fix: clone Headers before mutating them during prerendering

.changeset/curly-onions-compare.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: gracefully handle server endpoints that return `Response`s with immutable `Headers` when prerendering

packages/kit/src/runtime/server/endpoint.js

@@ -39,7 +39,7 @@ export async function render_endpoint(event, mod, state) {
 	}
 
 	try {
-		const response = await handler(
+		let response = await handler(
 			/** @type {import('@sveltejs/kit').RequestEvent<Record<string, any>>} */ (event)
 		);
 
@@ -50,6 +50,13 @@ export async function render_endpoint(event, mod, state) {
 		}
 
 		if (state.prerendering) {
+			// the returned Response might have immutable Headers
+			// so we should clone them before trying to mutate them
+			response = new Response(response.body, {
+				status: response.status,
+				statusText: response.statusText,
+				headers: new Headers(response.headers)
+			});
 			response.headers.set('x-sveltekit-prerender', String(prerender));
 		}
 

packages/kit/test/prerendering/basics/src/routes/immutable-headers/+server.js

@@ -0,0 +1,8 @@
+export const prerender = true;
+
+export const GET = () => {
+	const response = new Response('foo');
+	// this simulates immutable Response Headers, like those returned by undici
+	Object.defineProperty(response.headers, 'set', { value: null });
+	return response;
+};

packages/kit/test/prerendering/basics/test/tests.spec.js

@@ -223,3 +223,8 @@ test('prerendered.paths omits trailing slashes for endpoints', () => {
 		expect(content, `Missing ${path}`).toMatch(`"${path}"`);
 	}
 });
+
+test('prerenders responses with immutable Headers', () => {
+	const content = read('immutable-headers');
+	expect(content).toMatch('foo');
+});