diff --git a/.changeset/blue-deer-relax.md b/.changeset/blue-deer-relax.md
new file mode 100644
index 000000000000..9b7c9e6bcf6a
--- /dev/null
+++ b/.changeset/blue-deer-relax.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: serialize server `load` data before passing to universal `load`, to handle mutations and promises
diff --git a/packages/kit/src/runtime/server/data/index.js b/packages/kit/src/runtime/server/data/index.js
index 37fc409ab42d..83e16c3a9d12 100644
--- a/packages/kit/src/runtime/server/data/index.js
+++ b/packages/kit/src/runtime/server/data/index.js
@@ -2,11 +2,10 @@ import { text } from '@sveltejs/kit';
 import { HttpError, SvelteKitError, Redirect } from '@sveltejs/kit/internal';
 import { normalize_error } from '../../../utils/error.js';
 import { once } from '../../../utils/functions.js';
+import { server_data_serializer_json } from '../page/data_serializer.js';
 import { load_server_data } from '../page/load_data.js';
-import { clarify_devalue_error, handle_error_and_jsonify, serialize_uses } from '../utils.js';
+import { handle_error_and_jsonify } from '../utils.js';
 import { normalize_path } from '../../../utils/url.js';
-import * as devalue from 'devalue';
-import { create_async_iterator } from '../../../utils/streaming.js';
 import { text_encoder } from '../../utils.js';
 
 /**
@@ -120,7 +119,9 @@ export async function render_data(
 			)
 		);
 
-		const { data, chunks } = get_data_json(event, event_state, options, nodes);
+		const data_serializer = server_data_serializer_json(event, event_state, options);
+		for (let i = 0; i < nodes.length; i++) data_serializer.add_node(i, nodes[i]);
+		const { data, chunks } = data_serializer.get_data();
 
