Editable 404 Page

The starter treats 404 as managed content, not hardcoded UI. Editors can update the page body in Sanity, and the website renders that content when a route is not found.

What this gives you

• A dedicated singleton schema for 404 content (singleton.config.404)
• Modular content blocks for rich 404 layouts
• Site-scoped 404 documents, resolved through the site manifest
• Built-in 404 analytics tracking through GTM hooks

How it works

The not-found.tsx route reads X-Site-Id and X-Language-Code headers, validates them, then loads the site manifest. The manifest query includes a site-scoped notFound field that fetches the 404 singleton content.

If valid context is missing, the route falls back to plain text output (404 - Page not found). If data exists, it renders modular blocks via ModularContentRenderer.

Configure the 404 page in Sanity

1. Open the 404 Not Found singleton in Studio.
2. Add content in the modular content field.
3. Publish the document.
4. Verify your site has the correct site context in routing/proxy so X-Site-Id and X-Language-Code resolve.

Rendering and tracking details

• 404 content is loaded from the same manifest path used by layout-level site data.
• A client tracker component (Track404) runs on the 404 view and triggers GTM notFound() tracking.
• SEO fields exist on the 404 singleton schema and can be edited alongside content.

Implementation notes

• 404 singleton schema: apps/sanity/schema/types/singletons/notFound404.ts
• Site manifest query (notFound section): apps/web/src/features/site-manifest/queries/siteManifest.query.ts
• Not found page rendering: apps/web/src/app/(website)/~s/[siteId]/[languageCode]/not-found.tsx
• Tracking hook: apps/web/src/lib/gtm/hooks/useTrack404.ts
• Header constants used for site/language context: apps/web/src/constants/headers.ts