepts an input prompt, generates a continuation, and immediately terminates. In that single interaction, the model cannot run a build, inspect a filesystem, or verify whether its code compiles.
Comparing harnesses
Every harness runs the same loop. What separates them is where they run, what they will let the model do before asking you, and what you can bolt onto them. A demo will not tell you any of that. It comes out three weeks later, when the tool has to run in a pipeline, or point at a model the vendor did not sell you, or explain what it did to a file nobody expected it to touch.