Remix / React Router v7 Framework Patterns

Quick Guide: Each route exports a loader for reads and an action for writes. Both run on the server. Data flows through loaders, mutations go through actions, forms work without JavaScript, and nested routes enable parallel data loading. json() and defer() are deprecated in React Router v7 -- return raw objects instead, use data() for custom headers/status.

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<migration_notice>

IMPORTANT: React Router v7 Migration

Remix has merged into React Router v7. What was planned as Remix v3 is now React Router v7 "framework mode".

Remix v2 (Deprecated)	React Router v7 (Current)
json(data)	Return raw objects directly
json(data, { status, headers })	data(data, { status, headers })
defer({ key: promise })	Return { key: promise } with Single Fetch
@remix-run/node imports	react-router / @react-router/node
LoaderFunctionArgs	Route.LoaderArgs (generated types)
ActionFunctionArgs	Route.ActionArgs (generated types)
useLoaderData<typeof loader>()	loaderData prop via Route.ComponentProps
RemixServer	ServerRouter (from react-router)
RemixBrowser	HydratedRouter (from react-router/dom)
File-based routing (automatic)	routes.ts + optional @react-router/fs-routes

This skill covers both Remix v2 and React Router v7 patterns. Examples use Remix v2 imports by default with RR v7 equivalents documented in examples/react-router-v7.md.

Migration guide: Upgrading from Remix

</migration_notice>

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<critical_requirements>

CRITICAL: Before Using This Skill

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST export loaders and actions as named exports from route modules only -- they do not work in non-route files)

(You MUST throw Response objects for expected errors (404, 403) -- use ErrorBoundary for handling)

(You MUST await critical data and return non-critical data as Promises for streaming)

(You MUST use named constants for HTTP status codes -- no magic numbers)

</critical_requirements>

---

Auto-detection: Remix routes, React Router v7, loader function, action function, clientAction, clientLoader, useLoaderData, useActionData, useFetcher, defer, ErrorBoundary, Form component, meta function, links function, Single Fetch, ServerRouter, HydratedRouter, Route.LoaderArgs, Route.ComponentProps, shouldRevalidate

When to use:

• Building full-stack React applications with server-side rendering
• Implementing data loading with loaders and mutations with actions
• Creating progressively enhanced forms that work without JavaScript
• Streaming non-critical data with defer/Promises and Suspense
• Handling errors gracefully with route-level ErrorBoundary

When NOT to use:

• Static sites without server-side logic
• Simple SPAs without server rendering needs
• Projects already committed to a different meta-framework

Key patterns covered:

• File-based routing (routes/, _index, $params, _layout)
• Loaders for server-side data fetching
• Actions for mutations with progressive enhancement
• Streaming with defer() / raw Promises (RR v7)
• useFetcher for non-navigation mutations and optimistic UI
• Error boundaries with multi-status handling
• Meta and Links functions for SEO
• Resource routes (API endpoints, file downloads)
• Nested routing with parallel data loading
• React Router v7 migration (Single Fetch, type generation, clientAction)

---

Philosophy

Remix simplifies full-stack development to a single mental model: each route exports a loader for reads and an action for writes. Both functions execute exclusively on the server, enabling direct database access without exposing secrets to the client.

Core Principles:

1. Server-first data loading: Loaders run on the server before rendering, eliminating client-side data fetching waterfalls
2. Progressive enhancement: Forms work with plain POST requests -- JavaScript enhances but isn't required
3. HTTP semantics: Caching uses standard HTTP headers (Cache-Control), not framework-specific solutions
4. Nested routes: URL segments map to component hierarchy, enabling parallel data loading
5. Web standards: Uses Fetch API Request/Response objects throughout

Data Flow:

URL Change -> Loader(s) Execute -> Component Renders -> User Interacts
                                                            |
                                          Action Executes -> Loaders Revalidate

---

Core Patterns

Pattern 1: File-Based Routing

Files in app/routes/ become URL paths. File naming conventions control nesting, layouts, and dynamic segments.

