build-react-apps

name	build-react-apps
title	How to build React apps
description	Produce high-quality React apps from natural language descriptions.
allowed-tools	["files.*","sandbox.*","system.*"]

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. We use a light theme. Do not set background: black, background: #000, color: #fff, or any dark-theme values. All 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 light 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:

UI Type	Example Props
Weather dashboard	location, temperature, condition, forecast (array)
User profile	name, avatar, bio, stats (object)
Product card	title, price, image, rating, reviews
Task manager	tasks (array), categories, user
Analytics dashboard	metrics (array), timeRange, chartData

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

Category	Use	Never
Colors	var(--cg-color-...)	#hex, rgb(), named colors
Spacing	var(--cg-sp-...)	Raw pixel values for padding/margin/gap
Font sizes	var(--cg-text-...-size)	14px, 1rem
Border radius	var(--cg-radius-...) or var(--cg-card-radius)	12px, 24px
Shadows	var(--cg-elevation-...) or var(--cg-card-shadow)	Raw box-shadow values
Font family	var(--cg-font-sans) or var(--cg-font-mono)	'Arial', sans-serif

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,line-height,weight}, --cg-text-headline-{lg,md,sm}-{size,line-height,weight}, --cg-text-title-{lg,md,sm}-{size,line-height,weight}, --cg-text-body-{lg,md,sm}-{size,line-height,weight}, --cg-text-label-{lg,md,sm}-{size,line-height,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.
• Emit your data. The last view in your segment MUST call ark.emit() to hand collected data back to the orchestrator. Without this, the journey stalls.
• Receive context. Your segment may receive data from prior segments as props. Use it to personalize the experience.

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

Navigation

Within the segment, views navigate using window.opalSDK.navigateTo. At the boundary (the segment's final view), use window.opalSDK.emit to send data back to the orchestrator.

The SDK is available as window.opalSDK. It has three methods:

// Navigate to another view WITHIN this segment.
window.opalSDK.navigateTo("select_models", { teamProfile });

// Send data BACK TO THE ORCHESTRATOR (segment boundary).
// Use on the final view's CTA — this is what connects segments.
window.opalSDK.emit("journey:result", { decision, comparisonSet });

// Read a file from the shared workspace (async, returns text or null).
const data = await window.opalSDK.readFile("groceries.json");

Do not call any other methods on window.opalSDK. There is no onNavigation, subscribe, or event listener API. Navigation state is managed internally by your App component (e.g. useState + switch statement), not by the SDK.

Data Loading with readFile

Use readFile to load data files from the shared workspace at runtime instead of hardcoding data into your component. Paths are relative to the workspace root. You can read files written by any agent in the workspace — for example, a menu planner can read files produced by a diet researcher.

import React, { useState, useEffect } from "react";

export default function GroceryList() {
  const [items, setItems] = useState([]);
  const [loading, setLoading] = useState(true);

  useEffect(() => {
    window.opalSDK.readFile("groceries.json").then((text) => {
      if (text) {
        setItems(JSON.parse(text));
      }
      setLoading(false);
    });
  }, []);

  if (loading) return <div>Loading…</div>;
  return (
    <ul>
      {items.map((item, i) => (
        <li key={i}>{item}</li>
      ))}
    </ul>
  );
}

Rules for readFile:

• Returns Promise<string | null>. null means the file was not found.
• Paths are workspace-relative (e.g. "analysis/results.json", "diet_research/notes.md").
• Always handle the null case gracefully — the file may not exist yet.
• Show a loading state while data is being fetched.

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. Final view emits. The last view must include a CTA that calls window.opalSDK.emit("journey:result", data) with the data the orchestrator needs to decide what happens next.
6. 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.