Canonical URL is set on each page

The same page reached via /about, /about/, /about?ref=twitter, and https://www.yoursite.com/about is four separate URLs to Google, and PageRank splits across all of them. Duplicate indexing dilutes ranking signals, triggers canonical-selection on Google's side (often picking a URL you did not intend), and on e-commerce sites causes faceted URLs to consume crawl budget that should go to product pages. A canonical tag consolidates all signals to the version you choose.

Set alternates.canonical per page in Next.js metadata, pointing to the absolute URL you want indexed. For dynamic routes, use generateMetadata so each slug gets its own canonical.

// app/[slug]/page.tsx
export async function generateMetadata({ params }) {
  return {
    alternates: { canonical: `https://yoursite.com/${params.slug}` },
  }
}

For the Pages Router, emit a <link rel="canonical" href="..."> inside next/head. Always use the HTTPS, www-or-not variant that matches your production hostname.

• ID: seo-fundamentals.meta-tags.canonical-url

• Severity: medium

• What to look for: Check for <link rel="canonical" href="..."> tags or metadata.alternates.canonical in Next.js App Router. The canonical URL should point to the preferred version of each page.

• Pass criteria: Count all page routes. Pages have canonical URLs set, either explicitly per page or via a consistent pattern (e.g., a layout that generates canonical URLs from the current route). At least 1 canonical URL mechanism must exist. For each page route, classify as "has canonical" or "missing canonical."

• Fail criteria: No canonical URL mechanism found across the project. Report: "0 of Y page routes have canonical URLs configured."

• Skip (N/A) when: The project is purely an API with no HTML pages (project type detected as api in stack detection). Single-page marketing sites and SPAs still benefit from canonical URLs and should not be skipped.

• Detail on fail: "No canonical URL configuration found. Pages may be indexed under multiple URLs (with/without trailing slashes, query parameters, etc.)"

• Remediation: Canonical URLs tell search engines which version of a page is the "official" one, preventing duplicate content issues. In Next.js App Router:

  export const metadata = {
    alternates: {
      canonical: 'https://yoursite.com/about',
    },
  }
  

  Or generate them dynamically:

  export async function generateMetadata({ params }) {
    return {
      alternates: { canonical: `https://yoursite.com/${params.slug}` },
    }
  }