Creating Claude Hooks

Creating Claude Code Hooks

Use this skill when creating, improving, or publishing Claude Code hooks. Provides essential guidance on hook format, event handling, I/O conventions, and package structure.

When to Use This Skill

Activate this skill when:

• User asks to create a new Claude Code hook
• User wants to publish a hook as a PRPM package
• User needs to understand hook format or events
• User is troubleshooting hook execution
• User asks about hook vs skill vs command differences

Quick Reference

Two Hook Configuration Methods

Method 1: JSON Configuration (Recommended)

• Configure in .claude/settings.json, ~/.claude/settings.json, or plugin's hooks.json
• Supports both command hooks and prompt hooks
• More flexible, supports matchers and timeouts

Method 2: Executable Files (Legacy)

• Place executables in .claude/hooks/<event-name>
• Simpler but less configurable

Hook Types

Available Events

Exit Codes

JSON Hook Configuration

Settings-Based Hooks

Configure hooks in .claude/settings.json (project) or ~/.claude/settings.json (global):

{
  "hooks": {
    "PreToolUse": [{
      "matcher": "Write|Edit",
      "hooks": [{
        "type": "command",
        "command": "/path/to/validate-write.sh",
        "timeout": 5000
      }]
    }],
    "PostToolUse": [{
      "matcher": "Write|Edit",
      "hooks": [{
        "type": "command",
        "command": "/path/to/format-file.sh"
      }]
    }],
    "Stop": [{
      "matcher": "*",
      "hooks": [{
        "type": "prompt",
        "prompt": "Verify all requested changes were completed."
      }]
    }]
  }
}

Plugin hooks.json

For PRPM packages, use hook.json:

{
  "hooks": {
    "PreToolUse": [{
      "matcher": "Write",
      "hooks": [{
        "type": "command",
        "command": "${CLAUDE_PLUGIN_ROOT}/scripts/validate.sh",
        "timeout": 5000
      }]
    }]
  }
}

Matcher Patterns

Hook Options

{
  "type": "command",
  "command": "./my-hook.sh",
  "timeout": 5000,
  "once": true,
  "continue": true,
  "stopReason": "Message when blocked",
  "suppressOutput": false,
  "systemMessage": "Warning to show user"
}

Command Hooks

Command hooks run external scripts. They're fast and deterministic.

Shell Script Hook

#!/bin/bash
set -euo pipefail

# Read JSON input
INPUT=$(cat)
FILE=$(echo "$INPUT" | jq -r '.input.file_path // empty')

# Validate
[[ -n "$FILE" ]] || exit 0
[[ -f "$FILE" ]] || exit 0

# Block sensitive files
case "$FILE" in
  *.env|*.pem|*.key)
    echo "Blocked: $FILE is sensitive" >&2
    exit 2
    ;;
esac

exit 0

TypeScript Hook

#!/usr/bin/env node
import { readFileSync } from 'fs';

const input = JSON.parse(readFileSync(0, 'utf-8'));
const filePath = input.input?.file_path;

if (!filePath) process.exit(0);

// Block .env files
if (filePath.endsWith('.env')) {
  console.error('Blocked: Cannot modify .env files');
  process.exit(2);
}

process.exit(0);

Prompt Hooks

Prompt hooks use LLM reasoning for complex validation. Use sparingly - they take 2-10 seconds.

Basic Prompt Hook

{
  "hooks": {
    "PreToolUse": [{
      "matcher": "Write",
      "hooks": [{
        "type": "prompt",
        "prompt": "Check if the content being written contains hardcoded secrets, API keys, or credentials. If found, block the operation."
      }]
    }]
  }
}

Prompt Hook with Schema Validation

{
  "hooks": {
    "PreToolUse": [{
      "matcher": "Write|Edit",
      "hooks": [{
        "type": "prompt",
        "prompt": "Analyze the file content for security issues. Return your decision.",
        "schema": {
          "type": "object",
          "properties": {
            "decision": {
              "type": "string",
              "enum": ["allow", "block"]
            },
            "reason": {
              "type": "string"
            },
            "severity": {
              "type": "string",
              "enum": ["low", "medium", "high", "critical"]
            }
          },
          "required": ["decision"]
        }
      }]
    }]
  }
}

When to Use Prompt Hooks

Good use cases:

• Security analysis that requires understanding code context
• Detecting logic errors or anti-patterns
• Validating architectural decisions
• Complex permission checks

Avoid for:

• Simple pattern matching (use command hooks)
• File extension checks
• Path validation
• Anything that can be done with regex

File-Based Hooks (Legacy)

Simpler approach - place executables directly in hooks directory.

File Location

Project hooks:

.claude/hooks/PreToolUse
.claude/hooks/PostToolUse
.claude/hooks/SessionStart

User-global hooks:

~/.claude/hooks/PreToolUse
~/.claude/hooks/Stop

Requirements

Every file-based hook MUST:

1. Have a shebang line:

#!/bin/bash
#!/usr/bin/env node
#!/usr/bin/env python3

2. Be executable:

chmod +x .claude/hooks/PreToolUse

3. Handle JSON input from stdin

4. Exit with appropriate code

JSON Input Structure

Hooks receive JSON via stdin:

{
  "session_id": "abc123",
  "transcript_path": "/path/to/transcript.jsonl",
  "current_dir": "/path/to/project",
  "tool_name": "Write",
  "input": {
    "file_path": "/path/to/file.ts",
    "content": "file contents...",
    "command": "npm test",
    "old_string": "...",
    "new_string": "..."
  }
}

Tool-Specific Input Fields

Environment Variables

Available in hook execution:

