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

markdown
1---
2name: rule-identifier
3enabled: true
4event: bash|file|stop|prompt|all
5pattern: regex-pattern-here
6---
7
8Message to show Claude when this rule triggers.
9Can 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:

yaml
1event: bash
2pattern: rm\s+-rf

Advanced Format (Multiple Conditions)

For complex rules with multiple conditions:

markdown
1---
2name: warn-env-file-edits
3enabled: true
4event: file
5conditions:
6  - field: file_path
7    operator: regex_match
8    pattern: \.env$
9  - field: new_text
10    operator: contains
11    pattern: API_KEY
12---
13
14You're adding an API key to a .env file. Ensure this file is in .gitignore!

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:

markdown
1⚠️ **Console.log detected!**
2
3You're adding console.log to production code.
4
5**Why this matters:**
6- Debug logs shouldn't ship to production
7- Console.log can expose sensitive data
8- Impacts browser performance
9
10**Alternatives:**
11- Use a proper logging library
12- Remove before committing
13- Use conditional debug builds

Event Type Guide

bash Events

Match Bash command patterns:

markdown
1---
2event: bash
3pattern: sudo\s+|rm\s+-rf|chmod\s+777
4---
5
6Dangerous command detected!

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:

markdown
1---
2event: file
3pattern: console\.log\(|eval\(|innerHTML\s*=
4---
5
6Potentially problematic code pattern detected!

Match on different fields:

markdown
1---
2event: file
3conditions:
4  - field: file_path
5    operator: regex_match
6    pattern: \.tsx?$
7  - field: new_text
8    operator: regex_match
9    pattern: console\.log\(
10---
11
12Console.log in TypeScript file!

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):

markdown
1---
2event: stop
3pattern: .*
4---
5
6Before stopping, verify:
7- [ ] Tests were run
8- [ ] Build succeeded
9- [ ] Documentation updated

Use for:

• Reminders about required steps
• Completion checklists
• Process enforcement

prompt Events

Match user prompt content (advanced):

markdown
1---
2event: prompt
3conditions:
4  - field: user_prompt
5    operator: contains
6    pattern: deploy to production
7---
8
9Production deployment checklist:
10- [ ] Tests passing?
11- [ ] Reviewed by team?
12- [ ] Monitoring ready?

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:

rm\s+-rf         Matches: rm -rf, rm  -rf
console\.log\(   Matches: console.log(
(eval|exec)\(    Matches: eval( or exec(
chmod\s+777      Matches: chmod 777, chmod  777
API_KEY\s*=      Matches: API_KEY=, API_KEY =

Testing Patterns

Test regex patterns before using:

bash
1python3 -c "import re; print(re.search(r'your_pattern', 'test text'))"

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

Common Pitfalls

Too broad:

yaml
1pattern: log    # Matches "log", "login", "dialog", "catalog"

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

Too specific:

yaml
1pattern: rm -rf /tmp  # Only matches exact path

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:

markdown
1---
2name: my-rule
3enabled: true
4event: bash
5pattern: dangerous_command
6---
7
8Warning message here

Rule with conditions:

markdown
1---
2name: my-rule
3enabled: true
4event: file
5conditions:
6  - field: file_path
7    operator: regex_match
8    pattern: \.ts$
9  - field: new_text
10    operator: contains
11    pattern: any
12---
13
14Warning message

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