# 16.2 Choosing the right piece (/building-ai-workers/skills-and-plugins/choosing-the-right-piece)

---
type: Document
title: "16.2 Choosing the right piece"
description: "A built-in tool, your own code, a skill, a connector, a helper agent, or a hook or setting: what each is for, what it costs, and why a skill guides while a hook or setting enforces."
status: stable
order: 316.02
ksor:
  owner: team:panaversity
  audience: [ public ]
  approval:
    by: process:panaversity
    at: 2026-11-02T20:04:18Z
chapter: "16"
part: III
expert_status: co-build
concepts: [ "16.2" ]
last_verified: 2026-11-02
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  at: 2026-11-02T20:04:18Z
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**In everyday life.** A kitchen has a stove that came with the house and a knife you sharpened yourself. It has a recipe card and a delivery service for what you do not keep at home. It has an assistant cook for a side dish and a lock on the cabinet that holds the cleaning chemicals. Each does one kind of job. You would not write "do not open the chemicals cabinet" on a recipe card and call it locked.

**What you will learn.** How to decide what each new job for an agent needs. It may need what is already there, your own code, written instructions, a link to another system, a helper, or a rule that cannot be ignored.

**Why it matters.** Each piece does one kind of job and costs something different. Put a rule in the wrong piece, and it works only until the day the agent ignores it.

### Which pieces can I choose from?

Six. No single standard defines this list, because it covers several standards and every product's own features.

| Piece | What it is | Use it for | Who can change it |
| --- | --- | --- | --- |
| **Built-in tool** | A tool the agent already has: read a file, search, run a command | Anything the agent can already do well | The product's maker |
| **Custom tool** | Code you write and test, which the agent runs | Calculations, checks, anything that must be exact every time | You, through a reviewed change |
| **Skill** | Instructions and files that load when the work matches ([16.1](from-a-prompt-to-a-skill.md)) | A procedure with judgment in it: which steps, in what order, what to report | Whoever edits the skill |
| **Connector** | A server that gives the agent tools and data from another system, through the Model Context Protocol (MCP), an open standard | Systems the agent cannot reach on its own: an accounting system, a database, a shared drive | The connector's author |
| **Subagent** | A helper agent with its own context that reports back | A side task whose reading would crowd the main session | You |
| **Hook** or **setting** | A rule the product enforces before or after an action | Things that must never happen, whatever the instructions say | The person who controls the settings |

### What is the difference between a skill and a hook?

A skill guides. A hook or setting enforces. A skill is words. An agent usually follows them, and sometimes does not. A **hook** is code the product runs at a fixed moment, such as before every file edit, and it can block the action. A permission **setting** does the same without code. So "never edit the AP export" belongs in a setting that denies edits to `data/`, not only in a skill's "Never" list. Chapter 23 teaches hooks in depth.

### Where do calculations go?

Into tested code, never into a skill's words. This is the rule for every number from [Chapter 13](../programming-fundamentals/functions.md): an AI can reason about a problem, but deterministic code, which gives the same answer every time, enforces rules and arithmetic. A skill that says "add up the amounts" or "calculate the due date" asks the model to do the arithmetic itself. A skill that says "run the triage script and copy its totals" does not. So most good skills have a script beside them. The skill's job is to say when to run it and how to report what it printed. The number in that code is still a copy of a rule its owner decides ([16.6](shipping-a-version.md)).

### What does each piece cost?

A built-in tool costs nothing extra. A skill costs its short description on every turn. A connector costs its tool names or descriptions on every turn, depending on the product, and a running process. A subagent costs a second context. Choose the cheapest piece that does the job reliably.

### How do I choose?

Ask these questions in order, and stop at the first yes. If a job has parts, ask for each part. The Monday report's totals are tested code, and its steps are a skill.

![A decision ladder of six questions, asked in order. Must it never happen? Then a hook or setting. Is the data in another system? Then a connector. Must a number be calculated exactly? Then your own tested code. Is it side work that would crowd the session? Then a subagent. Can the agent already do it well? Then a built-in tool. Otherwise, it is a procedure with judgment: a skill.](img/choosing-the-piece.png)

*Figure 16.4. Choosing the piece. Reading a long contract is side work, so it goes to a subagent, even though the agent could read it in the main session.*

**At work.** Ana and the builder place six AP jobs, which means they choose a piece for each:

| Job | Piece | Why |
| --- | --- | --- |
| Read the weekly invoice export | Built-in tool | The agent can already read a file |
| Compute due dates | Custom tool: `src/due-date.ts` | A number people act on, so tested code |
| Sort invoices into piles and report | Skill, which runs a script | A procedure with steps and a report shape, with the sorting in code |
| Look up a vendor in the accounting system | Connector | Another system. Chapter 17 builds it |
| Never edit `data/` | Setting | It must work even if instructions are ignored |
| Pause before scheduling a payment | Hook | An approval gate. Chapter 23 builds it |

**The operating rule:** use the cheapest piece that is reliable enough. Put judgment in a skill, numbers in tested code, other systems behind a connector, and every "never" that must hold in a hook or setting.

**Practice it.** [Practice: choosing the right piece](both-ai-vendors.md#practice-choosing-the-right-piece), on your own, ten minutes.