Chapter 4: The Agentic Loop

You've seen the loop flash by on screen — gather, act, verify. This chapter slows it down, takes it apart, and puts it back together in your hands. When you understand why the agent reads before it writes, when to demand a plan before any action, and how to review machine work with confidence, you stop being a passenger in your own sessions and start being the director.

This is the chapter where the two lanes converge: from here on, everyone is doing agentic engineering — just on different projects.

What you will learn

• Trace each phase of the agentic loop and know what good looks like in each.
• Use plan mode to separate thinking from doing on anything non-trivial.
• Read diffs and agent explanations well enough to catch problems early.
• Design verification the agent runs itself — and the final check only you can do.
• Run the full Builder Loop on a real project, start to finish.

Builder principle

The loop is the unit of work. Master one turn of it and scale is just repetition.

Watch with this chapter

• Mastering Claude Code in 30 Minutes — Boris Cherny. Third viewing; this time watch the loop, nothing else.
• Claude Code in Action — Anthropic's free official course. Excellent companion material for this whole track.

Real talk

This chapter tests whether a disciplined loop improves a bounded task: clear brief, reviewed plan, inspected work, and independent verification. The result still depends on the model, context, tools, task, and reviewer.

Lessons in this chapter

• 4.1 Inside the Loop: Gather, Act, Verify
• 4.2 Plan Mode: Think Before You Build
• 4.3 Reading the Agent's Work
• 4.4 Verification: Never Trust, Always Check
• 4.5 The Builder Loop, End to End

Chapter checkpoint

Complete Chapter 4: Builder Loop with a caught failure, save the evidence, then score your first attempt with the shared rubric.