docs: document serverExternalPackages config for Next.js + server-util

Adds documentation for using @blocknote/server-util in Next.js App Router
API routes, including the required serverExternalPackages configuration
and shared schema pattern.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: YousefED

docs/content/docs/features/server-processing.mdx

@@ -35,6 +35,28 @@ Additionally, `ServerBlockNoteEditor` provides functions for processing Yjs docu
 - `yDocToBlocks` or `yXmlFragmentToBlocks`: use this to convert a Yjs document or XML Fragment to BlockNote blocks
 - `blocksToYDoc` or `blocksToYXmlFragment`: use this to convert a BlockNote document (blocks) to a Yjs document or XML Fragment
 
+## Next.js App Router
+
+If you're using `@blocknote/server-util` in a Next.js App Router API route (Route Handler), you need to add the BlockNote packages to `serverExternalPackages` in your `next.config.ts`:
+
+```typescript
+import type { NextConfig } from "next";
+
+const nextConfig: NextConfig = {
+  serverExternalPackages: [
+    "@blocknote/core",
+    "@blocknote/react",
+    "@blocknote/server-util",
+  ],
+};
+
+export default nextConfig;
+```
+
+This tells Next.js to load these packages using Node.js `require()` instead of bundling them with Turbopack, which is necessary because BlockNote's dependencies use APIs that aren't available in React Server Components.
+
+You can share the same custom schema (using `createReactBlockSpec`) between your API routes and client-side editor — see the [Next.js setup guide](/docs/getting-started/nextjs) for the full pattern.
+
 ## React compatibility
 
 If you use [custom schemas in React](/docs/features/custom-schemas), you can use the same schema on the server side.

docs/content/docs/getting-started/nextjs.mdx

@@ -58,6 +58,42 @@ function App() {
 }
 ```
 
+## Server-side processing (API Routes)
+
+If you want to use `@blocknote/server-util` in a Next.js API route (Route Handler) — for example, to convert blocks to HTML on the server — you need to add BlockNote packages to `serverExternalPackages` in your `next.config.ts`:
+
+```typescript
+import type { NextConfig } from "next";
+
+const nextConfig: NextConfig = {
+  serverExternalPackages: [
+    "@blocknote/core",
+    "@blocknote/react",
+    "@blocknote/server-util",
+  ],
+};
+
+export default nextConfig;
+```
+
+Then you can use `ServerBlockNoteEditor` in your API route as usual:
+
+```typescript
+// app/api/convert/route.ts
+import { ServerBlockNoteEditor } from "@blocknote/server-util";
+
+export async function POST(req: Request) {
+  const { blocks } = await req.json();
+  const editor = ServerBlockNoteEditor.create();
+  const html = await editor.blocksToFullHTML(blocks);
+  return Response.json({ html });
+}
+```
+
+If you use a [custom schema](/docs/features/custom-schemas) with `createReactBlockSpec`, you can share it between your API routes and your client-side editor. Just define the schema in a separate file and import it in both places — the `serverExternalPackages` config ensures it works in both contexts.
+
+See the [server-side processing docs](/docs/server-side-processing) for more details on `ServerBlockNoteEditor`.
+
 ## React 19 / Next 15 StrictMode
 
 BlockNote is not yet compatible with React 19 / Next 15 StrictMode. For now, disable StrictMode in your `next.config.ts`: