Hook Creator

Hook Creator

Create Claude Code hooks that execute shell commands at specific lifecycle events.

Hook Creation Workflow

1. Identify the use case - Determine what the hook should accomplish
2. Select the appropriate event - Choose from available hook events (see references/hook-events.md)
3. Design the hook command - Write shell command that processes JSON input from stdin
4. Configure the matcher - Set tool/event filter (use * for all, or specific tool names like Bash, Edit|Write)
5. Choose storage location - User settings (~/.claude/settings.json) or project (.claude/settings.json)
6. Test the hook - Verify behavior with a simple test case

Hook Configuration Structure

{
  "hooks": {
    "<EventName>": [
      {
        "matcher": "<ToolPattern>",
        "hooks": [
          {
            "type": "command",
            "command": "<shell-command>",
            "once": true
          }
        ]
      }
    ]
  }
}

Hook Types

• type: "command" - Execute shell command (default)
• type: "prompt" - LLM-based evaluation. Supported for: Stop, SubagentStop, UserPromptSubmit, PreToolUse, PermissionRequest

Hook Options

• once - Run only once per session (optional, default: false)

Common Patterns

Reading Input Data

Hooks receive JSON via stdin. Use jq to extract fields:

# Extract tool input field
jq -r '.tool_input.file_path'

# Extract with fallback
jq -r '.tool_input.description // "No description"'

# Conditional processing
jq -r 'if .tool_input.file_path
  then .tool_input.file_path else empty end'

Exit Codes for PreToolUse

• 0 - Allow the tool to proceed
• 2 - Block the tool and provide feedback to Claude

Matcher Patterns

• * - Match all tools
• Bash - Match only Bash tool
• Edit|Write - Match Edit or Write tools
• Read - Match Read tool

Quick Examples

Log all bash commands:

jq -r '"\(.tool_input.command)"' >> ~/.claude/bash-log.txt

Auto-format TypeScript after edit:

jq -r '.tool_input.file_path' | {
  read f
  [[ "$f" == *.ts ]] && npx prettier --write "$f"
}

Block edits to .env files:

python3 -c "
import json, sys
p = json.load(sys.stdin).get('tool_input', {}).get('file_path', '')
sys.exit(2 if '.env' in p else 0)
"

Prompt-Based Hooks

Use type: "prompt" for LLM-based evaluation. The prompt receives context and returns a decision.

Example - Stop hook with evaluation:

{
  "hooks": {
    "Stop": [
      {
        "hooks": [
          {
            "type": "prompt",
            "prompt": "Review if all user requirements are met. Context: {{context}}. Respond with JSON: {\"decision\": \"approve\" or \"block\", \"reason\": \"explanation\"}"
          }
        ]
      }
    ]
  }
}

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

Hooks in Skills/Agents/Commands

Define hooks directly in frontmatter for scoped lifecycle hooks:

---
name: secure-operations
description: Perform operations with security checks
hooks:
  PreToolUse:
    - matcher: "Bash"
      hooks:
        - type: command
          command: "./scripts/security-check.sh"
          once: true
---

Supported events in frontmatter: PreToolUse, PostToolUse, Stop

Hooks defined in Skills/Agents/Commands are scoped to that execution context and automatically cleaned up when finished.

Resources

• Hook Events Reference: See references/hook-events.md for detailed event documentation with input/output schemas

Triggers

This skill activates when users want to:

• Create a new hook for Claude Code
• Configure automatic formatting, linting, or code quality tools
• Set up logging or notification systems for tool usage
• Add file protection or access control rules
• Configure pre/post tool execution actions
• Set up session initialization or cleanup (SessionStart/SessionEnd)
• Auto-approve or deny permission requests (PermissionRequest)
• Understand hook events, input schemas, or exit codes

Anti-Patterns

Avoid these common mistakes when creating hooks:

• Blocking without feedback: Using exit code 2 in PreToolUse without providing stdout message leaves Claude confused about why the tool was blocked
• Ignoring stdin: Not reading JSON from stdin causes hooks to fail silently or behave unexpectedly
• Heavy synchronous operations: Running slow commands in PreToolUse blocks the entire workflow; prefer async logging or use PostToolUse
• Hardcoded paths: Using absolute paths like /home/user/... instead of $HOME or relative paths breaks portability
• Missing error handling: Not handling jq parse failures or missing fields causes cryptic errors
• Over-matching: Using * matcher when only specific tools need the hook wastes resources and may cause side effects

Extension Points

1. Custom event handlers: Add new hook configurations in settings.json for any of the 10 hook events
2. External integrations: Extend hooks to call external services (Slack, Discord, webhooks) via curl or custom scripts
3. Project-specific overrides: Layer project .claude/settings.json hooks over user ~/.claude/settings.json defaults
4. Script libraries: Create reusable shell scripts in project that hooks can call for complex logic

Design Rationale

Why JSON via stdin? Passing structured data through stdin allows hooks to access any tool input field without parsing command-line arguments. This is more flexible and handles special characters safely.

Why exit codes for control flow? Exit codes are the Unix-standard way to signal success/failure. Using 0/2 for PreToolUse allows simple shell commands to control tool execution without complex IPC.

Why separate PreToolUse and PostToolUse? Pre-hooks can block operations (validation, access control) while post-hooks handle side effects (formatting, logging). Mixing these concerns would complicate hook logic.

Why 10 distinct events? Each event represents a unique lifecycle moment with different input schemas and use cases. Fine-grained events allow precise hook targeting without over-triggering.