hook-developer

# Hook Developer Complete reference for developing Claude Code hooks. Use this to write hooks with correct input/output schemas. ## When to Use - Creating a new hook - Debugging hook input/output format - Understanding what fields are available - Setting up hook registration in settings.json - Learning what hooks can block vs inject context ## Quick Reference | Hook | Fires When | Can Block? | Primary Use | |------|-----------|------------|-------------| | **PreToolUse** | Before tool executes | YES | Block/modify tool calls | | **PostToolUse** | After tool completes | Partial | React to tool results | | **UserPromptSubmit** | User sends prompt | YES | Validate/inject context | | **PermissionRequest** | Permission dialog shows | YES | Auto-approve/deny | | **SessionStart** | Session begins | NO | Load context, set env vars | | **SessionEnd** | Session ends | NO | Cleanup/save state | | **Stop** | Agent finishes | YES | Force continuation | | **SubagentStart** | Subagent spawns | NO | Pattern coordination | | **SubagentStop** | Subagent finishes | YES | Force continuation | | **PreCompact** | Before compaction | NO | Save state | | **Notification** | Notification sent | NO | Custom alerts | **Hook type options:** `type: "command"` (bash) or `type: "prompt"` (LLM evaluation) --- ## Hook Input/Output Schemas ### PreToolUse **Purpose:** Block or modify tool execution before it happens. **Input:** ```json { "session_id": "strin...