 		if (!chunks) {
 			// use a normal JSON response where possible, so we get `content-length`
@@ -185,92 +186,3 @@ export function redirect_json_response(redirect) {
 		})
 	);
 }
-
-/**
- * If the serialized data contains promises, `chunks` will be an
- * async iterable containing their resolutions
- * @param {import('@sveltejs/kit').RequestEvent} event
- * @param {import('types').RequestState} event_state
- * @param {import('types').SSROptions} options
- * @param {Array<import('types').ServerDataSkippedNode | import('types').ServerDataNode | import('types').ServerErrorNode | null | undefined>} nodes
- *  @returns {{ data: string, chunks: AsyncIterable<string> | null }}
- */
-export function get_data_json(event, event_state, options, nodes) {
-	let promise_id = 1;
-	let count = 0;
-
-	const { iterator, push, done } = create_async_iterator();
-
-	const reducers = {
-		...Object.fromEntries(
-			Object.entries(options.hooks.transport).map(([key, value]) => [key, value.encode])
-		),
-		/** @param {any} thing */
-		Promise: (thing) => {
-			if (typeof thing?.then === 'function') {
-				const id = promise_id++;
-				count += 1;
-
-				/** @type {'data' | 'error'} */
-				let key = 'data';
-
-				thing
-					.catch(
-						/** @param {any} e */ async (e) => {
-							key = 'error';
-							return handle_error_and_jsonify(event, event_state, options, /** @type {any} */ (e));
-						}
-					)
-					.then(
-						/** @param {any} value */
-						async (value) => {
-							let str;
-							try {
-								str = devalue.stringify(value, reducers);
-							} catch {
-								const error = await handle_error_and_jsonify(
-									event,
-									event_state,
-									options,
-									new Error(`Failed to serialize promise while rendering ${event.route.id}`)
-								);
-
-								key = 'error';
-								str = devalue.stringify(error, reducers);
-							}
-
-							count -= 1;
-
-							push(`{"type":"chunk","id":${id},"${key}":${str}}\n`);
-							if (count === 0) done();
-						}
-					);
-
-				return id;
-			}
-		}
-	};
-
-	try {
-		const strings = nodes.map((node) => {
-			if (!node) return 'null';
-
-			if (node.type === 'error' || node.type === 'skip') {
-				return JSON.stringify(node);
-			}
-
-			return `{"type":"data","data":${devalue.stringify(node.data, reducers)},"uses":${JSON.stringify(
-				serialize_uses(node)
-			)}${node.slash ? `,"slash":${JSON.stringify(node.slash)}` : ''}}`;
-		});
-
-		return {
-			data: `{"type":"data","nodes":[${strings.join(',')}]}\n`,
-			chunks: count > 0 ? iterator : null
-		};
-	} catch (e) {
-		// @ts-expect-error
-		e.path = 'data' + e.path;
-		throw new Error(clarify_devalue_error(event, /** @type {any} */ (e)));
-	}
-}
diff --git a/packages/kit/src/runtime/server/page/data_serializer.js b/packages/kit/src/runtime/server/page/data_serializer.js
new file mode 100644
index 000000000000..2f0197ae648b
--- /dev/null
+++ b/packages/kit/src/runtime/server/page/data_serializer.js
@@ -0,0 +1,202 @@
+import * as devalue from 'devalue';
+import { create_async_iterator } from '../../../utils/streaming.js';
+import {
+	clarify_devalue_error,
+	get_global_name,
+	handle_error_and_jsonify,
+	serialize_uses
+} from '../utils.js';
+
+/**
+ * If the serialized data contains promises, `chunks` will be an
+ * async iterable containing their resolutions
+ * @param {import('@sveltejs/kit').RequestEvent} event
+ * @param {import('types').RequestState} event_state
+ * @param {import('types').SSROptions} options
+ * @returns {import('./types.js').ServerDataSerializer}
+ */
+export function server_data_serializer(event, event_state, options) {
+	let promise_id = 1;
+
+	const iterator = create_async_iterator();
+	const global = get_global_name(options);
+
+	/** @param {any} thing */
+	function replacer(thing) {
+		if (typeof thing?.then === 'function') {
+			const id = promise_id++;
+
+			const promise = thing
+				.then(/** @param {any} data */ (data) => ({ data }))
+				.catch(
+					/** @param {any} error */ async (error) => ({
+						error: await handle_error_and_jsonify(event, event_state, options, error)
+					})
+				)
+				.then(
+					/**
+					 * @param {{data: any; error: any}} result
+					 */
+					async ({ data, error }) => {
+						let str;
+						try {
+							str = devalue.uneval(error ? [, error] : [data], replacer);
+						} catch {
+							error = await handle_error_and_jsonify(
+								event,
+								event_state,
+								options,
+								new Error(`Failed to serialize promise while rendering ${event.route.id}`)
+							);
+							data = undefined;
+							str = devalue.uneval([, error], replacer);
+						}
+
+						return `${global}.resolve(${id}, ${str.includes('app.decode') ? `(app) => ${str}` : `() => ${str}`})`;
