fix(seo): noindex parameterised comparisons, keep bare /compare

25,193 schools make ~317 million pairs. The bare page stays indexable as the
landing page for the head term; the parameter space goes noindex, follow so
its outbound links still count.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015mWQnpye9F299NVRCCSRvj

nextjs-app/__tests__/app/metadata.test.ts

@@ -1,6 +1,7 @@
 import { metadata as homeMetadata } from '@/app/page';
 import { metadata as rankingsMetadata } from '@/app/rankings/page';
 import { metadata as admissionsMetadata } from '@/app/admissions/page';
+import { generateMetadata as compareMetadata } from '@/app/compare/page';
 
 describe('canonical URLs', () => {
   it('the homepage canonicalises to the bare root', () => {
@@ -21,3 +22,32 @@ describe('canonical URLs', () => {
       .toBe('https://www.schoolcompare.co.uk/admissions');
   });
 });
+
+describe('/compare indexability', () => {
+  it('the bare compare page is indexable and canonical to itself', async () => {
+    // This is the landing page for the "compare schools" head term.
+    const meta = await compareMetadata({ searchParams: Promise.resolve({}) });
+    expect(meta.alternates?.canonical)
+      .toBe('https://www.schoolcompare.co.uk/compare');
+    expect(meta.robots).toBeUndefined();
+  });
+
+  it('a comparison of specific schools is noindex, follow', async () => {
+    // ~317 million pairs before triples. Indexing the parameter space would
+    // swamp everything else in the corpus.
+    const meta = await compareMetadata({
+      searchParams: Promise.resolve({ urns: '100001,100002' }),
+    });
+    expect(meta.robots).toEqual({ index: false, follow: true });
+  });
+
+  it('a parameterised comparison still canonicalises to the bare path', async () => {
+    // follow:true plus a canonical means the outbound links to each school
+    // page still pass value even though this URL is not indexed.
+    const meta = await compareMetadata({
+      searchParams: Promise.resolve({ urns: '100001,100002' }),
+    });
+    expect(meta.alternates?.canonical)
+      .toBe('https://www.schoolcompare.co.uk/compare');
+  });
+});

nextjs-app/app/compare/page.tsx

@@ -5,6 +5,7 @@
 
 import { fetchComparison, fetchMetrics } from '@/lib/api';
 import { ComparisonView } from '@/components/ComparisonView';
+import { absoluteUrl } from '@/lib/site';
 import type { Metadata } from 'next';
 
 interface ComparePageProps {
@@ -14,13 +15,33 @@ interface ComparePageProps {
   }>;
 }
 
-export const metadata: Metadata = {
-  title: 'Compare Schools',
-  description:
-    'Compare schools in England side by side — Ofsted inspections, KS2 and GCSE results against the England average, admissions odds and school community.',
-  keywords:
-    'school comparison, compare schools, Ofsted comparison, school admissions, KS2 comparison, primary school performance',
-};
+/**
+ * Indexability depends on the query string, so this cannot be a static export.
+ *
+ * Bare /compare is the landing page for the "compare schools" head term and
+ * stays indexable. /compare?urns=… is an unbounded parameter space — 25,193
+ * schools make ~317 million pairs — so it goes noindex. It stays `follow` and
+ * keeps a canonical to the bare path, so the links out to each school page
+ * still count.
+ */
+export async function generateMetadata(
+  { searchParams }: ComparePageProps,
+): Promise<Metadata> {
+  const { urns } = await searchParams;
+
+  const base: Metadata = {
+    title: 'Compare Schools',
+    description:
+      'Compare schools in England side by side — Ofsted inspections, KS2 and GCSE results against the England average, admissions odds and school community.',
+    keywords:
+      'school comparison, compare schools, Ofsted comparison, school admissions, KS2 comparison, primary school performance',
+    alternates: { canonical: absoluteUrl('/compare') },
+  };
+
+  if (!urns) return base;
+
+  return { ...base, robots: { index: false, follow: true } };
+}
 
 // Dynamic via searchParams; remove force-dynamic so internal data fetches
 // can still use Next.js's per-call revalidate cache.