Build React Apps

How to build high-quality React apps

What You're Building

A multi-file React component bundle rendered in a sandboxed iframe. The bundle consists of:

• App.jsx — the root component that accepts configuration props
• components/*.jsx — reusable sub-components
• styles.css — shared styles using CSS custom properties

Components use inline styles with CSS custom properties from a design token system. Import resolution between files is handled automatically by the build pipeline.

Workflow

Your UI MUST work from 320px to 1200px+. This is not optional.

1. Write the app an its components as files in your directory.

2. Build the bundle with bundler.mjs:

node ./skills/{name of this skill}/scripts/bundler.mjs

If you do not run this exact command, the user will see a blank screen.

3. Once bundling is successful, return a brief confirmation that the UI was generated and bundled. Don't enclose produced files.

Developer Rules

Follow these rules strictly.

1. App.jsx is the entry point. It must be named exactly App.jsx and contain a function called App.
2. App carries the configuration. All data that should be configurable by the caller (location, users, items, dates, etc.) appears as props on App with realistic defaults.
3. Sub-components are reusable. Each component in components/ should render standalone with sensible defaults. Document all props with @prop JSDoc.
4. Every file imports what it uses. Include import React from "react" in every JSX file. Import sub-components with relative paths (e.g. import Header from "./components/Header").
5. CSS imports work. Use import "./styles.css" in App.jsx for shared styles.
6. Export default. Each component file must export default its component function.
7. All colors, spacing, typography, and radii MUST use --cg- design tokens. No hex colors, no rgb(), no named colors, no raw pixel values. Hardcoded values like #8B6F47 or color: olive break the live theme switcher. This is a build error, not a suggestion.
8. Your output renders inside a host application. Don't create app names, brand headers, splash screens, or taglines. Start with the actual task UI. The host provides the chrome.
9. No client storage APIs. Do not use localStorage, sessionStorage, IndexedDB, or any Web Storage APIs. The iframe environment may not have storage access due to origin restrictions. All state lives in React component state or is passed via props and the SDK.
10. Respect the host theme. Use design tokens. Backgrounds must use var(--cg-color-surface*) tokens and all text must use var(--cg-color-on-surface*) tokens. The tokens will natively map to their theme variants.
11. Use flex-wrap: wrap on any flex container with multiple children that should stack on narrow screens.
12. Use CSS Grid with auto-fit / minmax for multi-column layouts: grid-template-columns: repeat(auto-fit, minmax(min(100%, 280px), 1fr)).
13. Use clamp() for font sizes that should scale with viewport: font-size: clamp(1rem, 2vw + 0.5rem, 2rem).
14. Test your mental model at 375px, 768px, and 1200px before finishing.

These particular paterns are forbidden.

• No fixed pixel widths on containers or layout elements. Never write width: 800px, width: 600px, or similar. Use max-width with a percentage or min() instead: max-width: min(100%, 800px).
• No min-width exceeding 320px on any layout container. This prevents rendering on small screens.
• No horizontal overflow. If your layout causes a horizontal scrollbar at any viewport width ≥ 320px, it is broken.

CRITICAL FORBIDDEN PATTERN: NO TAILWIND!

• You MUST NOT use Tailwind CSS utility classes (e.g., flex, min-h-screen, p-4, bg-red-500). Tailwind is NOT INSTALLED.
• You MUST use standard semantic CSS class names (e.g., className="hero-container").
• You MUST write a separate styles.css file using Vanilla CSS.
• You MUST include import "./styles.css"; at the top of your App.jsx. If you don't write and import a CSS file, your design will be completely unstyled.

Configuration Props

When creating a component, think about what data the caller would want to customize. These become props on App:

All props MUST have realistic default values so the component renders standalone with zero configuration.

Design Token System

Reminder: this is a hard rule (see above). Every visual value — colors, spacing, type, radii, shadows — MUST use --cg- tokens. No exceptions.