+					}
+				);
+
+			iterator.add(promise);
+
+			return `${global}.defer(${id})`;
+		} else {
+			for (const key in options.hooks.transport) {
+				const encoded = options.hooks.transport[key].encode(thing);
+				if (encoded) {
+					return `app.decode('${key}', ${devalue.uneval(encoded, replacer)})`;
+				}
+			}
+		}
+	}
+
+	const strings = /** @type {string[]} */ ([]);
+
+	return {
+		add_node(i, node) {
+			try {
+				if (!node) {
+					strings[i] = 'null';
+					return;
+				}
+
+				/** @type {any} */
+				const payload = { type: 'data', data: node.data, uses: serialize_uses(node) };
+				if (node.slash) payload.slash = node.slash;
+
+				strings[i] = devalue.uneval(payload, replacer);
+			} catch (e) {
+				// @ts-expect-error
+				e.path = e.path.slice(1);
+				throw new Error(clarify_devalue_error(event, /** @type {any} */ (e)));
+			}
+		},
+
+		get_data(csp) {
+			const open = `<script${csp.script_needs_nonce ? ` nonce="${csp.nonce}"` : ''}>`;
+			const close = `</script>\n`;
+
+			return {
+				data: `[${strings.join(',')}]`,
+				chunks: promise_id > 1 ? iterator.iterate((str) => open + str + close) : null
+			};
+		}
+	};
+}
+
+/**
+ * If the serialized data contains promises, `chunks` will be an
+ * async iterable containing their resolutions
+ * @param {import('@sveltejs/kit').RequestEvent} event
+ * @param {import('types').RequestState} event_state
+ * @param {import('types').SSROptions} options
+ * @returns {import('./types.js').ServerDataSerializerJson}
+ */
+export function server_data_serializer_json(event, event_state, options) {
+	let promise_id = 1;
+
+	const iterator = create_async_iterator();
+
+	const reducers = {
+		...Object.fromEntries(
+			Object.entries(options.hooks.transport).map(([key, value]) => [key, value.encode])
+		),
+		/** @param {any} thing */
+		Promise: (thing) => {
+			if (typeof thing?.then !== 'function') {
+				return;
+			}
+
+			const id = promise_id++;
+
+			/** @type {'data' | 'error'} */
+			let key = 'data';
+
+			const promise = thing
+				.catch(
+					/** @param {any} e */ async (e) => {
+						key = 'error';
+						return handle_error_and_jsonify(event, event_state, options, /** @type {any} */ (e));
+					}
+				)
+				.then(
+					/** @param {any} value */
+					async (value) => {
+						let str;
+						try {
+							str = devalue.stringify(value, reducers);
+						} catch {
+							const error = await handle_error_and_jsonify(
+								event,
+								event_state,
+								options,
+								new Error(`Failed to serialize promise while rendering ${event.route.id}`)
+							);
+
+							key = 'error';
+							str = devalue.stringify(error, reducers);
+						}
+
+						return `{"type":"chunk","id":${id},"${key}":${str}}\n`;
+					}
+				);
+
+			iterator.add(promise);
+
+			return id;
+		}
+	};
+
+	const strings = /** @type {string[]} */ ([]);
+
+	return {
+		add_node(i, node) {
+			try {
+				if (!node) {
+					strings[i] = 'null';
+					return;
+				}
+
+				if (node.type === 'error' || node.type === 'skip') {
+					strings[i] = JSON.stringify(node);
+					return;
+				}
+
+				strings[i] =
+					`{"type":"data","data":${devalue.stringify(node.data, reducers)},"uses":${JSON.stringify(
+						serialize_uses(node)
+					)}${node.slash ? `,"slash":${JSON.stringify(node.slash)}` : ''}}`;
+			} catch (e) {
+				// @ts-expect-error
+				e.path = 'data' + e.path;
+				throw new Error(clarify_devalue_error(event, /** @type {any} */ (e)));
+			}
+		},
+
+		get_data() {
+			return {
+				data: `{"type":"data","nodes":[${strings.join(',')}]}\n`,
+				chunks: promise_id > 1 ? iterator.iterate() : null
+			};
+		}
+	};
+}
diff --git a/packages/kit/src/runtime/server/page/index.js b/packages/kit/src/runtime/server/page/index.js
index 7d24641f062d..9e05ab3a4334 100644
--- a/packages/kit/src/runtime/server/page/index.js
+++ b/packages/kit/src/runtime/server/page/index.js
@@ -10,10 +10,10 @@ import {
 	is_action_json_request,
 	is_action_request
 } from './actions.js';
+import { server_data_serializer, server_data_serializer_json } from './data_serializer.js';
 import { load_data, load_server_data } from './load_data.js';
 import { render_response } from './render.js';
 import { respond_with_error } from './respond_with_error.js';
-import { get_data_json } from '../data/index.js';
 import { DEV } from 'esm-env';
 import { get_remote_action, handle_remote_form_post } from '../remote.js';
 import { PageNodes } from '../../../utils/page_nodes.js';
@@ -147,7 +147,8 @@ export async function render_page(
 				options,
 				manifest,
 				state,
-				resolve_opts
+				resolve_opts,
+				data_serializer: server_data_serializer(event, event_state, options)
 			});
 		}
 
