withastro/astro · error · Error

Failed to prerender ${request.url}: ${prerenderError}

Error message

Failed to prerender ${request.url}: ${prerenderError}

What it means

Thrown by the Cloudflare prerenderer when the workerd handler reports a per-page prerender failure via the `x-astro-prerender-error` response header. Because pages may legitimately return non-2xx bodies (e.g. a custom 404), only this header marks an actual failure. The error message is the header value, prefixed with the failing URL.

Source

Thrown at packages/integrations/cloudflare/src/prerenderer.ts:278

				url: request.url,
				routeData: serializeRouteData(routeData, trailingSlash),
				incremental,
			};

			const response = await fetch(`${serverUrl}${PRERENDER_ENDPOINT}`, {
				method: 'POST',
				headers: { 'Content-Type': 'application/json' },
				body: JSON.stringify(body),
				redirect: 'manual',
			});

			// Check for prerender errors surfaced by the workerd handler via header
			// (the response body may be stripped by the Vite preview server).
			// Only the header marks a failure: pages may intentionally return
			// non-2xx responses while prerendering (e.g. a custom 404 page).
			const prerenderError = response.headers.get('x-astro-prerender-error');
			if (prerenderError) {
				throw new Error(`Failed to prerender ${request.url}: ${prerenderError}`);
			}

			// Incremental builds receive a `PrerenderEnvelope` wrapping the response
			// alongside the metadata collected in workerd, since a raw response
			// cannot carry it. Reconstruct the response and return it paired with
			// the metadata for the build to record.
			if (incremental) {
				const envelope: PrerenderEnvelope = await response.json();
				const reconstructed = new Response(Buffer.from(envelope.body, 'base64'), {
					status: envelope.status,
					statusText: envelope.statusText,
					headers: envelope.headers,
				});
				return { response: reconstructed, metadata: envelope.metadata };
			}

			return response;
		},

View on GitHub (pinned to d081033d5f)

Solutions

Read the prerender error message after the colon — it is the original throw from the page.
Open the failing route in `astro dev` (with the adapter) to get the full stack trace and HMR diagnostics.
Fix the page-level error (undefined reference, missing import, failed fetch) and rebuild.
If the error is binding-related, verify the binding exists and is wired for local prerender.

Example fix

// before — page throws on undefined
export const GET = ({ params }) => new Response(params.missing.id);

// after
export const GET = ({ params }) => new Response(params.id);

Defensive patterns

Strategy: try-catch

Try / catch

try {
  await build();
} catch (e) {
  if (/Failed to prerender/.test(e.message)) {
    const url = e.message.match(/prerender (\S+):/)?.[1];
    // open `url` in astro dev to reproduce with full stack trace
  }
  throw e;
}

Prevention

Render every route in astro dev with the adapter before building.
Keep page code defensive: null-check params, locals, and bindings.
Use the failing URL from the error to localize the bug quickly.

When it happens

Trigger: During build, rendering a specific route throws inside workerd; the handler catches it and surfaces the message through the `x-astro-prerender-error` header. The prerenderer detects the header and throws `Failed to prerender <url>: <message>`.

Common situations: A page references an undefined variable, a missing component, or a runtime API unavailable in workerd. A data fetch inside the page fails (e.g. a bound service is unreachable). Type errors that only manifest at runtime under workerd.

Related errors

AI-assisted analysis of withastro/astro@d081033d5f (2026-08-12). Data as JSON: /api/errors/ea7fb30939f4aec3. Report an issue: GitHub.