Support a custom SPA fallback file in Workers Static Assets

[Barbapapazes]

Hello 👋,

Currently, Workers Static Assets supports SPA routing through the not_found_handling option:

{
  "assets": {
    "directory": "./dist",
    "not_found_handling": "single-page-application"
  }
}

However, this behavior is hardcoded to serve /index.html for unmatched routes.

It would be useful to allow developers to configure the fallback HTML file instead of always using index.html.

Some frameworks, such as Nuxt, generate separate HTML files for prerendered routes and client-side navigation.

For example, when running nuxt generate, Nuxt produces:

• index.html: The prerendered homepage.
• 200.html: A generic SPA fallback used to render non-prerendered routes on the client.
• Other HTML files for prerendered routes.

When deploying this output to Cloudflare Workers Static Assets, enabling single-page-application causes unmatched routes to fall back to index.html rather than 200.html.

As a result, users navigating directly to a non-prerendered route receive the prerendered homepage HTML instead of the intended SPA shell.

This makes it difficult to combine prerendered pages with client-side routes. The current workaround is to use a worker but that means that the website isn't fully static.