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#How to apply this file

Each section opens with one imperative line; apply every rule in the section it introduces. Do not summarise or skip a section.

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#Purpose

Rules for React Server Components. The model: components render on the server by default, ship no JavaScript, and can read data directly. Interactivity opts in with "use client".

The two things worth internalising: the boundary is a security boundary, and everything crossing it is serialised into the HTML.

Client-component specifics are Frontend/client-components; framework wiring is Backend/nextjs.

---

#What belongs where

[INST] Apply every rule in this section: What belongs where. [/INST]

Default to a server component. Push "use client" down the tree, to the smallest interactive leaf:

tsx
// Bad: the whole page becomes a client component, and every child ships to the browser
"use client";
export default function ProductPage() { … }

// Good: the page stays on the server; only the button is interactive
export default async function ProductPage({ id }) {
  const product = await db.product.findUnique({ where: { id } });   // server-only
  return (<article><h1>{product.name}</h1><AddToCart id={product.id} /></article>);
}

"use client" marks an entry point, not a single file: everything it imports also ends up in the bundle. One "use client" at the top of a layout ships the entire tree below it.

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#The boundary is a security boundary

[INST] Apply every rule in this section: The boundary is a security boundary. [/INST]

ts
// lib/db.ts
import "server-only";        // importing this from a client component fails the build
export const db = new PrismaClient();

• Mark every module holding secrets, database access or internal service calls with server-only. It converts a silent leak into a build error.
• Props passed to a client component are serialised into the HTML and visible in view-source. Passing a whole user record sends the password hash to the browser.

tsx
// Leaks every column, including internal flags and hashes
<Profile user={user} />

// Explicit projection
<Profile user={{ id: user.id, name: user.name, avatarUrl: user.avatarUrl }} />

• Only serialisable values cross: primitives, plain objects, arrays, Date, Map, Set, and Server Action references. Functions, class instances and Symbols do not — a Prisma model with methods will fail or silently lose them.
• Authorization is enforced on the server, in the component or the data layer. A client component conditionally rendering nothing still received the data. → Backend/authorization

---

#Fetch without waterfalls

[INST] Apply every rule in this section: Fetch without waterfalls. [/INST]

Server components can await directly, which removes the client-side fetch-on-render waterfall. It also makes it easy to create a server-side waterfall by awaiting sequentially.

tsx
// Sequential — 300ms + 400ms
const user = await getUser(id);
const orders = await getOrders(id);

// Parallel — max(300ms, 400ms)
const [user, orders] = await Promise.all([getUser(id), getOrders(id)]);

• Start independent requests together.
• Fetch where the data is used, not high in the tree and drilled down. React deduplicates identical fetch calls within a render pass, so two components asking for the same thing cost one request.
• Stream slow sections with <Suspense> so the fast part of the page appears immediately:

tsx
<Suspense fallback={<OrdersSkeleton />}>
  <Orders userId={id} />       {/* the rest of the page does not wait */}
</Suspense>

• Provide a real skeleton with the same dimensions as the content, or streaming trades a slow page for a shifting one. → Frontend/performance

---

#Composition across the boundary

[INST] Apply every rule in this section: Composition across the boundary. [/INST]

A client component cannot import a server component — but it can render one passed as children:

tsx
// Server component page
<ClientTabs>
  <ServerRenderedPanel />     {/* stays on the server; only its output crosses */}
</ClientTabs>

This is the pattern that keeps a heavy dependency out of the bundle while still using an interactive shell around it. Use it before restructuring the tree.

Other rules:

• A server component cannot use hooks, state, effects, or browser APIs. If you need one, you need a client component.
• Context providers must be client components, but they can wrap server-rendered children.
• Server components re-render on navigation and revalidation, not on interaction. Anything that must change on click belongs in a client component.

---

#Anti-patterns

[INST] Apply every rule in this section: Anti-patterns. [/INST]

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#Checklist

• Verify: Components are server components by default
• Verify: "use client" appears at the smallest interactive leaves, not at layouts
• Verify: Every server-only module is marked with server-only
• Verify: No secret, database client or internal service is reachable from client code
• Verify: Props crossing to client components are explicitly projected
• Verify: Only serialisable values cross the boundary
• Verify: Authorization is enforced on the server, not by conditional rendering
• Verify: Independent data fetches run in parallel
• Verify: Data is fetched where it is used, relying on request deduplication
• Verify: Slow sections are wrapped in <Suspense> with correctly sized fallbacks
• Verify: Server content is passed to interactive shells as children
• Verify: No hooks, state or browser APIs appear in server components