Agent Skills: React Router Knowledge Patch

React Router Knowledge Patch

Use this skill before changing React Router applications, route modules, adapters, framework configuration, data APIs, middleware, generated types, or migrations. Determine the installed major and mode first: Framework, Data, Declarative, or RSC. Several names and defaults differ by major, and provisional names were stabilized in multiple steps.

Reference index

| Reference | Topics | | --- | --- | | Data loading and rendering | Single Fetch, loaders/actions, hydration, revalidation, fetchers, navigation, prerendering | | Framework routing and configuration | Route config, discovery, splitting, SPA/SSR, server bundles, paths, masks | | Middleware and context | Context providers, server/client middleware, next(), errors, request scope | | Migration, packages, and tooling | Package imports, runtime floors, Vite, adapters, removed APIs, scaffolding, major upgrades | | React Server Components | Data Mode and Framework Mode RSC APIs, exports, rendering, entrypoints | | Security, observability, and CSP | Action origins, patched vulnerabilities, nonces, SRI, error reporting, instrumentation | | Type safety and APIs | Typegen, href, route props, serialization, meta/matches, public API details |

Working method

1. Inspect package.json, the lockfile, and imports to identify the installed major.
2. Identify the routing mode and whether the app uses the framework Vite plugin.
3. Read the migration reference before changing imports, flags, runtime versions, or adapters.
4. Read only the topical references needed for the task.
5. Prefer stable spellings supported by the installed version; do not copy provisional names blindly.
6. Run route type generation before standalone TypeScript checks.
7. Test document requests, client navigations, submissions, and hydration separately when relevant.

Breaking changes first

Choose imports by major

For current major projects, import shared APIs such as Link, redirect, cookie helpers, and hooks from react-router. Import DOM renderers from react-router/dom.

import { Link, redirect } from "react-router";
import { HydratedRouter, RouterProvider } from "react-router/dom";

The compatibility react-router-dom package is absent in v8. During v7 migration it is only a re-export shim. Runtime-neutral APIs do not belong in adapter packages.

Respect runtime and module floors

Before upgrading a major, verify Node, React, Vite, ESM, and output-target requirements. The current major requires Node 22.22.0 or newer, React 19.2.7 or newer, Vite 7 or newer, ESM packages, and an ES2022 target. Adapter peer floors may be narrower.

Remove obsolete response and deferred helpers

Do not use removed json(), defer(), deferred-data symbols, or the removed multipart upload helpers. Return serializable values and promises directly; use Response.json() only when an actual response is needed. Loaders and actions may return undefined.

export function loader() {
  return { report: loadReport() };
}

Replace flags with current config

Do not retain old v7_*, v3_*, unstable_*, or adopted v8_* flags without checking the installed major. In v8, middleware, pass-through requests, trailing-slash-aware data requests, and the Vite Environment API are unconditional. Route splitting is top-level.

export default {
  splitRouteModules: true,
  subResourceIntegrity: true,
};

Use false for one chunk per route module or "enforce" to require splittability.

Use provider-based context

Middleware context is a typed RouterContextProvider, not an arbitrary object or Map. Custom getLoadContext functions must return a provider where middleware is enabled; that is unconditional in v8.

import { createContext, RouterContextProvider } from "react-router";

const userContext = createContext<User>();
const context = new RouterContextProvider();
context.set(userContext, user);

Use loaderData, not match data

The old match-level data fields were deprecated and then removed. Read loaderData from meta arguments, route component matches, and UIMatch, guarding it where an error boundary can render without completed loader data.

Treat route modules as split units

Framework route modules are automatically split. Shared local declarations referenced by multiple exports can prevent isolation; move them to another module. Root routes are the exception when enforced splitting is used because the root stays one chunk.

Framework quick reference

Configure routes explicitly

Export a RouteConfig from app/routes.ts. Use index, route, layout, and prefix from @react-router/dev/routes; spread the routes returned by prefix.

import { index, layout, prefix, route, type RouteConfig } from "@react-router/dev/routes";

export default [
  index("./home.tsx"),
  layout("./auth/layout.tsx", [route("login", "./auth/login.tsx")]),
  ...prefix("products", [route(":id", "./product.tsx")]),
] satisfies RouteConfig;

Use flatRoutes() only when file-route discovery is desired. Route ID root is reserved.

Wire generated types

Generated sibling modules live under .react-router/types; include that tree and configure TypeScript rootDirs. Import Route from each route's ./+types/<route> module.

import type { Route } from "./+types/product";

export async function loader({ params }: Route.LoaderArgs) {
  return { id: params.id };
}

export default function Product({ loaderData }: Route.ComponentProps) {
  return <h1>{loaderData.id}</h1>;
}

Run react-router typegen && tsc in CI and standalone checks.

Combine server and client data deliberately

Under SSR, loader supplies server or prerender data and clientLoader handles later navigations. Call serverLoader() to combine them. Set clientLoader.hydrate = true as const when it must run before hydration and provide HydrateFallback when the UI should wait.

Client-only loaders hydrate implicitly. If a hydrating client loader has no fallback, its first value must match server-rendered data to avoid a hydration mismatch.

Understand prerender and SPA output

Use prerender: true for all static routes or provide paths/callbacks for dynamic ones. Parameterized routes need explicit values. With ssr: false, including / changes which file is the generic fallback; inspect generated index.html and __spa-fallback.html.

Await router work

Navigation, submission, fetcher load/submit, and revalidation APIs expose completion promises. POP navigations can also be awaited.

await navigate(-1);
await fetcher.submit(formData, { method: "post" });

Middleware quick reference

Route middleware wraps server document/data work; clientMiddleware wraps browser work. A middleware may call next() at most once. Omitting it automatically continues, which is useful for pre-handler setup. A middleware that short-circuits may return Response or data().

Do not assume server middleware runs on every hydrated navigation: without a loader or action, no .data request is made. Add a loader returning null when the server middleware must run.

Do not expect server context to survive an SPA submission's POST-to-GET boundary. Each HTTP request receives a separate provider. Client middleware work can share one client context.

Security quick reference

Cross-origin UI-route actions are rejected by default. Configure only trusted allowedActionOrigins; ** allows every domain. With reverse proxies, validate the host on the adapter-constructed request because that host drives origin checks.

Pass a CSP nonce through ServerRouter, Links, PrefetchPageLinks, and Scripts as needed. Generated critical CSS, import maps, scripts, and module-preload links can inherit or receive it.

Use stable onError on RouterProvider or HydratedRouter for client reporting. Group telemetry by normalized url and route pattern, not by concrete URLs alone, and consult the observability reference before implementing instrumentations.

RSC caution

RSC support spans distinct Data Mode and Framework Mode APIs and remains provisional in several surfaces. Confirm entry-point placement and route export semantics before adopting it. In Framework Mode, client exports and Server* exports are explicitly independent; a ServerComponent does not implicitly make the error boundary, layout, or hydrate fallback server components.

Verification checklist

• Confirm imports resolve from the intended package entry points.
• Confirm config keys match the installed major and no removed flag remains.
• Regenerate route types and run TypeScript with the generated tree included.
• Exercise direct document loads, client navigation, actions, fetchers, and back/forward navigation.
• Exercise hydration and SPA fallback paths when SSR or prerender settings change.
• Exercise middleware both before and after next(), including error boundaries.
• Exercise mutation requests behind the production proxy or adapter.
• Verify CSP output in rendered HTML when nonces or SRI are enabled.
• Treat provisional RSC and router-state APIs as version-sensitive.