The hook system — shipped scripts that Claude Code runs on session and tool lifecycle events

Notes & tips

• Fail open on bad input. Exit 0 on any non-JSON / non-dict stdin; only a deliberate policy decision should exit 2.
• Only 2 blocks. Every other exit code allows the tool.
• Keep hooks fast. They run on the critical path under a timeout.
• One event name space. The event names (PreToolUse, …) are the Claude Code names.