@levnikolaevich/ln-623-code-principles-auditor

---
name: ln-623-code-principles-auditor
description: "Checks DRY (10 types), KISS/YAGNI, error handling, DI patterns. Returns findings with severity, location, effort, pattern_signature."
allowed-tools: Read, Grep, Glob, Bash
license: MIT
---

> **Paths:** File paths (`shared/`, `references/`, `../ln-*`) are relative to skills repo root. If not found at CWD, locate this SKILL.md directory and go up one level for repo root.

# Code Principles Auditor (L3 Worker)

Specialized worker auditing code principles (DRY, KISS, YAGNI) and design patterns.

## Purpose & Scope

- **Worker in ln-620 coordinator pipeline** - invoked by ln-620-codebase-auditor
- Audit **code principles** (DRY/KISS/YAGNI, error handling, DI)
- Return structured findings with severity, location, effort, pattern_signature, recommendations
- Calculate compliance score (X/10) for Code Principles category

## Inputs (from Coordinator)

**MANDATORY READ:** Load `shared/references/audit_worker_core_contract.md`.

Receives `contextStore` with: `tech_stack`, `best_practices`, `principles`, `codebase_root`, `output_dir`.

**Domain-aware:** Supports `domain_mode` + `current_domain` (see `audit_output_schema.md#domain-aware-worker-output`).

## Workflow

**MANDATORY READ:** Load `shared/references/two_layer_detection.md` for detection methodology.

1) **Parse context** — extract fields, determine `scan_path` (domain-aware if specified), extract `output_dir`
2) **Load detection patterns**
   - **MANDATORY READ:** Load `references/detection_patterns.md` for language-specific Grep/Glob patterns
   - Select patterns matching project's `tech_stack`
3) **Scan codebase for violations (Layer 1)**
   - All Grep/Glob patterns use `scan_path` (not codebase_root)
   - Follow step-by-step detection from `detection_patterns.md`
   - Apply exclusions from `detection_patterns.md#exclusions`
4) **Analyze context per candidate (Layer 2)**
   - DRY: read both code blocks to confirm true duplication (not just similar naming or shared interface)
   - KISS: check if abstraction serves DI pattern (valid single-impl interface) or is premature
   - YAGNI: check if feature flag was recently added (intentional) or dormant for months
5) **Generate recommendations**
   - **MANDATORY READ:** Load `references/refactoring_decision_tree.md` for pattern selection
   - Match each finding to appropriate refactoring pattern via decision tree
6) **Collect findings with severity, location, effort, pattern_id, pattern_signature, recommendation**
   - Tag each finding with `domain: domain_name` (if domain-aware)
   - Assign `pattern_signature` for cross-domain matching by ln-620
7) **Calculate score using penalty algorithm**
8) **Write Report:** Build full markdown report in memory per `shared/templates/audit_worker_report_template.md`, write to `{output_dir}/623-principles-{domain}.md` (or `623-principles.md` in global mode) in single Write call. **Include `<!-- FINDINGS-EXTENDED -->` JSON block** with pattern_signature fields for cross-domain DRY analysis
9) **Return Summary:** Return minimal summary to coordinator (see Output Format)

## Two-Layer Detection (MANDATORY)

**MANDATORY READ:** `shared/references/two_layer_detection.md`

All findings require Layer 2 context analysis. Layer 1 finding without Layer 2 = NOT a valid finding. Before reporting, ask: "Is this violation intentional or justified by design?"

| Finding Type | Layer 2 Downgrade Examples |
|-------------|--------------------------|
| DRY | Modules with different lifecycle/ownership → skip. Intentional duplication for decoupling → skip |
| KISS | Framework-required abstraction (e.g., DI in Spring) → downgrade. Single implementation today but interface for testing → skip |
| YAGNI | Feature flag used in A/B testing → skip. Config option used by ops team → skip |
| Error Handling | Centralized handler absent in 50-line script → downgrade to LOW |
| DI | Dependencies replaceable via params/closures → skip ARCH-DI |

