astro-canonical

Strict canonical URL enforcement for Astro projects.

astro-canonical validates every generated HTML page at build time to ensure:

• A canonical tag exists
• Canonical URLs are absolute
• Canonical URLs match your configured site
• Canonical URLs respect your trailingSlash policy
• Builds fail immediately if violations are found

This prevents silent SEO errors and guarantees canonical consistency across large sites.

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Features

• Zero runtime cost (build-time only)
• Reads your existing astro.config.mjs
• Enforces absolute canonical URLs
• Enforces trailing slash rules
• Ignores 404.html automatically
• Fails builds on invalid canonicals
• Works with static and hybrid output

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Installation

npm install astro-canonical

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Usage

Add the integration to your astro.config.mjs:

import { defineConfig } from "astro/config";
import astroCanonical from "astro-canonical";

export default defineConfig({
  site: "https://example.com/",
  trailingSlash: "always",
  integrations: [astroCanonical()],
});

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Trailing slash behaviour

astro-canonical reads your Astro configuration and enforces canonical URLs to match it exactly.

trailingSlash: "always"

All canonical URLs must end with a trailing slash:

<link rel="canonical" href="https://example.com/about/" />

If a canonical URL is missing the trailing slash, the build will fail.

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trailingSlash: "never"

All canonical URLs must not end with a trailing slash:

<link rel="canonical" href="https://example.com/about" />

If a canonical URL includes a trailing slash, the build will fail.

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No trailingSlash configured

If trailingSlash is not set in astro.config.mjs, astro-canonical will:

• Require a canonical tag
• Require an absolute URL
• Require the URL to start with site
• Not enforce trailing slash rules

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Validation rules

For each generated HTML file (excluding 404 pages):

• A <link rel="canonical"> tag must exist
• The href must be absolute
• The URL must start with the configured site
• Trailing slash rules must be respected (if configured)

If any page fails validation, the build fails.

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Example output

[astro-canonical] summary
[astro-canonical] site: https://example.com/
[astro-canonical] trailingSlash: always
[astro-canonical] HTML files scanned: 42
[astro-canonical] pages with canonical: 41
[astro-canonical] pages with invalid canonical: 1

[astro-canonical] invalid canonicals found:

• /about/index.html
  canonical: https://example.com/about
  issues: missing trailing slash

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Why this exists

Canonical URLs are easy to get wrong at scale:

• Mixed trailing slash usage
• Relative canonicals
• Incorrect domains
• Copy-paste mistakes
• CMS overrides

These issues are often invisible until SEO damage is done.

astro-canonical makes canonical correctness non-optional.

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License

MIT