sveltejs/kit · error · Error

Could not determine host. The request must have a value prov

Error message

Could not determine host. The request must have a value provided by the ${header_names}

What it means

adapter-node must construct an absolute origin for each request, which requires a host from the configured HOST_HEADER or the standard 'host' header. If neither is present the origin cannot be computed and the request fails. This guards against routing/URL generation breaking silently.

Source

Thrown at packages/adapter-node/src/handler.js:260

 */
function get_origin(headers) {
	const protocol = decodeURIComponent(
		normalise_header(protocol_header, headers[protocol_header]) || 'https'
	);

	// this helps us avoid host injections through the protocol header
	if (protocol.includes(':')) {
		throw new Error(
			`The ${protocol_header} header specified ${protocol} which is an invalid because it includes \`:\`. It should only contain the protocol scheme (e.g. \`https\`)`
		);
	}

	const host =
		normalise_header(host_header, headers[host_header]) ||
		normalise_header('host', headers['host']);
	if (!host) {
		const header_names = host_header ? `${host_header} or host headers` : 'host header';
		throw new Error(
			`Could not determine host. The request must have a value provided by the ${header_names}`
		);
	}

	const port = normalise_header(port_header, headers[port_header]);
	if (port && isNaN(+port)) {
		throw new Error(
			`The ${port_header} header specified ${port} which is an invalid port because it is not a number. The value should only contain the port number (e.g. 443)`
		);
	}

	return port ? `${protocol}://${host}:${port}` : `${protocol}://${host}`;
}

export const handler = sequence(
	/** @type {(RequestHandler | Middleware)[]} */
	([serve(path.join(dir, 'client'), true), serve_prerendered(), ssr].filter(Boolean))
);

View on GitHub (pinned to 03f1687fe6)

Solutions

Ensure your proxy always sets/forwards a Host header to the Node server
Set PROTOCOL_HEADER/HOST_HEADER (HOST_HEADER env) to a header your infrastructure reliably provides (e.g. x-forwarded-host)
Fix the client sending Host-less requests; per RFC 7230 HTTP/1.1 clients must send Host

Example fix

// nginx: before
proxy_pass http://node; # host header dropped
// after
proxy_set_header host $host;
proxy_pass http://node;

Defensive patterns

Strategy: validation

Validate before calling

if (!req.headers.host && !(process.env.HOST_HEADER && req.headers[process.env.HOST_HEADER.toLowerCase()])) {
  throw new Error('Request is missing a Host header; cannot determine origin');
}

Try / catch

try {
  const res = await server.respond(request);
} catch (err) {
  if (String(err.message).includes('Could not determine host')) {
    console.error('Ensure proxy forwards Host or set HOST_HEADER to a reliable header');
  } else {
    throw err;
  }
}

Prevention

Always forward Host through the proxy chain (proxy_set_header host $host)
Set HOST_HEADER env to x-forwarded-host when proxies rewrite Host
Reject Host-less requests at the load balancer

When it happens

Trigger: A request without a Host header arrives (HTTP/1.1 requires one; possible with hand-crafted or broken clients), while no alternative HOST_HEADER provides a value.

Common situations: Extremely low-level HTTP clients or health checks omitting Host, proxies stripping the Host header, or HTTP/2 requests relying on a :authority mapping your proxy doesn't forward as a host header.

Related errors

AI-assisted analysis of sveltejs/kit@03f1687fe6 (2026-09-02). Data as JSON: /api/errors/9eb3a1c7fc49864a. Report an issue: GitHub.