## Audit Rules

### 1. DRY Violations (Don't Repeat Yourself)

**MANDATORY READ:** Load `references/detection_patterns.md` for detection steps per type.

| Type | What | Severity | Exception (skip/downgrade) | Default Recommendation | Effort |
|------|------|----------|---------------------------|----------------------|--------|
| **1.1** Identical Code | Same functions/constants/blocks (>10 lines) in multiple files | HIGH: business-critical (auth, payment). MEDIUM: utilities. LOW: simple constants <5x | Different lifecycle/ownership modules → skip. Intentional decoupling → skip | Extract function → decide location by duplication scope | M |
| **1.2** Duplicated Validation | Same validation patterns (email, password, phone, URL) across files | HIGH: auth/payment. MEDIUM: user input 3+x. LOW: format checks <3x | Different security contexts (auth vs public) → skip | Extract to shared validators module | M |
| **1.3** Repeated Error Messages | Hardcoded error strings instead of centralized catalog | MEDIUM: critical messages hardcoded or no error catalog. LOW: <3 places | User-facing strings requiring per-context wording → downgrade | Create constants/error-messages file | M |
| **1.4** Similar Patterns | Functions with same call sequence/control flow but different names/entities | MEDIUM: business logic in critical paths. LOW: utilities <3x | Modules with divergent evolution expected → skip | Extract common logic (see decision tree for pattern) | M |
| **1.5** Duplicated SQL/ORM | Same queries in different services | HIGH: payment/auth queries. MEDIUM: common 3+x. LOW: simple <3x | Different bounded contexts; shared DB is worse than duplication → skip | Extract to Repository layer | M |
| **1.6** Copy-Pasted Tests | Identical setup/teardown/fixtures across test files | MEDIUM: setup in 5+ files. LOW: <5 files | Tests intentionally isolated for clarity/independence → downgrade | Extract to test helpers | M |
| **1.7** Repeated API Responses | Same response object shapes without DTOs | MEDIUM: in 5+ endpoints. LOW: <5 endpoints | Responses with different versioning lifecycle → skip | Create DTO/Response classes | M |
| **1.8** Duplicated Middleware Chains | Identical middleware/decorator stacks on multiple routes | MEDIUM: same chain on 5+ routes. LOW: <5 routes | Routes with different auth/rate-limit requirements → skip | Create named middleware group, apply at router level | M |
| **1.9** Duplicated Type Definitions | Interfaces/structs/types with 80%+ same fields | MEDIUM: in 5+ files. LOW: 2-4 files | Types with different ownership/evolution paths → skip | Create shared base type, extend where needed | M |
| **1.10** Duplicated Mapping Logic | Same entity→DTO / DTO→entity transformations in multiple locations | MEDIUM: in 3+ locations. LOW: 2 locations | Mappings with different validation/enrichment rules → skip | Create dedicated Mapper class/function | M |

**Recommendation selection:** Use `references/refactoring_decision_tree.md` to choose the right refactoring pattern based on duplication location (Level 1) and logic type (Level 2).

### 2. KISS Violations (Keep It Simple, Stupid)

| Violation | Detection | Severity | Exception (skip/downgrade) | Recommendation | Effort |
|-----------|-----------|----------|---------------------------|---------------|--------|
| Abstract class with 1 implementation | Grep `abstract class` → count subclasses | HIGH: prevents understanding core logic | Interface for DI/testing → skip. Framework-required (Spring, ASP.NET) → skip | Remove abstraction, inline | L |
| Factory for <3 types | Grep factory patterns → count branches | MEDIUM: unnecessary pattern | Factory used for DI/testing swap → downgrade | Replace with direct construction | M |
| Deep inheritance >3 levels | Trace extends chain | HIGH: fragile hierarchy | Framework-mandated hierarchy (UI widgets, ORM models) → downgrade | Flatten with composition | L |
| Excessive generic constraints | Grep `<T extends ... & ...>` | LOW: acceptable tradeoff | Type safety for public API boundary → skip | Simplify constraints | M |
| Wrapper-only classes | Read: all methods delegate to inner | MEDIUM: unnecessary indirection | Adapter pattern for external API isolation → skip | Remove wrapper, use inner directly | M |

### 3. YAGNI Violations (You Aren't Gonna Need It)

| Violation | Detection | Severity | Exception (skip/downgrade) | Recommendation | Effort |
|-----------|-----------|----------|---------------------------|---------------|--------|
| Dead feature flags (always true/false) | Grep flags → verify never toggled | LOW: cleanup needed | A/B testing flags → skip. Ops-controlled toggles → skip | Remove flag, keep active code path | M |
| Abstract methods never overridden | Grep abstract → search implementations | MEDIUM: unused extensibility | Plugin/extension point in public library → downgrade | Remove abstract, make concrete | M |
| Unused config options | Grep config key → 0 references | LOW: dead config | Env-specific configs (staging/prod) → verify before flagging | Remove option | S |
| Interface with 1 implementation | Grep interface → count implementors | MEDIUM: premature abstraction | Interface for DI/testing mock → skip | Remove interface, use class directly | M |
| Premature generics (used with 1 type) | Grep generic usage → count type params | LOW: over-engineering | Public library API designed for consumers → skip | Replace generic with concrete type | S |

### 4. Missing Error Handling

- Find async functions without try-catch
- Check API routes without error middleware
- Verify database calls have error handling

| Severity | Criteria |
|----------|----------|
| **CRITICAL** | Payment/auth without error handling |
| **HIGH** | User-facing operations without error handling |
| **MEDIUM** | Internal operations without error handling |

**Effort:** M

### 5. Centralized Error Handling

- Search for centralized error handler: `ErrorHandler`, `errorHandler`, `error-handler.*`
- Check if middleware delegates to handler
- Verify async routes use promises/async-await
- **Anti-pattern:** `process.on("uncaughtException")` usage

| Severity | Criteria |
|----------|----------|
| **HIGH** | No centralized error handler |
| **HIGH** | Using `uncaughtException` listener (Express anti-pattern) |
| **MEDIUM** | Middleware handles errors directly (no delegation) |
| **MEDIUM** | Async routes without proper error handling |
| **LOW** | Stack traces exposed in production |

**Outcome Goal:** All errors are logged with context and return clear user-facing messages. No error is silently swallowed. Stack traces never leak to production responses. Implementation choice (ErrorHandler class, middleware, decorator) depends on project stack and size.

