Claude Code Rules Directory: Modular Instructions That Scale

Problem: Your CLAUDE.md file has grown unwieldy. React patterns mixed with API guidelines mixed with testing rules. Everything loads every session, even when you're only working on database migrations.

Quick Win: Create your first modular rule:

mkdir -p .claude/rules
echo "# Testing Rules
- Run tests before committing
- Mock external services in unit tests" > .claude/rules/testing.md

That rule now loads automatically alongside your CLAUDE.md, keeping concerns separated.

What Is the Rules Directory?

The .claude/rules/ directory is a modular alternative to monolithic CLAUDE.md files. Instead of cramming everything into one file, you organize instructions into multiple markdown files that Claude loads as project memory.

Critical detail from Anthropic: Rules files load with the same high priority as CLAUDE.md. This matters because Claude's context window has a priority hierarchy - not all tokens are weighted equally.

Every .md file in .claude/rules/ automatically becomes part of your project context. No configuration needed.

.claude/rules/
├── code-style.md      # Formatting and conventions
├── testing.md         # Test requirements
├── security.md        # Security checklist
└── frontend/
    ├── react.md       # React-specific patterns
    └── styles.md      # CSS conventions

This structure gives you separation of concerns at the instruction level. Update your security rules without touching your styling guidelines.

Path-Specific Rules: The Power Feature

Here's where rules get interesting. You can target rules to specific file patterns using YAML frontmatter:

---
paths: src/api/**/*.ts
---
 
# API Development Rules
 
- All endpoints must validate input with Zod
- Return consistent error shapes: { error: string, code: number }
- Log all requests with correlation IDs

This rule only activates when Claude works on files matching src/api/**/*.ts. Your API guidelines stay out of the way when you're editing React components.

Multiple Path Patterns

Target multiple patterns in a single rule:

---
paths:
  - src/components/**/*.tsx
  - src/hooks/**/*.ts
---
 
# React Development Rules
 
- Use functional components exclusively
- Extract logic into custom hooks
- Memoize expensive computations

Common Path Patterns

Why Priority Matters: The Monolithic CLAUDE.md Problem

Claude's context window isn't flat. Different sources of information receive different priority levels in how the model weighs them during generation. Anthropic confirms that CLAUDE.md and rules files receive high priority - Claude treats these instructions as authoritative.

This created a problem with the old approach: stuffing everything into one massive CLAUDE.md meant all of that content received high priority. Your React patterns competed for attention with your API guidelines, even when you were working on database migrations.

High priority everywhere = priority nowhere.

When everything is marked important, Claude struggles to determine what's actually relevant to the current task. The result: instructions get ignored, context becomes noisy, and Claude's behavior becomes unpredictable.

The Context Priority Hierarchy

Understanding how Claude weighs different context sources:

The rules directory solves the monolithic problem by letting you distribute high-priority instructions across targeted files. Your API rules still get high priority - but only when you're working on API files.

Path Targeting = Priority Scoping

When a rule has paths frontmatter, it only loads (and receives high priority) when Claude is working on matching files:

---
paths: src/api/**/*.ts
---
# These instructions get high priority ONLY during API work

This is the key insight: you're not just organizing files, you're scoping when instructions receive elevated attention.

Rules vs CLAUDE.md vs Skills

When do you use each?

Use CLAUDE.md for what applies everywhere: routing logic, quality standards, coordination protocols. Keep it lean - everything here competes for high-priority attention.

Use rules for what applies to specific areas: API patterns for API files, test requirements for test files. Path targeting ensures high priority only when relevant.

Use skills for what applies across projects: deployment procedures, code review checklists, brand guidelines. Lower priority until explicitly triggered.

Practical Examples

Security Rules for Sensitive Directories

---
paths:
  - src/auth/**/*
  - src/payments/**/*
---
 
# Security-Critical Code Rules
 
- Never log sensitive data (passwords, tokens, card numbers)
- Validate all inputs at function boundaries
- Use parameterized queries exclusively
- Require explicit authorization checks before data access

Test File Standards

---
paths: **/*.test.ts
---
 
# Test Writing Standards
 
- Use descriptive test names: "should [action] when [condition]"
- One assertion per test when possible
- Mock external dependencies, never real APIs
- Include edge cases: empty inputs, null values, boundaries

Database Migration Rules

---
paths: prisma/migrations/**/*
---
 
# Migration Safety Rules
 
- Always include rollback instructions
- Test migrations on a copy of production data first
- Never delete columns in the same migration that removes code using them
- Add columns as nullable first, populate, then add constraints

Migration from Monolithic CLAUDE.md

If your CLAUDE.md has grown large, you're likely experiencing the priority saturation problem: too much high-priority content competing for attention. Extract domain sections into path-targeted rules:

Before (single 400-line CLAUDE.md):

# Project Context
...
 
## API Guidelines
- Validate inputs with Zod
- Return consistent errors
...
 
## React Patterns
- Use functional components
- Extract hooks
...
 
## Testing Rules
- Mock external services
...

After (lean CLAUDE.md + modular rules):

# CLAUDE.md - Operational Core Only
 
## Routing Logic
- Simple tasks: execute directly
- Complex tasks: delegate to sub-agents
 
## Quality Standards
- Correctness > Maintainability > Performance

.claude/rules/
├── api-guidelines.md      # API section with paths: src/api/**/*
├── react-patterns.md      # React section with paths: src/components/**/*
└── testing-rules.md       # Testing section with paths: **/*.test.*

Your CLAUDE.md stays focused on universal behavior. Domain knowledge lives in targeted rules that only receive high priority when relevant.

The result: cleaner priority distribution. Your core operational instructions always get attention. Domain-specific rules get attention only when Claude is working in their target areas.

Best Practices

Keep rules focused: One concern per file. Security rules separate from styling rules.

Use descriptive filenames: api-validation.md beats rules1.md.

Leverage path targeting: Rules without paths load everywhere. Add paths to reduce noise.

Version control everything: Rules are code. Review changes, track history, roll back mistakes.

Document rule purpose: Start each file with a brief comment explaining when it applies.

Next Steps

1. Audit your CLAUDE.md: Identify sections that apply only to specific file types
2. Extract one rule: Move your most domain-specific section to .claude/rules/
3. Add path targeting: Make that rule activate only where it matters
4. Iterate: As you work, notice when Claude receives irrelevant context and extract more rules

For the complete memory architecture and why monolithic files cause problems, see CLAUDE.md Mastery. To understand how priority fits into broader context strategy, explore context management and memory optimization.

The goal: Claude receives exactly the high-priority instructions it needs for the files it's touching. No more, no less. Priority where it matters, silence everywhere else.