Design Patterns — Detect · Diagnose · Design · Deliver

The complete design patterns skill. From function-level GoF to system-level Microservices. Not just theory — it scans your code, finds the pain, and generates the fix.

Announce at start: "I'm using the design-patterns skill to analyze your codebase architecture and apply professional design patterns."

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The Iron Laws

1. NO PATTERN WITHOUT PAIN
   — Don't apply Factory just because textbooks say so.
   — Show the EXACT line where current code breaks.

2. EVIDENCE BEFORE ELEGANCE
   — Measure complexity before proposing solutions.
   — If current code works, is testable, and is readable: LEAVE IT ALONE.

3. INCREMENTAL OVER REVOLUTIONARY
   — Never refactor 20 files in one PR.
   — One pattern, one module, one PR.

<HARD-GATE> Before proposing ANY pattern: 1. You MUST have scanned the target module's file tree 2. You MUST have identified at least ONE concrete anti-pattern with file:line evidence 3. You MUST have estimated the breaking impact (how many files change) If ANY of these is missing → STOP. Go back to scanning. </HARD-GATE>

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When to Trigger

Signal in Codebase	Severity	Pattern Category
Function > 100 lines	🟡 Medium	Code-Level (GoF)
Class doing 5+ unrelated things	🔴 High	SOLID violation
Business logic inside API route/controller	🔴 High	Architecture (Service Layer)
Database queries scattered across files	🟡 Medium	Architecture (Repository)
Duplicated logic in 3+ places	🟡 Medium	Code-Level (Strategy/Template)
Circular imports between modules	🔴 High	Architecture (Dependency Inversion)
Monolith > 50k LOC with mixed concerns	🔴 Critical	System (Microservices/DDD)
No error recovery or retry logic	🟡 Medium	System (Circuit Breaker)
Hardcoded config values	🟢 Low	Code-Level (Factory/Builder)

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The 4D Process: Detect → Diagnose → Design → Deliver

Phase 1: DETECT (Scan & Map)

digraph detect {
    rankdir=LR;
    scan [label="Scan\nFile Tree", shape=box, style=filled, fillcolor="#e6ffe6"];
    imports [label="Map\nImport Graph", shape=box];
    complexity [label="Score\nComplexity", shape=box];
    hotspots [label="Identify\nHotspots", shape=box, style=filled, fillcolor="#ffe6e6"];
    scan -> imports -> complexity -> hotspots;
}

What to scan:

1. File tree — list_dir recursively, understand module boundaries
2. Import graph — Who imports whom? Find coupling clusters
3. Function signatures — Count parameters (>5 = code smell)
4. File sizes — Lines per file (>300 = needs splitting)
5. Naming conventions — Are they consistent? (camelCase vs snake_case mix = chaos)

Output: A "Heat Map" of architectural debt:

🔴 HIGH DEBT:  src/api/routes.ts (450 lines, 12 functions, mixed DB + logic)
🟡 MED DEBT:   src/utils/helpers.ts (280 lines, "God module")
🟢 LOW DEBT:   src/models/user.ts (45 lines, clean data model)

Phase 2: DIAGNOSE (Identify Anti-Patterns)

For each hotspot, classify the anti-pattern:

Anti-Pattern	Description	Evidence Required
God Object	One class/module does everything	File > 300 LOC, > 8 methods
Spaghetti Code	No clear flow, tangled logic	Cyclomatic complexity > 15
Logic Leakage	Business rules in wrong layer	DB queries in route handlers
Shotgun Surgery	One change requires editing 5+ files	Grep shows same logic in multiple files
Feature Envy	Module uses another module's data more than its own	Import analysis
Primitive Obsession	Using strings/numbers instead of domain objects	No type definitions for core concepts
Tight Coupling	Modules directly depend on implementation details	No interfaces/abstractions

Output: Diagnosis table with file:line evidence.

Phase 3: DESIGN (Select Pattern)

Use the Pattern Selection Matrix to choose the right fix:

Scale 1: Code-Level (GoF + SOLID)

See: references/code-patterns.md

Problem	Pattern	When to Use
Multiple object creation paths	Factory Method	3+ constructors or conditional new
Complex object construction	Builder	Object with 5+ optional params
One global instance needed	Singleton	Config, DB connection pool
Algorithm varies at runtime	Strategy	if/else chains selecting behavior
React to state changes	Observer	Event systems, pub/sub
Add behavior without modifying	Decorator	Middleware, logging, caching
Simplify complex subsystem	Facade	External API wrapper
Undo/redo operations	Command	Action history, job queues
Process in steps	Template Method	ETL pipelines, test fixtures
Tree-like structures	Composite	UI components, file systems

