Remix Core Knowledge

Full Reference: See advanced.md for WebSocket integration, Server-Sent Events, Socket.IO patterns, and room management.

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: remix for comprehensive documentation.

Route Module

import type { LoaderFunctionArgs, ActionFunctionArgs } from '@remix-run/node';
import { json } from '@remix-run/node';
import { useLoaderData, Form } from '@remix-run/react';

// Server: Load data
export async function loader({ params }: LoaderFunctionArgs) {
  const user = await db.users.find(params.id);
  if (!user) throw new Response('Not Found', { status: 404 });
  return json({ user });
}

// Server: Handle mutations
export async function action({ request }: ActionFunctionArgs) {
  const formData = await request.formData();
  await db.users.update({ name: formData.get('name') });
  return json({ success: true });
}

// Client: UI
export default function UserPage() {
  const { user } = useLoaderData<typeof loader>();
  return (
    <Form method="post">
      <input name="name" defaultValue={user.name} />
      <button type="submit">Save</button>
    </Form>
  );
}

Key Concepts

Nested Routes

routes/
├── _index.tsx        → /
├── users.tsx         → /users (layout)
├── users._index.tsx  → /users (index)
└── users.$id.tsx     → /users/:id

Error Handling

export function ErrorBoundary() {
  const error = useRouteError();
  return <div>Error: {error.message}</div>;
}

Production Readiness

Security

// app/utils/csrf.server.ts
import { createCookie } from '@remix-run/node';

export const csrfCookie = createCookie('csrf', {
  httpOnly: true,
  secure: process.env.NODE_ENV === 'production',
  sameSite: 'strict',
});

// app/root.tsx - Security headers
export const headers: HeadersFunction = () => ({
  'X-Frame-Options': 'DENY',
  'X-Content-Type-Options': 'nosniff',
  'Referrer-Policy': 'strict-origin-when-cross-origin',
  'Content-Security-Policy': "default-src 'self'",
});

// Input validation
import { z } from 'zod';

const UserSchema = z.object({
  email: z.string().email(),
  name: z.string().min(2).max(100),
});

export async function action({ request }: ActionFunctionArgs) {
  const formData = await request.formData();
  const result = UserSchema.safeParse(Object.fromEntries(formData));

  if (!result.success) {
    return json({ errors: result.error.flatten() }, { status: 400 });
  }

  // Process valid data
}

Error Handling

// app/root.tsx
export function ErrorBoundary() {
  const error = useRouteError();

  if (isRouteErrorResponse(error)) {
    return (
      <div>
        <h1>{error.status} {error.statusText}</h1>
        <p>{error.data}</p>
      </div>
    );
  }

  return (
    <div>
      <h1>Unexpected Error</h1>
      <p>{error instanceof Error ? error.message : 'Unknown error'}</p>
    </div>
  );
}

// Per-route error boundary
// app/routes/users.$id.tsx
export function ErrorBoundary() {
  const error = useRouteError();

  if (isRouteErrorResponse(error) && error.status === 404) {
    return <div>User not found</div>;
  }

  throw error; // Re-throw to parent boundary
}

Performance

// Streaming with defer
import { defer } from '@remix-run/node';
import { Await, useLoaderData } from '@remix-run/react';

export async function loader() {
  const criticalData = await getCriticalData();
  const slowData = getSlowData(); // Don't await

  return defer({
    criticalData,
    slowData,
  });
}

export default function Page() {
  const { criticalData, slowData } = useLoaderData<typeof loader>();

  return (
    <div>
      <div>{criticalData.name}</div>
      <Suspense fallback={<Skeleton />}>
        <Await resolve={slowData}>
          {(data) => <SlowSection data={data} />}
        </Await>
      </Suspense>
    </div>
  );
}

// Caching
export const headers: HeadersFunction = () => ({
  'Cache-Control': 'public, max-age=300, s-maxage=3600',
});

Session Management

// app/utils/session.server.ts
import { createCookieSessionStorage, redirect } from '@remix-run/node';

const sessionStorage = createCookieSessionStorage({
  cookie: {
    name: '__session',
    httpOnly: true,
    maxAge: 60 * 60 * 24 * 7, // 1 week
    path: '/',
    sameSite: 'lax',
    secrets: [process.env.SESSION_SECRET!],
    secure: process.env.NODE_ENV === 'production',
  },
});

export async function requireUser(request: Request) {
  const session = await sessionStorage.getSession(request.headers.get('Cookie'));
  const userId = session.get('userId');

  if (!userId) {
    throw redirect('/login');
  }

  return userId;
}

Testing

// app/routes/users.test.ts
import { createRemixStub } from '@remix-run/testing';
import { render, screen } from '@testing-library/react';
import UsersPage, { loader } from './users';

describe('Users Page', () => {
  it('renders users', async () => {
    const RemixStub = createRemixStub([
      {
        path: '/',
        Component: UsersPage,
        loader: () => ({ users: [{ id: '1', name: 'John' }] }),
      },
    ]);

    render(<RemixStub />);

    expect(await screen.findByText('John')).toBeInTheDocument();
  });
});

// E2E with Playwright
test('user flow', async ({ page }) => {
  await page.goto('/users');
  await page.click('button:has-text("Add User")');
  await page.fill('input[name="name"]', 'Jane');
  await page.click('button:has-text("Save")');
  await expect(page.locator('text=Jane')).toBeVisible();
});

Monitoring Metrics

Checklist

• Security headers in root
• CSRF protection for forms
• Input validation with Zod
• ErrorBoundary per route
• Streaming with defer
• Cache headers for static data
• Secure session cookies
• Rate limiting on actions
• Testing with @remix-run/testing
• Progressive enhancement verified

When NOT to Use This Skill

This skill is for Remix (React meta-framework). DO NOT use for:

• React without Remix: Use frontend-react skill instead
• Next.js (React meta-framework): Use nextjs-app-router skill instead
• Nuxt (Vue meta-framework): Use nuxt3 skill instead
• SvelteKit: Use sveltekit skill instead
• Astro: Use astro skill instead
• React Router SPA: Use frontend-react with React Router documentation

Anti-Patterns

Quick Troubleshooting

Reference Documentation

• Route Conventions
• Forms & Actions