Build Your AgenticOS: Hooks Automate the Invisible

Hooks wire your AgenticOS to session events. They're the automation layer that removes the work you'd otherwise have to remember to ask for.

Memory tells agents what to remember. Hooks make the system act without being asked.

Every session, there is overhead you pay without thinking about it: describing the current git state, establishing which project is active, noting which deployment is live. None of it is hard. All of it is unnecessary. You know the system should have that context automatically. You keep meaning to set it up. Instead, you type it again.

Hooks are the automation layer that fixes this. They are shell commands wired to session events (SessionStart, PreToolUse, PostToolUse, Stop) and their output flows directly into the agent's context. No prompt required. The agent sees the hook output the same way it sees anything else you write. It just happened without you.

This post covers what hooks are, where they live, the event model, Glenn's real examples from .claude/settings.json, how to design a hook that works without surprising you, and the clean distinction between hooks, skills, and CLAUDE.md.