fix: cookies and immutable headers

.changeset/breezy-poets-grow.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: prevent immutable headers by using a new response object

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

@@ -370,6 +370,12 @@ export async function respond(request, options, manifest, state) {
 					// e.g. accessible when loading modules needed to handle the request
 					with_event(null, () =>
 						resolve(event, page_nodes, opts).then((response) => {
+							response = new Response(response.body, {
+								status: response.status,
+								statusText: response.statusText,
+								headers: new Headers(response.headers)
+							});
+
 							// add headers/cookies here, rather than inside `resolve`, so that we
 							// can do it once for all responses instead of once per `return`
 							for (const key in headers) {