CLAUDE.md Hierarchy: The 4-Tier Merge Order

Claude Code loads and merges CLAUDE.md files in four ordered locations: (1) ~/.claude/CLAUDE.md (global personal instructions), (2) ./CLAUDE.md (git-shared project root), (3) subdirectory CLAUDE.md files (scoped rules for specific codebase areas), and (4) .claude/CLAUDE.md. Keep personal preferences in ~/.claude/CLAUDE.md and domain-specific rules in subdirectory CLAUDE.md files.

File: ./apps/billing-service/CLAUDE.md (Subdirectory Scoped Rules)

- Run single test files in this service via: pnpm --filter billing-service test:unit <path>
- All monetary values in this subdirectory must use integer cents (bigint), never floats.