What makes an agent?

Everything you've built so far runs on rails you laid: the triage assistant classifies, the RAG pipeline retrieves-then-answers — you decided the steps; the model filled them in. An agent crosses one line: the model decides the next step. It reads the situation, picks a tool, observes the result, and picks again — looping until the task is done or a limit says stop.

PIPELINE (you decide the steps):
  input → [step you chose] → [step you chose] → output

AGENT (the model decides, inside limits you set):
  goal → ┌────────────────────────────────┐
         │ model: think → pick tool → act │──▶ done?  → result
         │        ▲ observe result ◀──────│
         └────────────────────────────────┘
         within: allowed tools, budgets, approval gates

What changes when the model steers

• Capability jumps. Tasks with branching ('look up the order — if it shipped, check the carrier; if not, check inventory') stop needing you to enumerate every branch. The model navigates.
• Variance jumps with it. A pipeline fails the same way twice; an agent can find novel paths to success and novel paths to failure. Run the same task twice, get two different trajectories — both maybe fine, maybe not.
• The unit of review changes. You stop reviewing answers and start reviewing trajectories — sequences of decisions and actions. Module 6 is built on this.
• Actions have consequences. A wrong answer misinforms; a wrong action refunds the wrong customer. The engineering weight shifts from output quality to action safety — which is why Module 5 is the heart of this course, not an appendix.

The autonomy dial (agents aren't binary)

Between 'pipeline' and 'fully autonomous' lies a dial you control per action: some tools the agent uses freely (read-only lookups), some it must ask before using (refunds), some it can only propose (account deletion). Setting this dial deliberately — per tool, per stakes — is most of what 'agent design' means in production. The reckless version isn't 'building agents'; it's leaving the dial at max everywhere. (You'll implement this dial in code in Module 5.)