x-void-page-data header causes fetch failed for pages with large props #19

Summary

The x-void-page-data response header (added in 636c71d) serializes the entire PageObject via JSON.stringify(pageObj) into an HTTP response header on every non-deferred SSR HTML response. For pages with large props, this exceeds HTTP header size limits and causes fetch failed errors.

Reproduction

• Branch: fix/x-void-page-data-large-props — adds a playground page (large-props) generating ~300KB of props and a failing integration test test: reproduce x-void-page-data header failure with large props #20
• Run: pnpm test:pages:react-vue -- -t "SSG prerender should not fail" --hookTimeout=60000
• Result: prerender /large-props failed: fetch failed

Real-world impact

A Notion-powered blog (musubi) where each page returns:

• ExtendedRecordMap (~30-70KB) — Notion page data for client-side React hydration
• Pre-rendered HTML (~50-200KB) — server-rendered Notion content

All 16 blog post pages fail with fetch failed during vite build with output: "static". The header also causes 500 errors in dev mode when props contain non-ASCII characters (Unicode text, emoji — common in CMS data).

Root cause

packages/void/src/pages/protocol.ts:221:

return new Response(html, {
  headers: {
    "content-type": "text/html; charset=utf-8",
    "x-void-page-data": JSON.stringify(pageObj), // ← entire page props in a header
    ...POWERED_BY,
  },
});

The header was added for dispatch to populate a separate JSON edge cache for SPA navigation. However:

1. HTTP headers are not designed for large payloads — Miniflare/workerd enforces size limits, Node/undici rejects non-ASCII header values
2. Affects all modes — SSG prerendering (via mf.dispatchFetch()), dev server (non-ASCII props), and would affect production Workers (~32KB header limit)
3. Page data is already in the HTML — the same data is embedded in the data-page attribute, making the header redundant for rendering

Environments affected

Suggested fix

Remove the header or use an alternative mechanism for edge cache population that doesn't rely on HTTP headers (e.g., write to Cache API directly, or parse data-page from the HTML body in dispatch).