Lesson 19Automation

Hooks System

Lifecycle events

What are Hooks

Hooks are shell commands that automatically execute on certain events in Claude Code.

Use cases

• Auto-format after Edit
• Bash command validation
• Slack notifications
• Action logging

Limitations

• Execute synchronously
• 60 second timeout
• Can block operations

Hook Events

SessionStart

When Claude Code session starts

Environment init, checks

SessionEnd

When session ends

Cleanup, send metrics

PreToolUse

Before tool is called

Validation, logging, blocking

PostToolUse

After tool is called

Formatting, post-processing

UserPromptSubmit

When user sends a message

Filtering, routing

Notification

On notifications from Claude

External alerts (Slack, email)

Stop

When Claude stops

Handle task completion

Example: Auto-formatting

Automatic file formatting after editing:

// .claude/hooks/format-on-edit.json
{
  "hooks": {
    "PostToolUse": [
      {
        "name": "auto-format",
        "match": {
          "tool": "Edit",
          "path": "**/*.{ts,tsx,js,jsx}"
        },
        "command": ["npx", "prettier", "--write", "{{path}}"]
      }
    ]
  }
}

💡{{path}} is replaced with the path to the edited file

Example: Blocking Dangerous Commands

PreToolUse can block operations by returning exit code 1:

// Блокировка опасных команд
{
  "hooks": {
    "PreToolUse": [
      {
        "name": "block-dangerous-commands",
        "match": {
          "tool": "Bash"
        },
        "command": ["bash", "-c", "
          if echo '{{command}}' | grep -qE 'rm -rf|drop database'; then
            echo 'BLOCKED: Dangerous command detected' >&2
            exit 1
          fi
        "]
      }
    ]
  }
}

Exit code 0

Operation continues

Exit code 1

Operation is blocked

Hook Configuration

Hooks are configured in .claude/hooks/*.json files:

.claude/hooks/
├── format.json      # Auto-formatting hooks
├── security.json    # Security validation hooks
├── notify.json      # Notification hooks
└── logging.json     # Logging hooks

nameHook name for logging

matchTrigger conditions (tool, path)

commandCommand to execute

Hook Best Practices

1.Keep hooks fast — they block the main thread
2.Use match for precise triggering
3.Log hook errors for debugging
4.Test hooks locally before committing

This lesson is part of a structured LLM course.

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