Scale 2: Architecture-Level

See: references/architecture-patterns.md

Problem	Pattern	When to Use
Logic mixed with infrastructure	Clean Architecture	New project or major refactor
External dependencies hard to swap	Hexagonal (Ports & Adapters)	Multiple data sources
Complex domain with many rules	DDD (Domain-Driven Design)	Enterprise, fintech, healthcare
Business logic in controllers	Service Layer	Any app with > 10 endpoints
DB access scattered	Repository Pattern	Any app with database
Read/Write have different needs	CQRS	High-traffic, analytics-heavy

Scale 3: System-Level

See: references/system-patterns.md

Problem	Pattern	When to Use
Monolith too large to deploy	Microservices	Team > 5, independent deploy needed
Services need loose coupling	Event-Driven Architecture	Async workflows, notifications
Cascading failures	Circuit Breaker	External API calls
Distributed transactions	Saga Pattern	Multi-service data consistency
Traffic spikes	Bulkhead + Rate Limiter	Public APIs
Config management across services	Sidecar / Ambassador	Kubernetes / containerized

Phase 4: DELIVER (Generate Code)

This is what makes this skill different. After selecting the pattern, the skill generates production-ready boilerplate code specific to your framework.

Output includes:

1. New file structure — Where each file goes
2. Boilerplate code — Ready to paste, framework-specific
3. Migration steps — How to move old code into new structure
4. Test template — Test file for the new pattern
5. Rollback plan — How to undo if something breaks

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Complexity Scoring Formula

Rate each module on 4 dimensions (1-5 each, max 20):

Dimension	1 (Clean)	5 (Critical)
Size	< 100 LOC	> 500 LOC
Coupling	0-2 imports	> 10 imports
Cohesion	Single responsibility	5+ unrelated functions
Testability	Unit testable	Requires full app to test

Score interpretation:

• 4-8: ✅ Healthy — No action needed
• 9-13: 🟡 Moderate debt — Plan refactor in next sprint
• 14-17: 🔴 High debt — Refactor before adding features
• 18-20: 🚨 Critical — Stop feature work, refactor NOW

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Red Flags — STOP and Rethink

• You're adding an interface for a class used in only 1 place → Over-engineering
• You're splitting a 50-line file into 3 files → Premature abstraction
• You're using DDD for a CRUD app → Wrong scale
• You're applying Microservices to a 2-person team → Organizational mismatch
• The refactored code is harder to read than the original → Failed refactor
• You can't explain the pattern in one sentence → You don't understand it

Common Rationalizations

Excuse	Reality
"We need Clean Architecture for everything"	Clean Architecture for a TODO app is a war crime
"Singleton is always bad"	DB connection pool is a legitimate Singleton
"We should use microservices"	If you can't build a monolith, you can't build microservices
"Interfaces everywhere for testability"	Mock the boundary, not every function
"This pattern is industry standard"	Industry standard for YOUR scale? YOUR team?

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Output Format

Every design-patterns analysis MUST follow:

## 📊 Architecture Health Score: [X/20]

## 🔍 Detected Anti-Patterns
| # | Anti-Pattern | File:Line | Severity | Evidence |
|---|-------------|-----------|----------|----------|
| 1 | [name]      | [path:L]  | 🔴/🟡/🟢 | [description] |

## 💡 Recommended Patterns
| Anti-Pattern | → Pattern | Scale | Impact |
|-------------|-----------|-------|--------|
| [current]   | [proposed]| Code/Arch/System | [files affected] |

## 🏗️ Implementation Plan
### Step 1: [action]
- Files to create: [list]
- Files to modify: [list]
- Tests to add: [list]

## 📝 Generated Code
[Framework-specific boilerplate]

## ⚠️ Breaking Changes
[List of files/tests that will break]

## ↩️ Rollback Plan
[How to undo safely]

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References

• references/code-patterns.md — 23 GoF patterns + 5 SOLID principles with production code examples
• references/architecture-patterns.md — Clean, Hexagonal, DDD, CQRS, Service Layer, Repository
• references/system-patterns.md — Microservices, Event-Driven, Circuit Breaker, Saga, Cloud patterns
• references/framework-catalog.md — Pattern implementations for Next.js, FastAPI, NestJS, Django, Express, Go
• references/testing-patterns.md — How to test every pattern (unit, integration, e2e) with real test code
• references/anti-patterns.md — 15 anti-patterns catalog with detection commands, severities, and cures
• references/migration-playbook.md — 5 step-by-step migration guides with verification checkpoints
• references/output-format.md — ADR template, Refactoring Report, Health Check, Pattern Comparison