File Name	URL	Description
_index.tsx	/	Index route (root)
about.tsx	/about	Static route
blog.$slug.tsx	/blog/:slug	Dynamic parameter
blog_.tsx	/blog	Pathless layout escape
_auth.tsx	(none)	Layout route (no URL segment)
_auth.login.tsx	/login	Route nested in layout
$.tsx	/*	Splat/catch-all route

// app/routes/blog.$slug.tsx -- dynamic route with loader
import type { LoaderFunctionArgs } from "@remix-run/node";
import { useLoaderData } from "@remix-run/react";

const HTTP_NOT_FOUND = 404;

export async function loader({ params }: LoaderFunctionArgs) {
  const post = await db.post.findUnique({ where: { slug: params.slug } });
  if (!post) throw new Response("Not Found", { status: HTTP_NOT_FOUND });
  return { post };
}

Why good: File names map directly to URLs, $ prefix for dynamic segments, loader params are typed, named constant for status code

See examples/core.md for complete route examples and examples/nested-routes.md for layout nesting patterns.

---

Pattern 2: Loaders for Data Fetching

Loaders are server-only functions that provide data to routes. They run on initial server render and on client navigation via fetch.

const HTTP_NOT_FOUND = 404;

export async function loader({ params, request }: LoaderFunctionArgs) {
  const user = await db.user.findUnique({ where: { id: params.userId } });
  if (!user) {
    throw json({ message: "User not found" }, { status: HTTP_NOT_FOUND });
  }
  return json({ user });
}

Key rules:

• Always throw Response for expected errors (triggers ErrorBoundary)
• Use useLoaderData<typeof loader>() for type-safe access (or Route.ComponentProps in RR v7)
• Loaders run on every navigation -- parent loaders re-run even for child route changes
• Use shouldRevalidate to optimize unnecessary re-runs

See examples/loaders.md for authentication, pagination, and caching examples.

---

Pattern 3: Actions for Mutations

Actions handle non-GET requests (POST, PUT, DELETE, PATCH). They run before loaders and enable progressive form handling.

export async function action({ request }: ActionFunctionArgs) {
  const formData = await request.formData();
  const intent = formData.get("intent");

  switch (intent) {
    case "update": {
      /* ... */ return json({ success: true });
    }
    case "delete": {
      /* ... */ return redirect("/items");
    }
    default:
      throw new Error(`Unknown intent: ${intent}`);
  }
}

Key rules:

• Use hidden intent field for multiple actions in one route
• Redirect after successful mutations to prevent double-submission
• Return validation errors with json({ errors }, { status: 400 })
• Forms work without JavaScript -- progressive enhancement by default

See examples/actions.md for validation and examples/forms.md for multi-form patterns.

---

Pattern 4: Streaming with defer / Promises

Await critical data, return Promises for non-critical data that can stream in.

// Remix v2: use defer()
return defer({
  user, // Awaited -- critical
  analytics: getAnalytics(), // Promise -- streams in
});

// React Router v7: return raw objects with Promises
return {
  user, // Awaited -- critical
  analytics: getAnalytics(), // Promise -- streams via Single Fetch
};

Render streamed data with <Suspense> + <Await>:

<Suspense fallback={<Skeleton />}>
  <Await resolve={analytics} errorElement={<p>Failed to load</p>}>
    {(data) => <Chart data={data} />}
  </Await>
</Suspense>

When to stream: Analytics, comments, recommendations, secondary content below the fold. When NOT to stream: Auth state, page title, SEO-critical content, data for page structure.

See examples/deferred.md for complete streaming examples.

---

Pattern 5: useFetcher for Non-Navigation Mutations

useFetcher enables data loading and mutations without page navigation. Essential for inline interactions.

const fetcher = useFetcher();

// Optimistic UI: show expected state immediately
const optimisticIsLiked = fetcher.formData
  ? fetcher.formData.get("liked") === "true"
  : isLiked;

Use <Form> for: Create/login/wizards -- actions that should change the URL. Use useFetcher for: Like buttons, toggles, inline editing, search autocomplete.

See examples/optimistic.md for optimistic UI and debounced search.

---

Pattern 6: Error Boundaries

Export ErrorBoundary from route modules. Distinguish between thrown Response errors and unexpected JavaScript errors.

export function ErrorBoundary() {
  const error = useRouteError();

  if (isRouteErrorResponse(error)) {
    // Thrown Response: render status-specific UI
    return <div role="alert"><h1>{error.status}</h1></div>;
  }

  if (error instanceof Error) {
    // Unexpected error: generic fallback
    return <div role="alert"><h1>Unexpected Error</h1></div>;
  }

  return <div role="alert"><h1>Unknown Error</h1></div>;
}

Key rules:

• Throw json({ message }, { status: 404 }) for expected errors
• ErrorBoundary is route-scoped -- rest of the page stays functional
• Use named constants for HTTP status codes
• isRouteErrorResponse() checks if error was a thrown Response

See examples/error-handling.md for multi-status error boundaries.

