/hook-development

name: Hook Development
description: This skill should be used when the user asks to "create a hook", "add a PreToolUse/PostToolUse/Stop hook", "validate tool use", "implement prompt-based hooks", "use ${CLAUDE_PLUGIN_ROOT}", "set up event-driven automation", "block dangerous commands", or mentions hook events (PreToolUse, PostToolUse, Stop, SubagentStop, SessionStart, SessionEnd, UserPromptSubmit, PreCompact, Notification). Provides comprehensive guidance for creating and implementing Claude Code plugin hooks with focus on advanced prompt-based hooks API.
version: 0.1.0

Hook Development for Claude Code Plugins

Overview

Hooks are event-driven automation scripts that execute in response to Claude Code events. Use hooks to validate operations, enforce policies, add context, and integrate external tools into workflows.

**Key capabilities:**

• Validate tool calls before execution (PreToolUse)
• React to tool results (PostToolUse)
• Enforce completion standards (Stop, SubagentStop)
• Load project context (SessionStart)
• Automate workflows across the development lifecycle

Hook Types

Prompt-Based Hooks (Recommended)

Use LLM-driven decision making for context-aware validation:

{
  "type": "prompt",
  "prompt": "Evaluate if this tool use is appropriate: $TOOL_INPUT",
  "timeout": 30
}

**Supported events:** Stop, SubagentStop, UserPromptSubmit, PreToolUse

**Benefits:**

• Context-aware decisions based on natural language reasoning
• Flexible evaluation logic without bash scripting
• Better edge case handling
• Easier to maintain and extend

Command Hooks

Execute bash commands for deterministic checks:

{
  "type": "command",
  "command": "bash ${CLAUDE_PLUGIN_ROOT}/scripts/validate.sh",
  "timeout": 60
}

**Use for:**

• Fast deterministic validations
• File system operations
• External tool integrations
• Performance-critical checks

Hook Configuration Formats

Plugin hooks.json Format

**For plugin hooks** in `hooks/hooks.json`, use wrapper format:

{
  "description": "Brief explanation of hooks (optional)",
  "hooks": {
    "PreToolUse": [...],
    "Stop": [...],
    "SessionStart": [...]
  }
}

**Key points:**

• `description` field is optional
• `hooks` field is required wrapper containing actual hook events
• This is the **plugin-specific format**

**Example:**

{
  "description": "Validation hooks for code quality",
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Write",
        "hooks": [
          {
            "type": "command",
            "command": "${CLAUDE_PLUGIN_ROOT}/hooks/validate.sh"
          }
        ]
      }
    ]
  }
}

Settings Format (Direct)

**For user settings** in `.claude/settings.json`, use direct format:

{
  "PreToolUse": [...],
  "Stop": [...],
  "SessionStart": [...]
}

**Key points:**

• No wrapper - events directly at top level
• No description field
• This is the **settings format**

**Important:** The examples below show the hook event structure that goes inside either format. For plugin hooks.json, wrap these in `{"hooks": {...}}`.

Hook Events

PreToolUse

Execute before any tool runs. Use to approve, deny, or modify tool calls.

**Example (prompt-based):**

{
  "PreToolUse": [
    {
      "matcher": "Write|Edit",
      "hooks": [
        {
          "type": "prompt",
          "prompt": "Validate file write safety. Check: system paths, credentials, path traversal, sensitive content. Return 'approve' or 'deny'."
        }
      ]
    }
  ]
}

**Output for PreToolUse:**

{
  "hookSpecificOutput": {
    "permissionDecision": "allow|deny|ask",
    "updatedInput": {"field": "modified_value"}
  },
  "systemMessage": "Explanation for Claude"
}

PostToolUse

Execute after tool completes. Use to react to results, provide feedback, or log.

**Example:**

{
  "PostToolUse": [
    {
      "matcher": "Edit",
      "hooks": [
        {
          "type": "prompt",
          "prompt": "Analyze edit result for potential issues: syntax errors, security vulnerabilities, breaking changes. Provide feedback."
        }
      ]
    }
  ]
}

**Output behavior:**

• Exit 0: stdout shown in transcript
• Exit 2: stderr fed back to Claude
• systemMessage included in context

Stop

Execute when main agent considers stopping. Use to validate completeness.

**Example:**

{
  "Stop": [
    {
      "matcher": "*",
      "hooks": [
        {
          "type": "prompt",
          "prompt": "Verify task completion: tests run, build succeeded, questions answered. Return 'approve' to stop or 'block' with reason to continue."
        }
      ]
    }
  ]
}

**Decision output:**

{
  "decision": "approve|block",
  "reason": "Explanation",
  "systemMessage": "Additional context"
}

SubagentStop

Execute when subagent considers stopping. Use to ensure subagent completed its task.

Similar to Stop hook, but for subagents.

UserPromptSubmit

Execute when user submits a prompt. Use to add context, validate, or block prompts.

**Example:**

{
  "UserPromptSubmit": [
    {
      "matcher": "*",
      "hooks": [
        {
          "type": "prompt",
          "prompt": "Check if prompt requires security guidance. If discussing auth, permissions, or API security, return relevant warnings."
        }
      ]
    }
  ]
}

SessionStart

Execute when Claude Code session begins. Use to load context and set environment.

**Example:**

{
  "SessionStart": [
    {
      "matcher": "*",
      "hooks": [
        {
          "type": "command",
          "command": "bash ${CLAUDE_PLUGIN_ROOT}/scripts/load-context.sh"
        }
      ]
    }
  ]
}

**Special capability:** Persist environment variables using `$CLAUDE_ENV_FILE`:

echo "export PROJECT_TYPE=nodejs" >> "$CLAUDE_ENV_FILE"

See `examples/load-context.sh` for complete example.

SessionEnd

Execute when session ends. Use for clean