Claude Code Hooks Explained: Config Structure, Matchers, and PreToolUse Guards

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Claude Code hooks are deterministic shell commands or endpoints that trigger at fixed points in a session, allowing developers to block, allow, or modify tool calls beyond standard permission prompts. Hooks are configured in a settings.json file using three levels of nesting: a hook event, a matcher group, and a hook handler. Matchers filter when a hook fires and support regex testing, while the optional 'if' field narrows execution further using permission-rule syntax. Command hooks report decisions via exit codes or JSON on stdout, and developers must choose one approach per hook. Notably, the PreToolUse event now requires decisions inside a hookSpecificOutput object with a permissionDecision field, replacing the deprecated top-level decision format used by other events.

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