Writing Rules Skill for Claude

Table of Contents

• Overview
• Quick Start
• Rule File Format
• Frontmatter Fields
• Event Types
• Advanced Conditions
• Operators
• Field Reference
• Pattern Writing
• Regex Basics
• Examples
• Test Patterns
• Example Rules
• Block Destructive Commands
• Warn About Debug Code
• Require Tests
• Protect Production Files
• Management
• Related Skills
• Best Practices

Hookify Rule Writing Guide

When To Use

• Creating behavioral rules to prevent unwanted actions
• Defining persistent guardrails for Claude Code sessions

When NOT To Use

• Complex multi-step workflows - use agents instead
• One-time operations that do not need persistent behavioral rules

Overview

Hookify rules are markdown files with YAML frontmatter that define patterns to watch for and messages to show when those patterns match. Rules are stored in .claude/hookify.{rule-name}.local.md files.

Quick Start

Create .claude/hookify.dangerous-rm.local.md:

---
name: dangerous-rm
enabled: true
event: bash
pattern: rm\s+-rf
action: block
---

🛑 **Dangerous rm command detected!**

This command could delete important files.

Verification: Run the command with --help flag to verify availability.

The rule activates immediately - no restart needed!

Rule File Format

Frontmatter Fields

name (required): Unique identifier (kebab-case)

enabled (required): true or false

event (required): bash, file, stop, prompt, or all

action (optional): warn (default) or block

pattern (simple): Regex pattern to match

Event Types

• bash: Bash tool commands
• file: Edit, Write, MultiEdit tools
• stop: When agent wants to stop
• prompt: User prompt submission
• all: All events

Advanced Conditions

For multiple field checks:

---
name: warn-env-edits
enabled: true
event: file
action: warn
conditions:
  - field: file_path
    operator: regex_match
    pattern: \.env$
  - field: new_text
    operator: contains
    pattern: API_KEY
---

🔐 **API key in .env file!**
Ensure file is in .gitignore.

Operators

• regex_match: Pattern matching
• contains: Substring check
• equals: Exact match
• not_contains: Must NOT contain
• starts_with: Prefix check
• ends_with: Suffix check

Field Reference

bash events: command

file events: file_path, new_text, old_text, content

prompt events: user_prompt

stop events: transcript

Pattern Writing

Regex Basics

• \s - whitespace
• \d - digit
• \w - word character
• . - any character (use \. for literal dot)
• + - one or more
• * - zero or more
• | - OR

Examples

rm\s+-rf          → rm -rf
console\.log\(    → console.log(
chmod\s+777       → chmod 777

Test Patterns

python3 -c "import re; print(re.search(r'pattern', 'text'))"

Example Rules

Block Destructive Commands

---
name: block-destructive
enabled: true
event: bash
pattern: rm\s+-rf|dd\s+if=|mkfs
action: block
---

🛑 **Destructive operation blocked!**
Can cause data loss.

Warn About Debug Code

---
name: warn-debug
enabled: true
event: file
pattern: console\.log\(|debugger;
action: warn
---

🐛 **Debug code detected!**
Remove before committing.

Require Tests

---
name: require-tests
enabled: true
event: stop
action: warn
conditions:
  - field: transcript
    operator: not_contains
    pattern: pytest|npm test
---

⚠️ **Tests not run!**
Please verify changes.

Protect Production Files

---
name: protect-prod
enabled: true
event: file
action: block
conditions:
  - field: file_path
    operator: regex_match
    pattern: /production/|\.prod\.
---

🚨 **Production file!**
Requires review.

Management

Enable/Disable:

Edit .local.md file: enabled: false

Delete:

rm .claude/hookify.my-rule.local.md

List:

/hookify:list

Related Skills

• abstract:hook-scope-guide - Hook placement decisions
• abstract:hook-authoring - SDK hook development
• abstract:hooks-eval - Hook evaluation

Best Practices

• Start with simple patterns
• Test regex thoroughly
• Use clear, helpful messages
• Prefer warnings over blocks initially
• Name rules descriptively
• Document intent in messages

Troubleshooting

Common Issues

If a rule doesn't trigger, verify that the event type matches the tool being used (e.g., use bash for command line tools). Check that the regex pattern is valid and matches the target text by testing it with a short Python script. If you encounter permission errors when creating rule files in .claude/, ensure that the directory is writable by your user.

Exit Criteria

• [ ] Rule file exists at .claude/hookify.<rule-name>.local.md

with all required frontmatter fields present: name (kebab-case),

enabled, event, and either pattern or conditions

• [ ] Regex pattern validated with

`python3 -c "import re; print(re.search(r'<pattern>', '<test>'))"

before the rule is saved; no untested patterns shipped

• [ ] Rule triggers correctly on a matching input: warn action

shows the message body, block action prevents the operation

• [ ] Rule does not fire on a non-matching input; false positive

tested with at least one known-safe example