Writing CLAUDE.md Files

Core Principle

CLAUDE.md files bridge Claude's statelessness. They preserve context so humans don't re-explain architectural intent every session.

Key distinction:

• Top-level: HOW to work in this codebase (commands, conventions)
• Subdirectory: WHY this piece exists and what it PROMISES (contracts, intent)

File Hierarchy

Claude automatically reads CLAUDE.md files from current directory up to root:

project/
├── CLAUDE.md                    # Project-wide: tech stack, commands, conventions
└── src/
    └── domains/
        ├── auth/
        │   └── CLAUDE.md        # Auth domain: purpose, contracts, invariants
        └── billing/
            └── CLAUDE.md        # Billing domain: purpose, contracts, invariants

Depth guideline: Typically one level (domain). Occasionally two (subdomain). Rarely more.

Top-Level CLAUDE.md

Focuses on project-wide WHAT and HOW.

What to Include

Template

# [Project Name]

Last verified: [DATE - use `date +%Y-%m-%d`]

## Tech Stack
- Language: TypeScript 5.x
- Framework: Next.js 14
- Database: PostgreSQL
- Testing: Vitest

## Commands
- `npm run dev` - Start dev server
- `npm run test` - Run tests
- `npm run build` - Production build

## Project Structure
- `src/domains/` - Domain modules (auth, billing, etc.)
- `src/shared/` - Cross-cutting utilities
- `src/infrastructure/` - External adapters (DB, APIs)

## Conventions
- Functional Core / Imperative Shell pattern
- Domain modules are self-contained
- See domain CLAUDE.md files for domain-specific guidance

## Boundaries
- Safe to edit: `src/`
- Never touch: `migrations/` (immutable), `*.lock` files

Subdirectory CLAUDE.md (Domain-Level)

Focuses on WHY and CONTRACTS. The code shows WHAT; these files explain intent.

What to Include

Template

# [Domain Name]

Last verified: [DATE - use `date +%Y-%m-%d`]

## Purpose
[1-2 sentences: WHY this domain exists, what problem it solves]

## Contracts
- **Exposes**: [public interfaces - what callers can use]
- **Guarantees**: [promises this domain keeps]
- **Expects**: [what callers must provide]

## Dependencies
- **Uses**: [domains/services this depends on]
- **Used by**: [what depends on this domain]
- **Boundary**: [what should NOT be imported here]

## Key Decisions
- [Decision]: [Rationale]

## Invariants
- [Thing that must always be true]

## Key Files
- `index.ts` - Public exports
- `types.ts` - Domain types
- `service.ts` - Main service implementation

## Gotchas
- [Non-obvious thing that will bite you]

Freshness Dates: MANDATORY

Every CLAUDE.md MUST include a "Last verified" date.

CRITICAL: Use Bash to get the actual date. Do NOT hallucinate dates.

date +%Y-%m-%d

Why mandatory: Stale CLAUDE.md files are worse than none. The date signals when contracts were last confirmed accurate.

Heuristics: Top-Level vs Subdirectory

Rule of thumb:

• Top-level = "How to work here"
• Subdirectory = "Why this exists and what it promises"

When to Create Subdirectory CLAUDE.md

Create when:

• Domain has non-obvious contracts with other parts
• Architectural decisions affect how code should evolve
• Invariants exist that aren't obvious from code
• New sessions consistently need the same context re-explained

Don't create for:

• Trivial utility folders
• Implementation details that change frequently
• Content better captured in code comments

Updating CLAUDE.md Files

When updating any CLAUDE.md:

1. Update the freshness date using Bash date +%Y-%m-%d
2. Verify contracts still hold - read the code, check invariants
3. Remove stale content - better short and accurate than long and wrong
4. Keep token-efficient - <300 lines top-level, <100 lines subdirectory

Common Mistakes

Checklist

Top-level:

• Tech stack listed
• Key commands documented
• Project structure overview
• Freshness date (from date +%Y-%m-%d)

Subdirectory:

• Purpose explains WHY (not what)
• Contracts: exposes, guarantees, expects
• Dependencies and boundaries clear
• Key decisions with rationale
• Invariants documented
• Freshness date (from date +%Y-%m-%d)
• Under 100 lines