---

Pattern 7: Meta and Links Functions

Export meta for SEO metadata and links for stylesheets/preloads.

export const meta: MetaFunction<typeof loader> = ({ data }) => {
  if (!data) return [{ title: "Not Found" }];
  return [
    { title: `${data.post.title} | ${SITE_NAME}` },
    { property: "og:title", content: data.post.title },
    { tagName: "link", rel: "canonical", href: url },
  ];
};

Gotcha: meta function receives null data on error -- always handle the missing data case. Gotcha: links function cannot access loader data -- use meta with tagName: "link" for dynamic links.

See examples/meta.md for Open Graph and Twitter Card patterns.

---

Pattern 8: Resource Routes

Routes without a default export become resource routes -- useful for APIs, webhooks, and file downloads.

// app/routes/api.health.ts (no default export = resource route)
export async function loader() {
  return json({ status: "healthy", timestamp: new Date().toISOString() });
}

See examples/resource-routes.md for webhook and file download examples.

---

Pattern 9: Nested Routes and Layouts

Nested routes share parent layouts and load data in parallel. Parent loaders provide shared data, child loaders run concurrently.

Pattern	Purpose
admin.tsx	Layout (has <Outlet />)
admin._index.tsx	Index route (renders at parent URL)
admin.users.tsx	Nested child route
admin_.settings.tsx	Escapes parent layout with trailing underscore
_auth.tsx	Pathless layout with leading underscore

See examples/nested-routes.md for admin layout and pathless layout examples.

---

Detailed Resources:

• examples/core.md - File-based routing, route naming, essential hooks
• examples/loaders.md - Protected routes, pagination, caching headers
• examples/actions.md - Signup forms, validation, delete with confirmation
• examples/forms.md - Multiple forms in one route, intent pattern
• examples/nested-routes.md - Layouts, pathless routes, admin panels
• examples/error-handling.md - Multi-status error boundaries
• examples/optimistic.md - Optimistic UI, debounced search
• examples/deferred.md - Streaming with defer/Promises
• examples/resource-routes.md - API endpoints, webhooks, file downloads
• examples/meta.md - SEO meta tags, Open Graph, Twitter Cards
• examples/react-router-v7.md - Migration: routes.ts, type generation, clientAction, Single Fetch
• reference.md - Decision frameworks, anti-patterns, route module exports

---

<red_flags>

RED FLAGS

High Priority Issues:

• Loaders/actions exported from non-route files -- Remix only runs these from route modules
• Missing type inference -- always use useLoaderData<typeof loader>() or Route.ComponentProps
• Client-side data fetching with useEffect + fetch -- use loaders for server data
• Returning null from loader instead of throwing Response -- every consumer must null-check

Medium Priority Issues:

• Streaming critical data (page title, auth state) -- causes content flicker
• useFetcher without optimistic UI -- makes interactions feel slow
• Magic numbers for HTTP status codes -- use named constants
• Form without method="post" -- defaults to GET, action not called

Gotchas & Edge Cases:

• Loader runs on every navigation -- even for child route changes, parent loaders re-run (use shouldRevalidate to optimize)
• Action runs before all loaders -- after action, all loaders revalidate by default
• defer() requires <Suspense> + <Await> wrapper -- forgetting causes errors
• Index routes need ?index query param for form actions targeting them
• meta function receives null data on error -- must handle missing data case
• links function cannot access loader data -- use meta with tagName: "link" for dynamic links
• In React Router v7, clientAction takes priority when both action and clientAction exist -- server action is completely skipped

</red_flags>

---

<critical_reminders>

CRITICAL REMINDERS

All code must follow project conventions in CLAUDE.md

(You MUST export loaders and actions as named exports from route modules only -- they do not work in non-route files)

(You MUST throw Response objects for expected errors (404, 403) -- use ErrorBoundary for handling)

(You MUST await critical data and return non-critical data as Promises for streaming)

(You MUST use named constants for HTTP status codes -- no magic numbers)

Failure to follow these rules will break data loading, type safety, and error handling.

</critical_reminders>