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",

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