Birgitta Böckeler, writing on Martin Fowler's site, sorted everything in a harness along two lines. They are the two most useful distinctions in this chapter. The first is about when a control acts.
Guide
Also called Feedforward control
A control that acts before the agent does, steering it towards the right thing: an instructions file, a skill, a code template, an example to copy.
A good guide means the agent gets it right first time, which is cheaper than being told it got it wrong. But a guide is only advice. The agent can read it and do something else.
Say it to your agent
“Add a guide for this: in AGENTS.md say that all database access goes through src/db/, and point to src/db/users.ts as the example to copy.”