**Effort:** M-L

### 6. Dependency Injection / Centralized Init

- Check for DI container: `inversify`, `awilix`, `tsyringe` (Node), `dependency_injector` (Python), Spring `@Autowired` (Java), ASP.NET `IServiceCollection` (C#)
- Grep for `new SomeService()` in business logic (direct instantiation)
- Check for bootstrap module: `bootstrap.ts`, `init.py`, `Startup.cs`, `app.module.ts`

| Severity | Criteria |
|----------|----------|
| **MEDIUM** | No DI container (tight coupling) |
| **MEDIUM** | Direct instantiation in business logic |
| **LOW** | Mixed DI and direct imports |

**Outcome Goal:** Dependencies are replaceable for testing without modifying production code. No tight coupling between service instantiation and business logic. Implementation choice (DI container, factory functions, parameter injection, closures) depends on project size and stack.

**Effort:** L

### 7. Missing Best Practices Guide

- Check for: `docs/architecture.md`, `docs/best-practices.md`, `ARCHITECTURE.md`, `CONTRIBUTING.md`

| Severity | Criteria |
|----------|----------|
| **LOW** | No architecture/best practices guide |

**Recommendation:** Create `docs/architecture.md` with layering rules, error handling patterns, DI usage, coding conventions.

**Effort:** S

## Scoring Algorithm

**MANDATORY READ:** Load `shared/references/audit_worker_core_contract.md` and `shared/references/audit_scoring.md`.

## Output Format

**MANDATORY READ:** Load `shared/references/audit_worker_core_contract.md` and `shared/templates/audit_worker_report_template.md`.

Write report to `{output_dir}/623-principles-{domain}.md` (or `623-principles.md` in global mode) with `category: "Architecture & Design"`.

**FINDINGS-EXTENDED block (required for this worker):** After the Findings table, include a `<!-- FINDINGS-EXTENDED -->` JSON block containing all DRY findings with `pattern_signature` for cross-domain matching by ln-620 coordinator. Follow `shared/templates/audit_worker_report_template.md`.

**pattern_id:** DRY type identifier (`dry_1.1` through `dry_1.10`). Omit for non-DRY findings.

**pattern_signature:** Normalized key for the detected pattern (e.g., `validation_email`, `sql_users_findByEmail`, `middleware_auth_validate_ratelimit`). Same signature in multiple domains triggers cross-domain DRY finding. Format is defined in `references/detection_patterns.md`.

Return summary to coordinator:
```
Report written: docs/project/.audit/ln-620/{YYYY-MM-DD}/623-principles-users.md
Score: X.X/10 | Issues: N (C:N H:N M:N L:N)
```

## Critical Rules

**MANDATORY READ:** Load `shared/references/audit_worker_core_contract.md`.

- **Do not auto-fix:** Report only
- **Domain-aware scanning:** If `domain_mode="domain-aware"`, scan ONLY `scan_path`
- **Tag findings:** Include `domain` field in each finding when domain-aware
- **Pattern signatures:** Include `pattern_id` + `pattern_signature` for every DRY finding
- **Context-aware:** Use project's `principles.md` to define what's acceptable
- **Effort realism:** S = <1h, M = 1-4h, L = >4h
- **Exclusions:** Skip generated code, vendor, migrations (see `detection_patterns.md#exclusions`)

## Definition of Done

**MANDATORY READ:** Load `shared/references/audit_worker_core_contract.md`.

- contextStore parsed (including domain_mode, current_domain, output_dir)
- scan_path determined (domain path or codebase root)
- Detection patterns loaded from `references/detection_patterns.md`
- All 7 checks completed (scoped to scan_path):
  - DRY (10 subcategories: 1.1-1.10), KISS, YAGNI, Error Handling, Centralized Errors, DI/Init, Best Practices Guide
- Recommendations selected via `references/refactoring_decision_tree.md`
- Findings collected with severity, location, effort, pattern_id, pattern_signature, recommendation, domain
- Score calculated per `shared/references/audit_scoring.md`
- Report written to `{output_dir}/623-principles-{domain}.md` with FINDINGS-EXTENDED block (atomic single Write call)
- Summary returned to coordinator

## Reference Files

- **Detection patterns:** [references/detection_patterns.md](references/detection_patterns.md)
- **Refactoring decision tree:** [references/refactoring_decision_tree.md](references/refactoring_decision_tree.md)

---
**Version:** 5.0.0
**Last Updated:** 2026-02-08