Non-HTML pages support exporting a getStaticPaths function just like .astro dynamic routes do, but we don’t currently document it. This should at least be mentioned in the non-HTML section of the Pages guide and probably deserves a comment in the getStaticPaths reference and Dynamic routes docs.

Particular gotcha to mention:

If the file route is src/pages/[api].json.ts, the api param returned by getStaticPaths need to include .json. Not sure if that’s intended or a bug — we should check with core. Here’s an example:

import type { APIRoute } from 'astro';

export const get: APIRoute = async ({ params }}) => {
  return {
    body: JSON.stringify({
      path: params.api,
    }),
  };
};

export function getStaticPaths() {
  return [
    { params: { api: 'a.json' } }, // ✅ works => outputs '/a.json'
    { params: { api: 'b' } },      // ❌ breaks => outputs '/b'
  ];
}

(I guess under the hood this is essentially working as if the .json part of the filename is not there.)