# Overview Hooks as first-class policy units and why HooKit exists. HooKit exists to make agent behavior governable by the people running it. The central abstraction is the **Hook**: a small, inspectable policy unit that subscribes to a Pi **Event**, runs a shell decision, and may own one outcome-selected Pi **Action**. Hooks live in your `.pi/hookit.json` **Hook Catalog**, come from a **Hook Source** (`local` or `owner/repo`), and produce immutable **Hook Results**, so a policy is always a plain, reviewable record rather than woven into the agent. Three properties make Hooks useful as policy units: **They are explicit and inspectable.** A Hook is `event` + `filter` + a shell decision, written in ordinary JSON. You can read every policy your session enforces, see exactly what it matched, and change it without touching Pi itself. There is no hidden policy layer. **They live at tight feedback loops.** Hooks subscribe at multiple points in a Pi lifecycle—before a tool runs, after a tool returns, at the end of a turn, when the agent settles, and before session changes that can still be cancelled. That means a policy can act *before* damage (block a tool call), *after* an operation (patch a result), and around the workflow (require a clean worktree each turn). The earlier and closer the decision, the less the agent does that a policy must undo. **Their decisions are observable.** Every Hook Evaluation can produce an Execution Report in the transcript: a flat, ordered record of which policies ran, what they decided, and how long they took. Policy is not judged by faith—you read the decision. The rest of the Concepts section explains the two ideas that shape how Hooks behave: - the [Hook Evaluation model](evaluation.md)—the exact sequence from Event to Hook Result Event, and the distinction between a Hook's individual decision and the Event's aggregate outcome; - the [composition model](composition.md)—how Presets, owned Actions, and `hook_result` combine policies, and where composition deliberately stops. And because Hooks execute shell commands, the [security page](security.md) states precisely what HooKit isolates and what remains trusted code.