Common Patterns

Pattern 1: Format on Save

{
  "hooks": {
    "PostToolUse": [{
      "matcher": "Write|Edit",
      "hooks": [{
        "type": "command",
        "command": "${CLAUDE_PLUGIN_ROOT}/scripts/format.sh",
        "timeout": 5000
      }]
    }]
  }
}

Pattern 2: Block Sensitive Files

{
  "hooks": {
    "PreToolUse": [{
      "matcher": "Write|Edit|Read",
      "hooks": [{
        "type": "command",
        "command": "${CLAUDE_PLUGIN_ROOT}/scripts/block-sensitive.sh"
      }]
    }]
  }
}

Pattern 3: Test Verification Before Stop

{
  "hooks": {
    "Stop": [{
      "matcher": "*",
      "hooks": [{
        "type": "prompt",
        "prompt": "Before finishing, verify: 1) All tests pass 2) No linting errors 3) Types check. If any issues, list them."
      }]
    }]
  }
}

Pattern 4: Session Context Loading

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

Pattern 5: Multi-Stage Validation

Combine PreToolUse (validate) with PostToolUse (verify):

{
  "hooks": {
    "PreToolUse": [{
      "matcher": "Write",
      "hooks": [{
        "type": "command",
        "command": "./validate-before.sh"
      }]
    }],
    "PostToolUse": [{
      "matcher": "Write",
      "hooks": [{
        "type": "command",
        "command": "./verify-after.sh"
      }]
    }]
  }
}

Common Mistakes

Best Practices

1. Keep Hooks Fast

Target < 100ms for PreToolUse hooks:

• Cache results where possible
• Run heavy operations in background
• Use specific matchers, not wildcards

2. Handle Errors Gracefully

# Check dependencies exist
if ! command -v jq &> /dev/null; then
  echo "jq not installed, skipping" >&2
  exit 0
fi

# Validate input
FILE=$(echo "$INPUT" | jq -r '.input.file_path // empty')
if [[ -z "$FILE" ]]; then
  echo "No file path provided" >&2
  exit 1
fi

3. Use Shebangs

Always start with shebang:

#!/bin/bash
#!/usr/bin/env node
#!/usr/bin/env python3

4. Secure Sensitive Files

BLOCKED=(".env" ".env.*" "*.pem" "*.key")
for pattern in "${BLOCKED[@]}"; do
  case "$FILE" in
    $pattern)
      echo "Blocked: $FILE is sensitive" >&2
      exit 2
      ;;
  esac
done

5. Quote All Variables

# WRONG - breaks on spaces
prettier --write $FILE

# RIGHT - handles spaces
prettier --write "$FILE"

6. Log for Debugging

LOG_FILE=~/.claude-hooks/debug.log

# Log to file
echo "[$(date)] Processing $FILE" >> "$LOG_FILE"

# Log to stderr (shows in transcript)
echo "Hook running..." >&2

Publishing as PRPM Package

Package Structure

my-hook/
├── prpm.json          # Package manifest
├── HOOK.md            # Hook documentation
└── hook-script.sh     # Hook executable

prpm.json

{
  "name": "@username/hook-name",
  "version": "1.0.0",
  "description": "Brief description shown in search",
  "author": "Your Name",
  "format": "claude",
  "subtype": "hook",
  "tags": ["automation", "security", "formatting"],
  "main": "HOOK.md"
}

HOOK.md Format

---
name: session-logger
description: Logs session start/end times for tracking
event: SessionStart
language: bash
hookType: hook
---

# Session Logger Hook

Logs Claude Code session activity for tracking and debugging.

## Installation

This hook will be installed to `.claude/hooks/session-start`.

## Behavior

- Logs session start time to `~/.claude/session.log`
- Displays environment status
- Runs silent dependency checks

## Requirements

- bash 4.0+
- write access to `~/.claude/`

## Source Code

\`\`\`bash
#!/bin/bash
echo "Session started at $(date)" >> ~/.claude/session.log
echo "Environment ready"
exit 0
\`\`\`

Publishing Process

# Test locally first
prpm test

# Publish to registry
prpm publish

# Version bumps
prpm publish patch  # 1.0.0 -> 1.0.1
prpm publish minor  # 1.0.0 -> 1.1.0
prpm publish major  # 1.0.0 -> 2.0.0

Security Requirements

Input Validation

# Parse JSON safely
INPUT=$(cat)
if ! FILE=$(echo "$INPUT" | jq -r '.input.file_path // empty' 2>&1); then
  echo "JSON parse failed" >&2
  exit 1
fi

# Validate field exists
[[ -n "$FILE" ]] || exit 1

Path Sanitization

# Prevent directory traversal
if [[ "$FILE" == *".."* ]]; then
  echo "Path traversal detected" >&2
  exit 2
fi

# Keep in project directory
if [[ "$FILE" != "$CLAUDE_PROJECT_DIR"* ]]; then
  echo "File outside project" >&2
  exit 2
fi

User Confirmation

Claude Code automatically:

• Requires confirmation before installing hooks
• Shows hook source code to user
• Warns about hook execution
• Displays hook output in transcript

Hooks vs Skills vs Commands

Examples:

• Hook: Auto-format files on save
• Skill: Reference guide for testing patterns
• Command: /review-pr quick code review

Related Resources

• claude-hook-writer skill - Detailed hook development guidance
• typescript-hook-writer skill - TypeScript-specific hook development
• Claude Code Docs
• Schema

Checklist for New Hooks

Before publishing:

• Shebang line included
• File is executable (chmod +x)
• Validates all stdin input
• Quotes all variables
• Handles missing dependencies gracefully
• Uses appropriate exit codes
• Logs errors to stderr or file
• Tests with edge cases (spaces, Unicode, missing fields)
• Documents dependencies in HOOK.md
• Includes installation instructions
• Source code included in documentation
• Clear description and tags in prpm.json
• Version number is semantic

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