App Builder

Full-stack application builder that creates web apps, APIs, mobile apps, and more from natural language requests. Use when the user wants to build a new application, add features to existing projects, scaffold a project structure, or plan an implementation.

App Builder Skill

This skill provides structured knowledge for building full-stack applications from scratch or enhancing existing projects. It covers project detection, tech stack selection, scaffolding patterns, implementation planning, and feature building.

Contents

1. Project Detection - Keyword matrix for identifying project types
2. Tech Stack - 2025 default technologies and alternatives
3. Scaffolding - Directory structures and core files
4. Implementation Planning - Requirement analysis, task breakdown, and plan format
5. Feature Building - Analysis and implementation patterns
6. Coordination - Multi-phase development workflow

---

1. Project Detection

Keyword Matrix

Keywords	Project Type	Recommended Stack
blog, post, article	Blog	Next.js Static
e-commerce, product, cart, payment	E-commerce	Next.js + Stripe
dashboard, panel, management	Admin Dashboard	Next.js + Supabase
api, backend, service, rest	API Service	Express or FastAPI
python, fastapi, django	Python API	FastAPI
mobile, android, ios, react native	Mobile App	React Native (Expo)
portfolio, personal, cv	Portfolio	Next.js Static
crm, customer, sales	CRM	Next.js + Supabase
saas, subscription, stripe	SaaS	Next.js + Stripe + Auth
landing, promotional, marketing	Landing Page	Next.js Static
extension, plugin, chrome	Browser Extension	Chrome MV3
cli, command line, terminal	CLI Tool	Node.js

Detection Process

1. Tokenize user request
2. Extract keywords and match to project type
3. Identify missing information → ask clarifying questions
4. Suggest appropriate tech stack
5. Confirm with user before proceeding

---

2. Tech Stack Selection (2025)

Default Web App Stack

Frontend:
  framework: Next.js 16 (Stable)
  language: TypeScript 5.7+
  styling: Tailwind CSS v4
  state: React 19 Actions / Server Components
  bundler: Turbopack (Dev Mode)

Backend:
  runtime: Node.js 23
  framework: Next.js API Routes
  validation: Zod

Database:
  primary: PostgreSQL
  provider: Supabase
  orm: Prisma

Auth:
  provider: Supabase Auth or Clerk

Deployment:
  tool: Built-in deploy command

Alternative Options

Need	Default	Alternative
Real-time	Supabase Realtime	Socket.io
File storage	Supabase Storage	Cloudinary, S3
Payment	Stripe	LemonSqueezy, Paddle
Email	Resend	SendGrid
Search	-	Algolia, Typesense

---

3. Project Scaffolding

Next.js Full-Stack Structure

project-name/
├── src/
│   ├── app/                        # Routes only (thin layer)
│   │   ├── layout.tsx
│   │   ├── page.tsx
│   │   ├── globals.css
│   │   ├── (auth)/                 # Route group - auth pages
│   │   │   ├── login/page.tsx
│   │   │   └── register/page.tsx
│   │   ├── (dashboard)/            # Route group - dashboard
│   │   │   ├── layout.tsx
│   │   │   └── page.tsx
│   │   └── api/
│   │       └── [resource]/route.ts
│   │
│   ├── features/                   # Feature-based modules
│   │   ├── auth/
│   │   │   ├── components/
│   │   │   ├── hooks/
│   │   │   ├── actions.ts          # Server Actions
│   │   │   ├── queries.ts          # Data fetching
│   │   │   └── types.ts
│   │   └── [other-features]/
│   │
│   ├── shared/                     # Shared utilities
│   │   ├── components/ui/          # Reusable UI components
│   │   ├── lib/                    # Utils, helpers
│   │   └── hooks/                  # Global hooks
│   │
│   └── server/                     # Server-only code
│       ├── db/                     # Database client
│       ├── auth/                   # Auth config
│       └── services/               # External API integrations
│
├── prisma/
│   ├── schema.prisma
│   └── seed.ts
│
├── public/
├── .env.example
├── package.json
├── tailwind.config.ts
└── tsconfig.json

Structure Principles

Principle	Implementation
Feature isolation	Each feature in features/ with its own components, hooks, actions
Server/Client separation	Server-only code in server/, prevents accidental client imports
Thin routes	app/ only for routing, logic lives in features/
Route groups	(groupName)/ for layout sharing without URL impact
Shared code	shared/ for truly reusable UI and utilities

Path Aliases (tsconfig.json)

{
  "compilerOptions": {
    "paths": {
      "@/*": ["./src/*"],
      "@/features/*": ["./src/features/*"],
      "@/shared/*": ["./src/shared/*"],
      "@/server/*": ["./src/server/*"]
    }
  }
}

---

4. Implementation Planning

When planning a new project or feature, follow this structured approach before writing any code.

