building-fullstack-features

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name: building-fullstack-features description: Guides end-to-end implementation of features in side projects, from design decision to deployment. Use when building a new feature from scratch, connecting frontend to backend, setting up a new side project architecture, or following a complete implementation checklist. Triggers on: "build this feature", "implement end-to-end", "new feature from scratch", "set up project structure", "ship this". Covers the Next.js 15 + React 19 + Supabase + Vercel stack.

Building Fullstack Features

Core Philosophy

Ship end-to-end, then iterate. A half-built feature is a bug waiting to happen — it creates confused state between the DB, API, and UI that takes longer to debug than building it right from the start.

Define the contract before writing a single line of code. The order is: scope → API contract → DB schema → frontend → deploy. Never skip to UI before the API is defined.

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Workflow

Step 1 — Feature Scope

Write one sentence that answers: what does this feature do, who uses it, and what problem does it solve?

Feature: [Name]
Sentence: [User] can [action] so that [outcome].

Then list explicit non-goals — features that sound related but are out of scope for this iteration. Non-goals prevent scope creep during implementation.

Output: annotate the sentence + non-goals at the top of your implementation doc.

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Step 2 — API Contract (before writing code)

Define every endpoint or server action before implementing any of them:

// Resource: [resource name]
// Base: /api/[resource]

POST   /api/[resource]          // Create
GET    /api/[resource]          // List (cursor-based pagination)
GET    /api/[resource]/:id      // Get one
PATCH  /api/[resource]/:id      // Update (partial)
DELETE /api/[resource]/:id      // Soft delete

For each endpoint, document:

• Request body: TypeScript type
• Response (success): type + status code
• Response (error): { error: string, code: string, message: string }
• Auth requirement: public / authenticated / role-scoped

Define error codes as string constants, not raw HTTP codes. RESOURCE_NOT_FOUND is more useful than 404 in client error handling.

Output: docs/features/YYYY-MM-DD-[feature-name]/api.md

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Step 3 — Database Schema

Design tables before writing any SQL migration:

-- [table_name]
CREATE TABLE [table_name] (
  id          UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  user_id     UUID NOT NULL REFERENCES auth.users(id) ON DELETE CASCADE,
  [fields]    [types] NOT NULL,
  created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
  updated_at  TIMESTAMPTZ NOT NULL DEFAULT NOW()
);

-- Indexes
CREATE INDEX idx_[table]_user_id ON [table_name](user_id);
CREATE INDEX idx_[table]_created_at ON [table_name](created_at DESC);

-- RLS
ALTER TABLE [table_name] ENABLE ROW LEVEL SECURITY;

CREATE POLICY "[table]: users read own rows"
  ON [table_name] FOR SELECT
  USING (auth.uid() = user_id);

CREATE POLICY "[table]: users insert own rows"
  ON [table_name] FOR INSERT
  WITH CHECK (auth.uid() = user_id);

Rules to follow:

• Enable RLS on every table — skipping this is a security vulnerability, not a shortcut
• Index every foreign key column — Postgres does not do this automatically
• Use TIMESTAMPTZ, not TIMESTAMP — timezone-aware by default
• Add a NOT NULL constraint unless NULL has explicit semantic meaning for that field

Output: docs/features/YYYY-MM-DD-[feature-name]/schema.md

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Step 4 — Frontend Implementation

Implement UI in this order: data shape → server component → client component → loading state → error state.

Data shape first:

// types/[feature].ts
export interface FeatureItem {
  id: string
  userId: string
  // ...fields from schema
  createdAt: string
}

Server component (fetch + layout):

// app/[route]/page.tsx
export default async function FeaturePage() {
  const data = await getFeatureItems() // server-side fetch
  return <FeatureList items={data} />
}

Client component (interactions):

"use client"
// Only interactive parts need "use client"
// Keep server boundary as high as possible

Loading state: Every fetch must have a loading.tsx or Suspense fallback.

Error state: Every mutation must have explicit error handling shown to the user — no silent failures.

Form validation: Use Zod schema on both client (for UX) and server (for security):

const schema = z.object({ field: z.string().min(1) })
// Validate on client with react-hook-form + zodResolver
// Re-validate on server action before DB write

Output: components in components/[feature]/, hooks in hooks/use-[feature].ts

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Step 5 — Deploy & Monitor

Before pushing to production:

Environment variables checklist:

• All new env vars added to Vercel project settings
• Local .env.local updated
• .env.example updated with key names (not values)

Feature flag (if gradual rollout):

// lib/flags.ts
export const FEATURE_FLAGS = {
  newFeature: process.env.NEXT_PUBLIC_FF_NEW_FEATURE === 'true'
}

First metric: Define what success looks like in numbers before launch. Example: "This feature succeeds if 30% of active users use it within 7 days." Add the tracking event:

// Track the primary action
analytics.track('feature_action_completed', { userId, featureId })

Error tracking: Verify errors in the new code paths are captured by your error monitoring (Sentry, etc.).

Output: docs/features/YYYY-MM-DD-[feature-name]/implementation-notes.md

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Tech Stack Reference

Layer	Technology
Framework	Next.js 15 (App Router)
UI	React 19 + TypeScript
Styling	Tailwind CSS v4 + shadcn/ui
Database	Supabase (Postgres + Auth + Realtime)
Deployment	Vercel
Validation	Zod
Forms	react-hook-form
API client	SWR (client-side)

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Output Artifacts

docs/features/YYYY-MM-DD-[feature-name]/
├── api.md               # Endpoint contracts, request/response types
├── schema.md            # DB tables, indexes, RLS policies
└── implementation-notes.md  # Decisions, gotchas, first metric

Quality Checklist

• Feature scope defined in one sentence with explicit non-goals
• API contract documented before any implementation code
• All tables have RLS enabled
• All foreign key columns have an index
• Every fetch has a loading state; every mutation has an error state
• Zod validation on both client and server
• First metric and tracking event defined before launch
• Environment variables documented in .env.example

Common Antipatterns

• Building UI before API contract — The UI ends up driving the DB shape instead of the domain model. Define the contract first.
• Skipping RLS "for now" — There is no "later" for security. Enable RLS in the migration that creates the table.
• No loading/error states — Users see blank screens. Every async operation needs a visual state for loading and failure.
• Magic strings for error codes — Use string constants. SUBSCRIPTION_LIMIT_REACHED is debuggable; 422 is not.

See Also

• api-design-principles — For REST and GraphQL API design conventions
• supabase-postgres — For database schema, RLS, and query optimization
• react-best-practices — For React and Next.js performance patterns
• vercel-composition-patterns — For component architecture and React 19 patterns
• product-launch — For go-to-market checklist after the feature ships