Claude Hooks
Shell commands Claude Code runs automatically at lifecycle events (before/after a tool call, on session start, on stop) to enforce policies or automate work.
Hooks are configured in ~/.claude/settings.json (global) or .claude/settings.json (per-project) under the hooks key. Each entry has a matcher (event name + tool pattern) and a command. The runtime executes the command synchronously and waits for it to finish before continuing.
Common patterns: auto-format on PostToolUse Edit (run biome/prettier after every file edit), block destructive commands on PreToolUse Bash (exit 1 if you spot rm -rf), inject context on SessionStart (cat docs/CONTEXT.md), notify Slack on Stop (curl webhook). A PreToolUse hook that exits with code 1 blocks the tool call, and the error message is surfaced to Claude so it can adjust its plan.
Hooks run with your user's permissions, they can read, write and execute anything you can. Keep them fast (a few seconds max), since a hung hook hangs Claude.