Customizing Claude Code with CLAUDE.md

Every project has its own conventions, every team has its own standards, and every developer has their preferences. Out of the box, Claude Code works well with generic best practices. But the real power unlocks when you configure it specifically for your environment. The CLAUDE.md configuration file is how you teach Claude Code about your project, your rules, and your preferences. This article covers everything you need to know to set up effective configurations.

The Configuration Hierarchy

Claude Code reads configuration from multiple locations, each with a different scope.

Global configuration lives at ~/CLAUDE.md in your home directory. Settings here apply to every project and every session. This is where you put personal preferences, default model choices, and universal rules that apply regardless of what you are working on.

Project configuration lives at CLAUDE.md in your project root. This is committed to version control and shared with your team. It defines project-specific conventions, technology choices, and standards.

Directory configuration can exist in subdirectories for module-specific rules. A CLAUDE.md in your /server directory might have backend-specific conventions while one in /client covers frontend patterns.

When configurations exist at multiple levels, they are merged. Project settings override global settings where they conflict. Directory settings override project settings for work within that directory. This hierarchy lets you define broad rules globally and refine them per project.

Essential Project Configuration

A well-structured project CLAUDE.md covers several key areas.

Project Context

Start by telling Claude Code what the project is:

## Project Overview
- This is an e-commerce API built with Node.js and Express
- Database: PostgreSQL with Drizzle ORM
- Authentication: JWT-based with refresh tokens
- Testing: Vitest for unit tests, Supertest for API tests
- Deployment: Docker containers on AWS ECS

This context shapes every decision Claude Code makes. When it generates code, it uses your ORM. When it writes tests, it uses your test framework. When it suggests solutions, it accounts for your deployment environment.

Coding Conventions

Define the patterns you expect:

## Conventions
- Use TypeScript strict mode everywhere
- All functions must have explicit return types
- Use async/await, never raw promises with .then()
- Error handling: always use custom error classes from src/errors/
- Naming: camelCase for variables, PascalCase for types, kebab-case for files
- Imports: group by external, internal, types (in that order)
- No default exports except for pages and layouts

These conventions are enforced consistently. Claude Code follows them when generating new code and flags violations when reviewing existing code.

Architecture Rules

Define boundaries and patterns:

## Architecture
- Controllers handle HTTP concerns only (request parsing, response formatting)
- Services contain business logic (no HTTP concepts)
- Repositories handle database access (no business logic)
- Use dependency injection for all services
- No circular dependencies between modules

Architecture rules prevent Claude Code from generating code that works but violates your design principles.

Things to Avoid

Negative rules are as important as positive ones:

## Do Not
- Do not use any package (ORM) for raw SQL queries
- Do not install new dependencies without checking the license first
- Do not use console.log for logging (use the Logger service)
- Do not access environment variables directly (use the Config service)
- Do not store secrets in code or configuration files

These guardrails prevent common mistakes that might technically work but violate project standards.

Global Configuration Patterns

Your global CLAUDE.md captures preferences that apply everywhere.

## Preferences
- Default to TypeScript over JavaScript
- Prefer functional programming patterns
- Use conventional commits format
- Always generate tests alongside new code
- Prefer explicit over implicit (no magic)

## Git Workflow
- Branch naming: feature/, fix/, chore/ prefixes
- Commit messages: conventional commits (feat:, fix:, refactor:, etc.)
- Always create a branch, never commit directly to main

Global configuration reduces repetitive instructions. Instead of telling Claude Code your commit message format in every session, you define it once.

Advanced Configuration Techniques

Model Selection Rules

You can define when different models should be used. For more on effective prompting strategies, tailor your model selection to task complexity:

## Model Selection
- Use Opus for security reviews and architecture decisions
- Use Sonnet for feature implementation and standard code review
- Use Haiku for git operations, file management, and quick fixes

Environment-Specific Rules

## Environments
- Development: use local PostgreSQL on port 5432
- Testing: use in-memory SQLite for speed
- Production: connection string from DATABASE_URL env var
- Never reference production credentials in code or config

External Service Configuration

## APIs and Services
- Payment processing: Stripe (use test keys in development)
- Email: SendGrid (mock in tests, sandbox in staging)
- Storage: S3 (use MinIO locally)
- All API keys must come from environment variables

Review Checklists

Define what Claude Code should check during reviews. Agents can automate these checks:

## Review Checklist
- [ ] All new endpoints have input validation
- [ ] Error responses follow the standard format
- [ ] Database queries use parameterized values
- [ ] New features have corresponding tests
- [ ] No TODO or FIXME comments in committed code
- [ ] TypeScript strict mode passes without errors

Team Configuration Best Practices

When multiple developers share a project, the CLAUDE.md becomes a living standards document. For more on team collaboration strategies, maintain shared standards effectively.

Keep it current. When the team adopts a new pattern, update the configuration. When a convention changes, reflect it immediately. Stale configuration leads to inconsistent code.

Be specific over general. Instead of “write clean code,” specify what clean means: explicit return types, no nested ternaries, maximum function length, required error handling patterns.

Include examples. For complex patterns, include a code example in your configuration:

## API Response Format
All API responses must follow this structure:
```json
{
  "data": {},
  "meta": { "timestamp": "ISO8601" },
  "errors": []
}

Document the why. When a rule exists for a non-obvious reason, explain it:

## No Default Exports
We avoid default exports because they make refactoring harder.
Named exports enable better IDE support for auto-imports and rename refactoring.

Maintaining Your Configuration

Your CLAUDE.md should evolve with your project. Review it periodically:

• After adding new dependencies, update the project context
• After team discussions about patterns, capture the decisions
• After discovering Claude Code produces suboptimal output for a recurring pattern, add specific guidance
• After completing a major feature, check if new conventions were established that should be documented

The Claude Code documentation provides additional configuration options and examples. A well-maintained CLAUDE.md is one of the highest-leverage investments you can make in your development workflow because it compounds over every session.