Token Rules

Available Tokens

Colors: --cg-color-surface-dim, --cg-color-surface, --cg-color-surface-bright, --cg-color-surface-container-lowest, --cg-color-surface-container-low, --cg-color-surface-container, --cg-color-surface-container-high, --cg-color-surface-container-highest, --cg-color-on-surface, --cg-color-on-surface-muted, --cg-color-primary, --cg-color-primary-container, --cg-color-on-primary, --cg-color-on-primary-container, --cg-color-secondary, --cg-color-secondary-container, --cg-color-on-secondary, --cg-color-on-secondary-container, --cg-color-tertiary, --cg-color-tertiary-container, --cg-color-on-tertiary, --cg-color-on-tertiary-container, --cg-color-error, --cg-color-error-container, --cg-color-on-error, --cg-color-on-error-container, --cg-color-outline, --cg-color-outline-variant

Typography: --cg-font-sans, --cg-font-mono, --cg-text-display-{lg,md,sm}-{size,weight}, --cg-text-headline-{lg,md,sm}-{size,weight}, --cg-text-title-{lg,md,sm}-{size,weight}, --cg-text-body-{lg,md,sm}-{size,weight}, --cg-text-label-{lg,md,sm}-{size,weight}

Spacing (4px grid): --cg-sp-0 through --cg-sp-16

Radius: --cg-radius-{xs,sm,md,lg,xl,full}

Elevation: --cg-elevation-{1,2,3}

Motion: --cg-motion-duration-{short,medium,long}, --cg-motion-easing-{standard,decel,accel}

Component tokens: Card: --cg-card-{bg,radius,padding,shadow}, Button: --cg-button-{radius,padding,bg,color,font-size,font-weight}, Input: --cg-input-{bg,border,radius,padding,color,placeholder}, Badge: --cg-badge-{bg,color,radius,padding,font-size}, Divider: --cg-divider-{color,thickness,style}

Expressive: --cg-border-{style,width}, --cg-heading-{transform,letter-spacing}, --cg-img-{radius,border,shadow,filter}, --cg-hover-{scale,brightness,shadow}

Component Design

Decomposition

• Compose, don't monolith. A dashboard should be built from Header, MetricsGrid, ForecastCard, etc.
• Each component renders standalone with realistic defaults.
• The top-level App composes everything into a cohesive layout.

Icons

Google Material Symbols Outlined is available via a web font:

<span className="material-symbols-outlined" style={{ fontSize: "20px" }}>
  search
</span>

CRITICAL: DO NOT import third-party icon packages. You do not have lucide-react, heroicons, or react-icons installed. Using them will break the build. ONLY use the material-symbols-outlined span.

Interactivity

Components should be interactive where appropriate. Use useState, useEffect with cleanup. Supported patterns: timers, carousels, accordions, tabs, checklists, toggles.

Stable Defaults

Never use Date.now(), Math.random(), or new Date() in default parameters. Compute once at module level or use useState(() => ...).

Critical: You're building a Segment, Not a Standalone App

Your React app is one segment of a wider orchestrated journey. Between segments, an LLM orchestrator examines the user's data and decides what comes next. This means:

• Don't brand it. No app names, no splash screens, no taglines. The host application provides the chrome and framing. Start with the task UI.

File Structure

Each state gets its own component file named after the state:

• App.jsx — shell that renders the initial state
• views/InputRequirements.jsx — one view per state
• views/SelectModels.jsx
• views/DetailedComparison.jsx
• views/DecisionReport.jsx
• components/*.jsx — shared sub-components (reusable across views)
• styles.css — shared styles

View Contract

Each view component receives two props:

• data — the journey context relevant to this state
• onTransition — callback for state transitions (wired to window.opalSDK.navigateTo)

export default function SelectModels({ data = {}, onTransition }) {
  const handleSelect = (item) => {
    onTransition("detailed_comparison", {
      ...data,
      shortlist: [...(data.shortlist || []), item],
    });
  };
  // ...
}

Rules

1. One view per state. Don't merge states into a single component.
2. Views are self-contained. Each view renders standalone with defaults.
3. Shared components go in components/. Headers, cards, buttons used across multiple views should be extracted.
4. Context flows forward. Each transition carries the data the next view needs. Views may also use readFile to load shared workspace data.
5. Responsive. The user may view this UI on a mobile device, so ensure that you make every component and the app itself responsive.

Available Globals

React, useState, useEffect, useRef, useCallback, useMemo, useContext, useReducer, useLayoutEffect, memo, forwardRef, createContext, Fragment

Be creative and visually impressive.

Source: breadboard-ai/breadboard — distributed by TomeVault.