Frontend builder spec

Authoritative specification for all PolicyEngine frontend projects (dashboards and interactive tools). Any agent building or validating a frontend MUST load this skill and follow every requirement below. Where another agent's instructions conflict with this spec, this spec wins.

Mandatory requirements

1. Tailwind CSS (v4+)

The application MUST use Tailwind CSS v4 for all styling. Tailwind utility classes are the primary styling mechanism.

• MUST install tailwindcss (v4+)
• MUST have a globals.css containing:

  @import "tailwindcss";
  @import "@policyengine/ui-kit/theme.css";
  

• MUST NOT have a tailwind.config.ts or tailwind.config.js — Tailwind v4 uses @theme in CSS instead
• MUST NOT have a postcss.config.js or postcss.config.mjs — Tailwind v4 does not require PostCSS
• MUST NOT use @tailwind base; @tailwind components; @tailwind utilities; — use @import "tailwindcss" instead
• MUST NOT use plain CSS files or CSS modules (*.module.css) for layout or styling
• MUST NOT use other CSS-in-JS libraries (styled-components, emotion, vanilla-extract)
• MUST NOT use other component frameworks for styling (Mantine, Chakra UI, Material UI)
• The only CSS files allowed are globals.css (which imports ui-kit theme)

2. @policyengine/ui-kit (component library + theme)

The application MUST install @policyengine/ui-kit and use it as the primary component library and design token source. MUST use ui-kit components when an equivalent exists — do NOT rebuild components that ui-kit already provides.

• MUST install: bun add @policyengine/ui-kit
• MUST import theme in globals.css: @import "@policyengine/ui-kit/theme.css";
• MUST use ui-kit components for all standard UI patterns (see availability table below)
• MAY build custom components only when no ui-kit equivalent exists

Component availability table:

Dashboard need	ui-kit component
Page shell	DashboardShell
Header with logo + nav	Header (light/dark variants, navLinks prop)
Two-column layout	SidebarLayout + InputPanel + ResultsPanel
Single-column narrative	SingleColumnLayout
Buttons	Button (4 variants, 3 sizes)
Cards	Card, CardHeader, CardTitle, CardContent, CardFooter
Badges	Badge (6 variants)
Tab navigation	Tabs, TabsList, TabsTrigger, TabsContent
Currency input	CurrencyInput
Number input	NumberInput
Select dropdown	SelectInput
Checkbox	CheckboxInput
Slider	SliderInput
Input grouping	InputGroup
KPI display	MetricCard (currency/percent, trends)
Summary text	SummaryText
Data tables	DataTable
Charts	ChartContainer, PEBarChart, PELineChart, PEAreaChart, PEWaterfallChart
Branding	PolicyEngineWatermark, logos.*
Utilities	formatCurrency, formatPercent, formatNumber

Component precedence rule: When building UI:

1. First: Use @policyengine/ui-kit if it has the component
2. Second: Use shadcn/ui primitives (Dialog, Popover, Tooltip, Select, DropdownMenu, etc.) styled with Tailwind semantic classes
3. Third: Build custom from scratch with Tailwind utility classes

3. Design tokens via ui-kit theme

The application MUST load design tokens from @policyengine/ui-kit/theme.css. This single CSS import provides all colors, spacing, typography, and chart tokens.

• MUST import theme in globals.css: @import "@policyengine/ui-kit/theme.css";
• MUST NOT load tokens via CDN <link> — the theme is bundled with ui-kit
• MUST NOT hardcode hex color values when a design token exists
• MUST NOT hardcode pixel spacing values when a Tailwind spacing class exists
• MUST NOT hardcode font-family values — use var(--font-sans)
• MAY use custom values when no token covers the need (e.g., chart-specific dimensions, animation durations)

Token usage patterns:

Context	Approach	Example
React components	Tailwind semantic classes	className="bg-primary text-foreground"
Brand palette	Tailwind direct classes	className="bg-teal-500 text-gray-600"
Recharts (SVG)	CSS vars directly in fill/stroke	fill="var(--chart-1)"
Inline styles	CSS vars	style={{ color: "var(--primary)" }}

4. Framework: Next.js (App Router)

The application MUST use Next.js with the App Router.

