REACT Loops & Harnesses

Days 1–2

The first two days of Week 2 introduce agentic systems from the inside, by building one.

Anatomy of an agentic system

• The core loop: reason, act, observe, repeat.
• Tools and actions: how an agent affects the world and reads back results.
• Stopping conditions and control flow.

Building a REACT loop

• Implement a basic REACT (reason + act) loop from scratch.
• Wire in a small set of tools and watch the loop use them.
• Inspect each step so the system's behavior stays legible to you.

Building a simple harness

• Wrap the loop in a harness that manages prompts, tools, and state.
• Add basic logging so you can see what the agent did and why.
• Establish a clean seam between the agent and the rest of your code.

Outcomes

You'll finish these two days with a working REACT loop and a simple harness you understand end to end — the substrate for the more capable systems built later in the week.