claude-code/claude/rules/common/coding-style.md

Coding Style

Immutability (CRITICAL)

ALWAYS create new objects, NEVER mutate existing ones:

// Pseudocode
WRONG:  modify(original, field, value) → changes original in-place
CORRECT: update(original, field, value) → returns new copy with change

Rationale: Immutable data prevents hidden side effects, makes debugging easier, and enables safe concurrency.

File Organization

MANY SMALL FILES > FEW LARGE FILES:

• High cohesion, low coupling
• 200-400 lines typical, 800 max
• Extract utilities from large modules
• Organize by feature/domain, not by type

Error Handling

ALWAYS handle errors comprehensively:

• Handle errors explicitly at every level
• Provide user-friendly error messages in UI-facing code
• Log detailed error context on the server side
• Never silently swallow errors

Input Validation

ALWAYS validate at system boundaries:

• Validate all user input before processing
• Use schema-based validation where available
• Fail fast with clear error messages
• Never trust external data (API responses, user input, file content)

Code Quality Checklist

Before marking work complete:

• Code is readable and well-named
• Functions are small (<50 lines)
• Files are focused (<800 lines)
• No deep nesting (>4 levels)
• Proper error handling
• No hardcoded values (use constants or config)
• No mutation (immutable patterns used)