• MUST use create-next-app or equivalent to scaffold with App Router
• MUST have next.config.ts at the project root
• MUST have an app/ directory with layout.tsx and page.tsx
• MUST use TypeScript (.ts/.tsx files, tsconfig.json)
• MUST NOT use the Pages Router (pages/ directory)
• MUST NOT use Vite as the application bundler (Vite is only used by Vitest for testing)
• MUST NOT use other bundlers (Webpack, Parcel, esbuild, etc.)
• MUST NOT use other meta-frameworks (Remix, Gatsby, Astro, etc.)

5. Package manager: bun

The application MUST use bun as the package manager.

• MUST use bun install instead of npm install
• MUST use bun run dev, bun run build instead of npm run dev, npm run build
• MUST use bunx vitest run instead of npx vitest run
• MUST have a bun.lock lockfile (not package-lock.json)
• MUST NOT use npm, yarn, or pnpm

6. Vercel deployment

The application MUST be deployed using Vercel.

• MUST have a vercel.json at the project root with appropriate configuration
• MUST use output: 'export' in next.config.ts for static export, unless the dashboard requires server-side rendering
• MUST configure the Vercel project to build from the repository root (not a subdirectory)
• MUST set any required environment variables in the Vercel project settings using the NEXT_PUBLIC_* prefix
• MUST deploy under the policy-engine Vercel scope
• MUST NOT deploy using other hosting platforms (Netlify, AWS Amplify, GitHub Pages, etc.) for the frontend

7. shadcn/ui for custom components

When building custom components not available in @policyengine/ui-kit, the application SHOULD use shadcn/ui primitives as the base layer.

• SHOULD initialize shadcn/ui: bunx shadcn@latest init
• SHOULD use shadcn/ui for: Dialog, Popover, Tooltip, Select, DropdownMenu, Accordion, Sheet, and other interaction primitives
• MUST style shadcn/ui components with Tailwind semantic classes (the ui-kit theme already defines shadcn/ui semantic tokens like background, foreground, primary, muted)
• MUST NOT use shadcn/ui when an equivalent @policyengine/ui-kit component exists

Tailwind v4 + ui-kit theme integration pattern

globals.css

@import "tailwindcss";
@import "@policyengine/ui-kit/theme.css";

body {
  font-family: var(--font-sans);
  color: var(--foreground);
  background: var(--background);
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

The single @import "@policyengine/ui-kit/theme.css" provides:

1. :root variables — shadcn/ui semantic tokens (--primary, --background, --chart-1, etc.)
2. @theme inline — Bridges :root vars to Tailwind utilities (bg-primary, text-foreground)
3. @theme — Brand palette (bg-teal-500, text-gray-600), font sizes, spacing, breakpoints

Next.js: app/layout.tsx

import './globals.css'
import { Inter } from 'next/font/google'
import type { Metadata } from 'next'

const inter = Inter({ subsets: ['latin'] })

export const metadata: Metadata = {
  title: 'DASHBOARD_TITLE - PolicyEngine',
  description: 'DASHBOARD_DESCRIPTION',
  icons: { icon: '/favicon.svg' },
}

export default function RootLayout({ children }: { children: React.ReactNode }) {
  return (
    <html lang="en">
      <body className={inter.className}>{children}</body>
    </html>
  )
}

Usage in components

// Prefer ui-kit components:
import { MetricCard, Button, Card, CardContent } from '@policyengine/ui-kit';
import { formatCurrency } from '@policyengine/ui-kit';

// Use Tailwind classes with semantic and brand tokens for custom layouts:
<div className="bg-background border border-border rounded-lg p-4 flex flex-col gap-1">
  <span className="text-sm text-muted-foreground font-medium">Metric title</span>
  <span className="text-2xl font-bold text-foreground">{formatCurrency(1234)}</span>
</div>

// Brand colors when needed:
<div className="bg-teal-500 text-white hover:bg-teal-600">Primary teal</div>

// Responsive design uses Tailwind breakpoint prefixes:
<main className="max-w-content mx-auto px-6 py-4 font-sans text-foreground">
  <div className="flex gap-6 md:flex-col">
    {/* sidebar collapses at md breakpoint */}
  </div>
</main>

