# Engineering Standards > Core software engineering principles (SOLID, DRY, YAGNI, KISS) with detection patterns and refactoring guidance. Load when reviewing code quality, planning architecture, or identifying technical debt. Use when this capability is needed. - Skill: `tomevault-io/engineering-standards` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds@latest add tomevault-io/engineering-standards` - Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/engineering-standards/raw - Safety review: pending (external: skill-scanner PASS, skillspector PASS) - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - Author: tomevault-io (https://skillmd.com/u/tomevault-io) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/tomevault-io/engineering-standards --- # Engineering Standards Foundational principles for high-quality software. Use this skill when: - Reviewing code for quality issues - Planning architectural changes - Identifying refactoring opportunities - Evaluating technical debt ## SOLID Principles ### Single Responsibility (SRP) A class/module should have one reason to change. **Detection patterns:** - Class with 5+ public methods doing unrelated things - Method longer than 50 lines - Class name contains "And" or "Manager" with mixed concerns - File imports from 10+ unrelated modules **Refactoring:** - Extract class for each responsibility - Split into focused modules - Use composition over inheritance ### Open/Closed (OCP) Open for extension, closed for modification. **Detection patterns:** - Switch/case on type with frequent additions - if/else chains checking instance types - Modifying existing code to add new features **Refactoring:** - Strategy pattern for varying behaviors - Plugin architecture for extensions - Dependency injection for configurability ### Liskov Substitution (LSP) Subtypes must be substitutable for their base types. **Detection patterns:** - Override that throws "not implemented" - Subclass that ignores parent behavior - Type checks before calling inherited methods **Refactoring:** - Favor composition over inheritance - Extract interface for true polymorphism - Use abstract base with required overrides ### Interface Segregation (ISP) Clients shouldn't depend on methods they don't use. **Detection patterns:** - Interface with 10+ methods - Implementing classes that stub methods as no-ops - "Fat" interfaces with unrelated method groups **Refactoring:** - Split into role-specific interfaces - Use mixins/traits for optional behaviors - Compose multiple focused interfaces ### Dependency Inversion (DIP) Depend on abstractions, not concretions. **Detection patterns:** - Direct instantiation of dependencies (`new ConcreteClass()`) - Hard-coded database/API connections - Test files creating production instances **Refactoring:** - Constructor injection - Factory pattern for complex creation - Interface-based dependencies --- ## DRY (Don't Repeat Yourself) **Detection patterns:** - Copy-pasted code blocks (3+ occurrences) - Similar functions with minor variations - Duplicated validation logic - Repeated configuration values **Refactoring:** - Extract shared function/class - Parameterize variations - Create configuration constants - Use template method pattern **Exceptions (acceptable duplication):** - Test code clarity (explicit over DRY) - Cross-boundary isolation (microservices) - Performance-critical paths --- ## YAGNI (You Aren't Gonna Need It) **Detection patterns:** - Unused parameters "for future use" - Abstract classes with single implementation - Configuration options never used - Speculative generalization **Guidance:** - Build for current requirements - Refactor when needs emerge - Delete dead code immediately - Prefer simple over flexible --- ## KISS (Keep It Simple, Stupid) **Detection patterns:** - Cyclomatic complexity > 10 - Nested callbacks/promises 4+ deep - Generic solutions for specific problems - Framework overkill for simple tasks **Refactoring:** - Flatten control flow - Extract named functions - Use early returns - Choose boring technology --- ## Code Smells Quick Reference | Smell | Symptom | Fix | |-------|---------|-----| | Long Method | >50 lines, multiple concerns | Extract method | | Large Class | >500 lines, many responsibilities | Extract class | | Feature Envy | Method uses other class more than own | Move method | | Data Clumps | Same fields appear together | Extract object | | Primitive Obsession | Strings/ints for domain concepts | Value objects | | Switch Statements | Type-based switching | Polymorphism | | Parallel Inheritance | Every subclass needs partner subclass | Merge hierarchies | | Lazy Class | Class doing too little | Inline class | | Speculative Generality | Unused abstraction | Remove it | | Temporary Field | Field only set sometimes | Extract class | --- ## When to Apply **Always apply:** - SRP, DRY for production code - KISS for all code **Apply with judgment:** - OCP when extension points are clear - ISP when interfaces grow beyond 5 methods - DIP at module boundaries **Defer:** - YAGNI violations until pattern emerges 3+ times See [references/refactoring-catalog.md](references/refactoring-catalog.md) for detailed refactoring techniques. --- > Converted and distributed by [TomeVault](https://tomevault.io/claim/groupzer0) — claim your Tome and manage your conversions.