Agent = Model + Harness.
If you're not building the model, you're building the harness. That's Claude Code. Hermes Agent. Codex. Cursor. Pick the layer that matches the work.
If you can't draw the line between these four, you will:
buy a chatbot and call it an agent
buy an assistant and expect completed work
build an agent and skip the harness, then wonder why it loops at 3am
The video is 80 seconds. The line is the one sentence.
The rest is just making you believe it.