@@ -157,6 +158,12 @@ export async function render_page(
 		/** @type {Error | null} */
 		let load_error = null;
 
+		const data_serializer = server_data_serializer(event, event_state, options);
+		const data_serializer_json =
+			state.prerendering && should_prerender_data
+				? server_data_serializer_json(event, event_state, options)
+				: null;
+
 		/** @type {Array<Promise<import('types').ServerDataNode | null>>} */
 		const server_promises = nodes.data.map((node, i) => {
 			if (load_error) {
@@ -172,7 +179,7 @@ export async function render_page(
 						throw action_result.error;
 					}
 
-					return await load_server_data({
+					const server_data = await load_server_data({
 						event,
 						event_state,
 						state,
@@ -187,6 +194,11 @@ export async function render_page(
 							return data;
 						}
 					});
+
+					data_serializer.add_node(i, server_data);
+					data_serializer_json?.add_node(i, server_data);
+
+					return server_data;
 				} catch (e) {
 					load_error = /** @type {Error} */ (e);
 					throw load_error;
@@ -287,7 +299,8 @@ export async function render_page(
 									data: null,
 									server_data: null
 								}),
-								fetched
+								fetched,
+								data_serializer: server_data_serializer(event, event_state, options)
 							});
 						}
 					}
@@ -303,14 +316,9 @@ export async function render_page(
 			}
 		}
 
-		if (state.prerendering && should_prerender_data) {
+		if (state.prerendering && data_serializer_json) {
 			// ndjson format
-			let { data, chunks } = get_data_json(
-				event,
-				event_state,
-				options,
-				branch.map((node) => node?.server_data)
-			);
+			let { data, chunks } = data_serializer_json.get_data();
 
 			if (chunks) {
 				for await (const chunk of chunks) {
@@ -339,7 +347,8 @@ export async function render_page(
 			error: null,
 			branch: ssr === false ? [] : compact(branch),
 			action_result,
-			fetched
+			fetched,
+			data_serializer
 		});
 	} catch (e) {
 		// if we end up here, it means the data loaded successfully
diff --git a/packages/kit/src/runtime/server/page/load_data.js b/packages/kit/src/runtime/server/page/load_data.js
index a7b63235305a..520d6ff5bb85 100644
--- a/packages/kit/src/runtime/server/page/load_data.js
+++ b/packages/kit/src/runtime/server/page/load_data.js
@@ -1,12 +1,11 @@
 import { DEV } from 'esm-env';
-import * as devalue from 'devalue';
 import { disable_search, make_trackable } from '../../../utils/url.js';
 import { validate_depends, validate_load_response } from '../../shared.js';
 import { with_request_store, merge_tracing } from '@sveltejs/kit/internal/server';
 import { record_span } from '../../telemetry/record_span.js';
-import { clarify_devalue_error, get_node_type } from '../utils.js';
 import { base64_encode, text_decoder } from '../../utils.js';
 import { NULL_BODY_STATUS } from '../constants.js';
+import { get_node_type } from '../utils.js';
 
 /**
  * Calls the user's server `load` function.
@@ -234,38 +233,11 @@ export async function load_data({
 		},
 		fn: async (current) => {
 			const traced_event = merge_tracing(event, current);
-			return await with_request_store({ event: traced_event, state: event_state }, () => {
-				/** @type {Record<string, any> | null} */
-				let data = null;
-
-				return load.call(null, {
+			return await with_request_store({ event: traced_event, state: event_state }, () =>
+				load.call(null, {
 					url: event.url,
 					params: event.params,
-					get data() {
-						if (data === null && server_data_node?.data != null) {
-							/** @type {Record<string, (value: any) => any>} */
-							const reducers = {};
-
-							/** @type {Record<string, (value: any) => any>} */
-							const revivers = {};
-
-							for (const key in event_state.transport) {
-								reducers[key] = event_state.transport[key].encode;
-								revivers[key] = event_state.transport[key].decode;
-							}
-
-							// run it through devalue so that the developer can't accidentally mutate it
-							try {
-								data = devalue.parse(devalue.stringify(server_data_node.data, reducers), revivers);
-							} catch (e) {
-								// @ts-expect-error
-								e.path = e.path.slice(1);
-								throw new Error(clarify_devalue_error(event, /** @type {any} */ (e)));
-							}
-						}
-
-						return data;
-					},
+					data: server_data_node?.data ?? null,
 					route: event.route,
 					fetch: create_universal_fetch(event, state, fetched, csr, resolve_opts),
 					setHeaders: event.setHeaders,
@@ -273,8 +245,8 @@ export async function load_data({
 					parent,
 					untrack: (fn) => fn(),
 					tracing: traced_event.tracing
-				});
-			});
+				})
+			);
 		}
 	});
 
