Canonical URLs & Duplicate Content Prevention

Added in v1.0.376

Overview

As of v1.0.376, every page in the application declares a canonical URL. This prevents search engines from indexing the same content under multiple origins — such as www vs non-www variants, or Vercel preview deployment URLs — and ensures all link equity is consolidated to the single production domain.

How it works

Root-level canonical (all pages)

A default canonical is set in src/app/layout.tsx via Next.js metadata.alternates.canonical. This automatically applies to every page that does not declare its own override:

// src/app/layout.tsx
export const metadata: Metadata = {
  alternates: {
    canonical: process.env.NEXT_PUBLIC_APP_URL,
  },
};

The value is read from the NEXT_PUBLIC_APP_URL environment variable, which must be set to your production origin (e.g. https://www.yourdomain.com).

Per-page canonical overrides

Pages with distinct URL paths export their own metadata.alternates.canonical to produce a precise canonical tag:

// src/app/pricing/page.tsx
export const metadata: Metadata = {
  alternates: {
    canonical: `${process.env.NEXT_PUBLIC_APP_URL}/pricing`,
  },
};

This ensures the rendered <link rel="canonical" href="..."> tag in each page's <head> points to the exact canonical path, not just the root.

Vercel preview deployment suppression

A X-Robots-Tag: noindex response header is added to Vercel preview deployments via vercel.json using environment-conditional header rules. This prevents preview URLs (e.g. https://my-app-git-feature-org.vercel.app) from being crawled or indexed by search engines while leaving production unaffected.

Environment variable

Variable	Required	Description
`NEXT_PUBLIC_APP_URL`	Yes	The canonical production origin. Example: `https://www.yourdomain.com`. Do not include a trailing slash.

Note: If NEXT_PUBLIC_APP_URL is not set, the canonical tag will render as undefined and provide no benefit. Always set this variable in your production and staging environment configurations.

Deployment checklist

NEXT_PUBLIC_APP_URL is set in Vercel production environment variables
NEXT_PUBLIC_APP_URL uses the exact preferred domain (including or excluding www) that you want search engines to index
Vercel preview environments do not have NEXT_PUBLIC_APP_URL set to the production domain
Any new top-level page routes include a metadata.alternates.canonical export pointing to their full URL

Background

This work addresses SEO control SEO-05, which identified that no metadata.alternates.canonical was configured on any page. The risk was that Vercel preview deployment URLs could be indexed as duplicate content and that www vs non-www variants could split ranking signals. Canonical tags are the standard mechanism recommended by Google and other search engines to disambiguate the preferred URL for a piece of content.