Requirement Analysis

1. Break down user requests into concrete features
2. Identify data models and their relationships
3. Determine API endpoints or server actions needed
4. List UI components required
5. Identify potential blockers early

Task Breakdown

• Create an ordered task list with dependencies
• Estimate complexity for each task (simple / medium / complex)
• Flag tasks that require user decisions or external services

Plan Output Format

When presenting a plan to the user, use this structure:

# Implementation Plan: [Project Name]

## Overview
[Brief description of what will be built]

## Data Models
- Model1: [fields and relationships]
- Model2: [fields and relationships]

## API Routes / Server Actions
- POST /api/resource - [description]
- GET /api/resource - [description]

## Pages & Components
- /page-name
  - ComponentA
  - ComponentB

## Implementation Order
1. [ ] Database schema
2. [ ] API routes / server actions
3. [ ] UI components
4. [ ] Integration & wiring
5. [ ] Testing & error fixing

## Dependencies
- [package1]
- [package2]

Planning Guidelines

• Be specific and actionable in each task
• Consider edge cases and error states
• Plan for input validation and error handling
• Keep security in mind (auth, authorization, input sanitization)
• Present the plan to the user and get approval before proceeding

---

5. Feature Building

Feature Analysis Template

Request: "[user feature request]"

Analysis:
├── Required Changes:
│   ├── Database: [tables/columns needed]
│   ├── Backend: [API routes/actions needed]
│   ├── Frontend: [components/pages needed]
│   └── Config: [environment variables needed]
│
├── Dependencies:
│   ├── [npm packages]
│   └── [existing features required]
│
└── Implementation Steps:
    1. [Step 1]
    2. [Step 2]
    ...

Iterative Enhancement Process

1. Analyze existing project structure
2. Create detailed change plan
3. Present plan to user for approval
4. Get confirmation
5. Apply changes incrementally
6. Test after each change
7. Deploy and show preview

Error Handling

Error Type	Solution Strategy
TypeScript Error	Fix type, add missing import
Missing Dependency	Install with npm/pnpm
Build Error	Check syntax, verify imports
Database Error	Check schema, validate migrations
Runtime Error	Debug logic, check API responses

Recovery Strategy

1. Detect error from build/runtime output
2. Attempt automatic fix
3. If failed, report to user with context
4. Suggest alternative approach
5. Rollback if necessary (git)

---

6. Development Coordination

Core Workflow

1. ANALYZE → Understand user request, detect project type
2. PLAN → Create detailed implementation plan, get user approval
3. BUILD → Scaffold structure, implement features (database → backend → frontend)
4. TEST → Run build, verify functionality, fix errors
5. DEPLOY → Deploy application and provide URL

Execution Phases

Phase	Focus	Checkpoint
1. Analysis	Understand requirements	Clear spec confirmed
2. Planning	Create implementation plan	Plan approved by user
3. Database	Schema design, migrations	Schema created
4. Backend	API routes, server actions	Endpoints working
5. Frontend	Components, pages, styling	UI complete
6. Testing	Build check, error fixing	Build passes
7. Deployment	Deploy to production	Live URL provided

Quality Gates

• Before Phase 3: User must approve the plan
• Before Phase 7: Build must pass without errors
• After Deployment: Verify the live site works

---

Usage Guidelines

1. Always start with project type detection - Match keywords to determine the best approach
2. Present plans before implementing - Get user confirmation on significant decisions
3. Build incrementally - Test after each major change
4. Deploy early - Get a working version deployed quickly, then iterate
5. Handle errors gracefully - Fix issues and continue, don't give up
6. Type safety throughout - Full TypeScript coverage with Zod validation for inputs
7. Progressive enhancement - Start simple, add complexity as needed