diff --git a/packages/kit/src/runtime/server/page/render.js b/packages/kit/src/runtime/server/page/render.js
index 928be29255d3..2ad4908508d0 100644
--- a/packages/kit/src/runtime/server/page/render.js
+++ b/packages/kit/src/runtime/server/page/render.js
@@ -8,15 +8,14 @@ import { serialize_data } from './serialize_data.js';
 import { s } from '../../../utils/misc.js';
 import { Csp } from './csp.js';
 import { uneval_action_response } from './actions.js';
-import { clarify_devalue_error, handle_error_and_jsonify, serialize_uses } from '../utils.js';
 import { public_env } from '../../shared-server.js';
-import { create_async_iterator } from '../../../utils/streaming.js';
 import { SVELTE_KIT_ASSETS } from '../../../constants.js';
 import { SCHEME } from '../../../utils/url.js';
 import { create_server_routing_response, generate_route_object } from './server_routing.js';
 import { add_resolution_suffix } from '../../pathname.js';
 import { with_request_store } from '@sveltejs/kit/internal/server';
 import { text_encoder } from '../../utils.js';
+import { get_global_name } from '../utils.js';
 
 // TODO rename this function/module
 
@@ -40,6 +39,7 @@ const updated = {
  *   event_state: import('types').RequestState;
  *   resolve_opts: import('types').RequiredResolveOptions;
  *   action_result?: import('@sveltejs/kit').ActionResult;
+ *   data_serializer: import('./types.js').ServerDataSerializer
  * }} opts
  */
 export async function render_response({
@@ -54,7 +54,8 @@ export async function render_response({
 	event,
 	event_state,
 	resolve_opts,
-	action_result
+	action_result,
+	data_serializer
 }) {
 	if (state.prerendering) {
 		if (options.csp.mode === 'nonce') {
@@ -295,16 +296,8 @@ export async function render_response({
 		}
 	}
 
-	const global = DEV ? '__sveltekit_dev' : `__sveltekit_${options.version_hash}`;
-
-	const { data, chunks } = get_data(
-		event,
-		event_state,
-		options,
-		branch.map((b) => b.server_data),
-		csp,
-		global
-	);
+	const global = get_global_name(options);
+	const { data, chunks } = data_serializer.get_data(csp);
 
 	if (page_config.ssr && page_config.csr) {
 		body += `\n\t\t\t${fetched
@@ -644,95 +637,3 @@ export async function render_response({
 				}
 			);
 }
-
-/**
- * If the serialized data contains promises, `chunks` will be an
- * async iterable containing their resolutions
- * @param {import('@sveltejs/kit').RequestEvent} event
- * @param {import('types').RequestState} event_state
- * @param {import('types').SSROptions} options
- * @param {Array<import('types').ServerDataNode | null>} nodes
- * @param {import('./csp.js').Csp} csp
- * @param {string} global
- * @returns {{ data: string, chunks: AsyncIterable<string> | null }}
- */
-function get_data(event, event_state, options, nodes, csp, global) {
-	let promise_id = 1;
-	let count = 0;
-
-	const { iterator, push, done } = create_async_iterator();
-
-	/** @param {any} thing */
-	function replacer(thing) {
-		if (typeof thing?.then === 'function') {
-			const id = promise_id++;
-			count += 1;
-
-			thing
-				.then(/** @param {any} data */ (data) => ({ data }))
-				.catch(
-					/** @param {any} error */ async (error) => ({
-						error: await handle_error_and_jsonify(event, event_state, options, error)
-					})
-				)
-				.then(
-					/**
-					 * @param {{data: any; error: any}} result
-					 */
-					async ({ data, error }) => {
-						count -= 1;
-
-						let str;
-						try {
-							str = devalue.uneval(error ? [, error] : [data], replacer);
-						} catch {
-							error = await handle_error_and_jsonify(
-								event,
-								event_state,
-								options,
-								new Error(`Failed to serialize promise while rendering ${event.route.id}`)
-							);
-							data = undefined;
-							str = devalue.uneval([, error], replacer);
-						}
-
-						const nonce = csp.script_needs_nonce ? ` nonce="${csp.nonce}"` : '';
-						push(
-							`<script${nonce}>${global}.resolve(${id}, ${str.includes('app.decode') ? `(app) => ${str}` : `() => ${str}`})</script>\n`
-						);
-						if (count === 0) done();
-					}
-				);
-
-			return `${global}.defer(${id})`;
-		} else {
-			for (const key in options.hooks.transport) {
-				const encoded = options.hooks.transport[key].encode(thing);
-				if (encoded) {
-					return `app.decode('${key}', ${devalue.uneval(encoded, replacer)})`;
-				}
-			}
-		}
-	}
-
-	try {
-		const strings = nodes.map((node) => {
-			if (!node) return 'null';
-
-			/** @type {any} */
-			const payload = { type: 'data', data: node.data, uses: serialize_uses(node) };
-			if (node.slash) payload.slash = node.slash;
-
-			return devalue.uneval(payload, replacer);
-		});
-
-		return {
-			data: `[${strings.join(',')}]`,
-			chunks: count > 0 ? iterator : null
-		};
-	} catch (e) {
-		// @ts-expect-error
-		e.path = e.path.slice(1);
-		throw new Error(clarify_devalue_error(event, /** @type {any} */ (e)));
-	}
-}
diff --git a/packages/kit/src/runtime/server/page/respond_with_error.js b/packages/kit/src/runtime/server/page/respond_with_error.js
index b1a6ae9ae606..0767e177d124 100644
--- a/packages/kit/src/runtime/server/page/respond_with_error.js
+++ b/packages/kit/src/runtime/server/page/respond_with_error.js
@@ -4,6 +4,7 @@ import { load_data, load_server_data } from './load_data.js';
 import { handle_error_and_jsonify, static_error_page, redirect_response } from '../utils.js';
 import { get_status } from '../../../utils/error.js';
 import { PageNodes } from '../../../utils/page_nodes.js';
+import { server_data_serializer } from './data_serializer.js';
 
