writing-hookify-rules

Writing Hookify 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.

Rule File Format

Basic Structure

---
name: rule-identifier
enabled: true
event: bash|file|stop|prompt|all
pattern: regex-pattern-here
---
 
Message to show Claude when this rule triggers.
Can include markdown formatting, warnings, suggestions, etc.

Frontmatter Fields

name (required): Unique identifier for the rule

• Use kebab-case: warn-dangerous-rm, block-console-log
• Be descriptive and action-oriented
• Start with verb: warn, prevent, block, require, check

enabled (required): Boolean to activate/deactivate

• true: Rule is active
• false: Rule is disabled (won't trigger)
• Can toggle without deleting rule

event (required): Which hook event to trigger on

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

action (optional): What to do when rule matches

• warn: Show message but allow operation (default)
• block: Prevent operation (PreToolUse) or stop session (Stop events)
• If omitted, defaults to warn

pattern (simple format): Regex pattern to match

• Used for simple single-condition rules
• Matches against command (bash) or new_text (file)
• Python regex syntax

Example:

event: bash

Advanced Format (Multiple Conditions)

For complex rules with multiple conditions:

Condition fields:

• field: Which field to check
  • For bash: command
  • For file: file_path, new_text, old_text, content
• operator: How to match
  • regex_match: Regex pattern matching
  • contains: Substring check
  • equals: Exact match
  • not_contains: Substring must NOT be present
  • starts_with: Prefix check
  • ends_with: Suffix check
• pattern: Pattern or string to match

All conditions must match for rule to trigger.

Message Body

The markdown content after frontmatter is shown to Claude when the rule triggers.

Good messages:

• Explain what was detected
• Explain why it's problematic
• Suggest alternatives or best practices
• Use formatting for clarity (bold, lists, etc.)

Example:

Event Type Guide

bash Events

Match Bash command patterns:

Common patterns:

• Dangerous commands: rm\s+-rf, dd\s+if=, mkfs
• Privilege escalation: sudo\s+, su\s+
• Permission issues: chmod\s+777, chown\s+root

file Events

Match Edit/Write/MultiEdit operations:

Match on different fields:

Common patterns:

• Debug code: console\.log\(, debugger, print\(
• Security risks: eval\(, innerHTML\s*=, dangerouslySetInnerHTML
• Sensitive files: \.env$, credentials, \.pem$
• Generated files: node_modules/, dist/, build/

stop Events

Match when agent wants to stop (completion checks):

Use for:

• Reminders about required steps
• Completion checklists
• Process enforcement

prompt Events

Match user prompt content (advanced):

Pattern Writing Tips

Regex Basics

Literal characters: Most characters match themselves

• rm matches "rm"
• console.log matches "console.log"

Special characters need escaping:

• . (any char) → \. (literal dot)
• ( ) → \( \) (literal parens)
• [ ] → \[ \] (literal brackets)

Common metacharacters:

• \s - whitespace (space, tab, newline)
• \d - digit (0-9)
• \w - word character (a-z, A-Z, 0-9, _)
• . - any character
• + - one or more
• * - zero or more
• ? - zero or one
• | - OR

Examples:

Testing Patterns

Test regex patterns before using:

Or use online regex testers (regex101.com with Python flavor).

Common Pitfalls

Too broad:

Better: console\.log\(|logger\.

Too specific:

Better: rm\s+-rf

Escaping issues:

• YAML quoted strings: "pattern" requires double backslashes \\s
• YAML unquoted: pattern: \s works as-is
• Recommendation: Use unquoted patterns in YAML

File Organization

Location: All rules in .claude/ directory Naming: .claude/hookify.{descriptive-name}.local.md Gitignore: Add .claude/*.local.md to .gitignore

Good names:

• hookify.dangerous-rm.local.md
• hookify.console-log.local.md
• hookify.require-tests.local.md
• hookify.sensitive-files.local.md

Bad names:

• hookify.rule1.local.md (not descriptive)
• hookify.md (missing .local)
• danger.local.md (missing hookify prefix)

Workflow

Creating a Rule

1. Identify unwanted behavior
2. Determine which tool is involved (Bash, Edit, etc.)
3. Choose event type (bash, file, stop, etc.)
4. Write regex pattern
5. Create .claude/hookify.{name}.local.md file in project root
6. Test immediately - rules are read dynamically on next tool use

Refining a Rule

1. Edit the .local.md file
2. Adjust pattern or message
3. Test immediately - changes take effect on next tool use

Disabling a Rule

Temporary: Set enabled: false in frontmatter Permanent: Delete the .local.md file

Examples

See ${CLAUDE_PLUGIN_ROOT}/examples/ for complete examples:

• dangerous-rm.local.md - Block dangerous rm commands
• console-log-warning.local.md - Warn about console.log
• sensitive-files-warning.local.md - Warn about editing .env files

Quick Reference

Minimum viable rule:

Rule with conditions:

Event types:

• bash - Bash commands
• file - File edits
• stop - Completion checks
• prompt - User input
• all - All events

Field options:

• Bash: command
• File: file_path, new_text, old_text, content
• Prompt: user_prompt

Operators:

• regex_match, contains, equals, not_contains, starts_with, ends_with_