Serve props JSON for /api data requests via middleware rewrite

src/proxy.js

@@ -1,10 +1,46 @@
 import { NextResponse } from 'next/server';
 
+// Client-side navigations fetch page props from /_next/data/<buildId>/api/....json.
+// Vercel's routing applies the /api/:path* -> /ipa/:path* rewrite from
+// next.config.mjs to those requests but loses the data-request context and
+// serves the page HTML instead of the props JSON
+// (https://github.com/vercel/next.js/issues/39669), so the router never
+// receives pageProps. Proxy rewrites keep data-request semantics, so map
+// /api/* to /ipa/* here for data requests only; regular page requests fall
+// through to the config rewrites, and the /ipa -> /api canonical redirect
+// is unchanged.
+function rewriteApiDataRequest(req) {
+    const isDataRequest =
+        req.headers.get('x-nextjs-data') !== null ||
+        req.nextUrl.pathname.startsWith('/_next/data/');
+    if (!isDataRequest) return null;
+
+    const url = req.nextUrl.clone();
+    if (url.pathname.startsWith('/_next/data/')) {
+        // Raw data-request path, in case the runtime matches it un-normalized.
+        // Bare api.json maps to ipa/introduction.json to mirror the /api ->
+        // /ipa/introduction rewrite; there is no /ipa index page.
+        url.pathname = url.pathname
+            .replace(
+                /^(\/_next\/data\/[^/]+)\/api\.json$/,
+                '$1/ipa/introduction.json',
+            )
+            .replace(/^(\/_next\/data\/[^/]+)\/api\//, '$1/ipa/');
+    } else if (url.pathname === '/api') {
+        url.pathname = '/ipa/introduction';
+    } else if (url.pathname.startsWith('/api/')) {
+        url.pathname = url.pathname.replace(/^\/api\//, '/ipa/');
+    }
+
+    if (url.pathname === req.nextUrl.pathname) return null;
+    return NextResponse.rewrite(url);
+}
+
 export function proxy(req) {
     if (req.nextUrl.href.includes('/docs-static/_next/'))
         return NextResponse.rewrite(
             req.nextUrl.href.replace('/docs-static/_next/', '/_next/'),
         );
 
-    return null;
-}
+    return rewriteApiDataRequest(req);
+}