[vite-plugin] Surface prerender response-body streaming errors

[mikepage]

Adds an opt-in experimental.bufferPreviewResponses option to the Cloudflare Vite plugin that surfaces response-body streaming errors during vite preview.

Problem

When a Worker returns a streaming Response and the body stream errors after the 200 status and headers have been committed (for example a component that throws mid-render during framework prerendering), workerd terminates the chunked response cleanly. The HTTP consumer sees a successful but silently-truncated 200 with no error — so astro build–style prerendering writes truncated HTML and exits 0. The underlying runtime behaviour is tracked upstream in cloudflare/workerd#6818; downstream report: withastro/astro#17047.

Change

When experimental.bufferPreviewResponses is enabled, the preview Worker's entry is wrapped so its response body is fully buffered in-worker (the only place the stream error is catchable). A buffered body error — or any thrown handler error — is returned as a 500 carrying an x-vite-cloudflare-worker-error marker header.

• The marker is set on the Worker's own response, so it reliably traverses the proxy and preview server back to the caller (a thrown dispatchFetch would otherwise be swallowed/reshaped by the proxy and lose custom headers).
• It lets consumers (e.g. a framework's prerenderer) distinguish a thrown error from a response the Worker intentionally returned with a non-2xx status (a custom 404 still passes through unchanged).
• Off by default — buffering breaks legitimate streaming responses in a normal preview.
• The wrapper re-exports named exports (Durable Objects, entrypoints, Workflows) and non-fetch handlers; it assumes a plain-object default export.

The companion @astrojs/cloudflare change (separate repo) enables this option and consumes the marker header.

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• Tests
  • Tests included/updated
  • Automated tests not possible - manual testing has been completed as follows:
  • Additional testing not necessary because:
• Public documentation
  • Cloudflare docs PR(s):
  • Documentation not necessary because: the option is experimental and consumed by framework adapters (e.g. @astrojs/cloudflare) rather than used directly; it can be documented when/if stabilised.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 280b5ac

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@cloudflare/vite-plugin	Minor

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[workers-devprod]

Codeowners approval required for this PR:

• @cloudflare/wrangler

Show detailed file reviewers

• .changeset/cuddly-pillows-buffer.md: [@cloudflare/wrangler]
• packages/vite-plugin-cloudflare/src/tests/fixtures/streaming-error/index.ts: [@cloudflare/wrangler]
• packages/vite-plugin-cloudflare/src/tests/fixtures/streaming-error/wrangler.jsonc: [@cloudflare/wrangler]
• packages/vite-plugin-cloudflare/src/tests/preview-stream-errors.spec.ts: [@cloudflare/wrangler]
• packages/vite-plugin-cloudflare/src/miniflare-options.ts: [@cloudflare/wrangler]
• packages/vite-plugin-cloudflare/src/plugin-config.ts: [@cloudflare/wrangler]