A Skill Is Not a Tool
Chapter 1 said a skill is like a line on a resume. For that line to hold up, we first have to pull skills out of the tool pile. Skills and tools are not the same kind of thing, and as long as they're lumped together, an agent's capabilities can't be described clearly.
A Tool Is a Call, a Skill Is a Capability
A tool is a call. Given an input, it returns an output — stateless, discarded after use. Checking the weather, sending a request, running a computation: all tools. A tool is the hand an agent uses to reach the outside world. It does not, by itself, constitute knowing how to do anything.
A skill is a capability. It packages the methods, procedures, and knowledge needed to do a whole class of work. It has a name, it can be reused, and it can be moved from one agent to another without much loss. It behaves the way skills behave in a person: someone who can build slide decks can still build slide decks at their next company. The capability travels with the person, not with any single task.
The difference is the difference between "can use a screwdriver" and "can renovate a house." The screwdriver is a tool; renovation is a skill. A renovator obviously uses plenty of tools, but when you hire one, you're hiring the ability to renovate — not the possession of a screwdriver.
An Agent's Capabilities Are Defined by Its Skills
Once this distinction takes hold, you look at agents differently.
An agent may have access to many tools — it can query databases, send email, run scripts. But that's just the toolbox at its elbow. It doesn't tell you what the agent can actually do. What defines its capabilities are the skills it holds: it can reconcile accounts, write weekly reports, review code. Tools are the hands; skills are the craft.
So "adding a capability" to an agent, most of the time, does not mean wiring up another tool. It means distilling a skill — turning "how this class of work gets done" into a nameable, reusable unit. That is when the agent's capability actually grows. The first book's treatment of portability already made the case that skills migrate across agents without falling apart; within the abstraction of "a person," a skill's place is the craft — the line you can actually write on a resume.
Where does craft come from? Either the company teaches it, which is onboarding training, or it was earned earlier and travels with the worker, which is a professional certification.