// Recharts uses CSS vars directly:
<Line stroke="var(--chart-1)" />
<Bar fill="var(--chart-2)" />
<CartesianGrid stroke="var(--border)" />

// Header with logo and structured nav links:
import { Header, logos } from '@policyengine/ui-kit';

<Header
  variant="dark"
  logo={<img src={logos.whiteWordmark} alt="PolicyEngine" className="h-5" />}
  navLinks={[
    { slug: 'research', text: 'Research', href: 'https://policyengine.org/research' },
  ]}
>
  <span className="ml-2">Dashboard Title</span>
</Header>

Project structure

DASHBOARD_NAME/
├── app/
│   ├── layout.tsx              # Root layout — Inter font + globals.css
│   ├── page.tsx                # Main dashboard page
│   ├── globals.css             # @import "tailwindcss" + @import ui-kit theme
│   └── providers.tsx           # React Query provider (client component)
├── components/
│   └── (from plan.yaml)        # Custom dashboard components (only if not in ui-kit)
├── lib/
│   ├── api/
│   │   ├── client.ts           # API client (stubs or real)
│   │   ├── types.ts            # Request/response TypeScript types
│   │   └── fixtures.ts         # Mock data for stubs
│   ├── embedding.ts            # Country detection, hash sync, share URLs
│   └── hooks/
│       └── useCalculation.ts   # React Query hooks
├── public/
├── next.config.ts
├── tsconfig.json
├── package.json
├── vitest.config.ts
├── plan.yaml
├── CLAUDE.md
├── README.md
├── vercel.json
└── .gitignore

Package dependencies

Production:

• next
• react, react-dom
• tailwindcss (v4+)
• @policyengine/ui-kit
• @tanstack/react-query
• recharts (if custom charts beyond ui-kit)
• react-plotly.js, plotly.js-dist-min (if maps)
• axios

Development:

• typescript, @types/react, @types/react-dom, @types/node
• vitest, @vitejs/plugin-react, jsdom
• @testing-library/react, @testing-library/jest-dom

Testing

Vitest is the test runner. Configure vitest.config.ts:

import { defineConfig } from 'vitest/config'
import react from '@vitejs/plugin-react'

export default defineConfig({
  plugins: [react()],
  test: {
    environment: 'jsdom',
    globals: true,
    setupFiles: ['./vitest.setup.ts'],
  },
})

Note: @vitejs/plugin-react is only used by Vitest for JSX transform during testing — Vite is NOT used as the application bundler.

What NOT to do

• MUST NOT use Vite as the application bundler — only Next.js is allowed (Vite is used only by Vitest for testing)
• MUST NOT use other bundlers (Webpack, Parcel, esbuild)
• MUST NOT use other meta-frameworks (Remix, Gatsby, Astro)
• MUST NOT use the Next.js Pages Router — use App Router only
• MUST NOT have tailwind.config.ts or postcss.config.js — Tailwind v4 uses @theme in CSS
• MUST NOT use @tailwind base; @tailwind components; @tailwind utilities; — use @import "tailwindcss"
• MUST NOT use plain CSS files or CSS modules for layout/styling
• MUST NOT use styled-components, emotion, or vanilla-extract
• MUST NOT use Mantine, Chakra UI, Material UI, or other component frameworks for styling
• MUST NOT hardcode hex color values when a design token exists
• MUST NOT hardcode pixel spacing values when a Tailwind spacing class exists
• MUST NOT hardcode font-family values — use var(--font-sans) via Tailwind
• MUST NOT deploy to platforms other than Vercel
• MUST NOT use npm, yarn, or pnpm — use bun
• MUST NOT rebuild components that exist in @policyengine/ui-kit
• MUST NOT use getCssVar() — it no longer exists. SVG accepts var() directly.

Related skills

• policyengine-design-skill — Complete design token reference (colors, typography, spacing, chart branding)
• policyengine-interactive-tools-skill — Embedding, hash sync, country detection patterns
• policyengine-app-skill — app-v2 component architecture reference