 /**
  * @typedef {import('./types.js').Loaded} Loaded
@@ -45,6 +46,7 @@ export async function respond_with_error({
 		const nodes = new PageNodes([default_layout]);
 		const ssr = nodes.ssr();
 		const csr = nodes.csr();
+		const data_serializer = server_data_serializer(event, event_state, options);
 
 		if (ssr) {
 			state.error = true;
@@ -59,6 +61,7 @@ export async function respond_with_error({
 			});
 
 			const server_data = await server_data_promise;
+			data_serializer.add_node(0, server_data);
 
 			const data = await load_data({
 				event,
@@ -101,7 +104,8 @@ export async function respond_with_error({
 			fetched,
 			event,
 			event_state,
-			resolve_opts
+			resolve_opts,
+			data_serializer
 		});
 	} catch (e) {
 		// Edge case: If route is a 404 and the user redirects to somewhere from the root layout,
diff --git a/packages/kit/src/runtime/server/page/types.d.ts b/packages/kit/src/runtime/server/page/types.d.ts
index 300e05fefbe9..bfe975fb37a9 100644
--- a/packages/kit/src/runtime/server/page/types.d.ts
+++ b/packages/kit/src/runtime/server/page/types.d.ts
@@ -1,5 +1,12 @@
 import { CookieSerializeOptions } from 'cookie';
-import { SSRNode, CspDirectives, ServerDataNode } from 'types';
+import {
+	CspDirectives,
+	ServerDataNode,
+	SSRNode,
+	ServerDataSkippedNode,
+	ServerErrorNode
+} from 'types';
+import { Csp } from './csp.js';
 
 export interface Fetched {
 	url: string;
@@ -34,3 +41,16 @@ export interface Cookie {
 	value: string;
 	options: CookieSerializeOptions & { path: string };
 }
+
+export type ServerDataSerializer = {
+	add_node(i: number, node: ServerDataNode | null): void;
+	get_data(csp: Csp): { data: string; chunks: AsyncIterable<string> | null };
+};
+
+export type ServerDataSerializerJson = {
+	add_node(
+		i: number,
+		node: ServerDataSkippedNode | ServerDataNode | ServerErrorNode | null | undefined
+	): void;
+	get_data(): { data: string; chunks: AsyncIterable<string> | null };
+};
diff --git a/packages/kit/src/runtime/server/respond.js b/packages/kit/src/runtime/server/respond.js
index bcb0defd0846..2e97adaaf441 100644
--- a/packages/kit/src/runtime/server/respond.js
+++ b/packages/kit/src/runtime/server/respond.js
@@ -35,6 +35,7 @@ import {
 	strip_data_suffix,
 	strip_resolution_suffix
 } from '../pathname.js';
+import { server_data_serializer } from './page/data_serializer.js';
 import { get_remote_id, handle_remote_call } from './remote.js';
 import { record_span } from '../telemetry/record_span.js';
 import { otel } from '../telemetry/otel.js';
@@ -542,7 +543,8 @@ export async function internal_respond(request, options, manifest, state) {
 					error: null,
 					branch: [],
 					fetched: [],
-					resolve_opts
+					resolve_opts,
+					data_serializer: server_data_serializer(event, event_state, options)
 				});
 			}
 
diff --git a/packages/kit/src/runtime/server/utils.js b/packages/kit/src/runtime/server/utils.js
index 1c66bd25196c..f7c33124a57b 100644
--- a/packages/kit/src/runtime/server/utils.js
+++ b/packages/kit/src/runtime/server/utils.js
@@ -44,6 +44,13 @@ export function allowed_methods(mod) {
 	return allowed;
 }
 
+/**
+ * @param {import('types').SSROptions} options
+ */
+export function get_global_name(options) {
+	return DEV ? '__sveltekit_dev' : `__sveltekit_${options.version_hash}`;
+}
+
 /**
  * Return as a response that renders the error.html
  *
diff --git a/packages/kit/src/utils/streaming.js b/packages/kit/src/utils/streaming.js
index cfb0d6a04f93..449507ac6695 100644
--- a/packages/kit/src/utils/streaming.js
+++ b/packages/kit/src/utils/streaming.js
@@ -16,36 +16,50 @@ function defer() {
 
 /**
  * Create an async iterator and a function to push values into it
+ * @template T
  * @returns {{
- *   iterator: AsyncIterable<any>;
- *   push: (value: any) => void;
- *   done: () => void;
+ *   iterate: (transform?: (input: T) => T) => AsyncIterable<T>;
+ *   add: (promise: Promise<T>) => void;
  * }}
  */
 export function create_async_iterator() {
+	let count = 0;
+
 	const deferred = [defer()];
 
 	return {
-		iterator: {
-			[Symbol.asyncIterator]() {
-				return {
-					next: async () => {
-						const next = await deferred[0].promise;
-						if (!next.done) deferred.shift();
-						return next;
-					}
-				};
-			}
+		iterate: (transform = (x) => x) => {
+			return {
+				[Symbol.asyncIterator]() {
+					return {
+						next: async () => {
+							const next = await deferred[0].promise;
+
+							if (!next.done) {
+								deferred.shift();
+								return { value: transform(next.value), done: false };
+							}
+
+							return next;
+						}
+					};
+				}
+			};
 		},
-		push: (value) => {
-			deferred[deferred.length - 1].fulfil({
-				value,
-				done: false
+		add: (promise) => {
+			count += 1;
+
+			void promise.then((value) => {
+				deferred[deferred.length - 1].fulfil({
+					value,
+					done: false
+				});
+				deferred.push(defer());
+
+				if (--count === 0) {
+					deferred[deferred.length - 1].fulfil({ done: true });
+				}
 			});
-			deferred.push(defer());
-		},
-		done: () => {
-			deferred[deferred.length - 1].fulfil({ done: true });
 		}
 	};
 }
diff --git a/packages/kit/src/utils/streaming.spec.js b/packages/kit/src/utils/streaming.spec.js
index 320fbe3e2769..2a94ad178bef 100644
--- a/packages/kit/src/utils/streaming.spec.js
+++ b/packages/kit/src/utils/streaming.spec.js
@@ -2,16 +2,15 @@ import { expect, test } from 'vitest';
 import { create_async_iterator } from './streaming.js';
 
 test('works with fast consecutive promise resolutions', async () => {
-	const iterator = create_async_iterator();
+	const { iterate, add } = create_async_iterator();
 
-	void Promise.resolve(1).then((n) => iterator.push(n));
-	void Promise.resolve(2).then((n) => iterator.push(n));
-	void Promise.resolve().then(() => iterator.done());
+	add(Promise.resolve(1));
+	add(Promise.resolve(2));
 
 	const actual = [];
-	for await (const value of iterator.iterator) {
+	for await (const value of iterate((n) => n * 10)) {
 		actual.push(value);
 	}
 
-	expect(actual).toEqual([1, 2]);
+	expect(actual).toEqual([10, 20]);
 });
diff --git a/packages/kit/test/apps/basics/src/routes/streaming/server/+page.js b/packages/kit/test/apps/basics/src/routes/streaming/server/+page.js
new file mode 100644
index 000000000000..a5673a92a7b2
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/streaming/server/+page.js
@@ -0,0 +1,3 @@
+export function load({ data }) {
+	return { ...data };
+}