From e1ba14a439a0a44564d4c07113ca2fb439beb845 Mon Sep 17 00:00:00 2001
From: Patrick <Patrick@ShowYou.us>
Date: Wed, 10 Sep 2025 01:30:51 +0200
Subject: [PATCH 01/13] test

---
 packages/kit/test/apps/basics/src/routes/+layout.js       | 3 ++-
 packages/kit/test/apps/basics/src/routes/+layout.svelte   | 1 +
 packages/kit/test/apps/basics/test/cross-platform/test.js | 1 +
 3 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/packages/kit/test/apps/basics/src/routes/+layout.js b/packages/kit/test/apps/basics/src/routes/+layout.js
index 113e4a079bd0..547834859adf 100644
--- a/packages/kit/test/apps/basics/src/routes/+layout.js
+++ b/packages/kit/test/apps/basics/src/routes/+layout.js
@@ -1,7 +1,8 @@
 /** @type {import('./$types').LayoutLoad} */
-export async function load() {
+export async function load({ data }) {
 	// Do NOT make this load function depend on something which would cause it to rerun
 	return {
+		...data,
 		foo: {
 			bar: 'Custom layout'
 		}
diff --git a/packages/kit/test/apps/basics/src/routes/+layout.svelte b/packages/kit/test/apps/basics/src/routes/+layout.svelte
index 969229a5f678..e4d1e773b1e9 100644
--- a/packages/kit/test/apps/basics/src/routes/+layout.svelte
+++ b/packages/kit/test/apps/basics/src/routes/+layout.svelte
@@ -9,6 +9,7 @@
 
 <slot />
 
+<div id="root-layout">{data.rootlayout}</div>
 <footer>{data.foo.bar}</footer>
 
 <style>
diff --git a/packages/kit/test/apps/basics/test/cross-platform/test.js b/packages/kit/test/apps/basics/test/cross-platform/test.js
index 7de5222bece0..8827f50e8cdc 100644
--- a/packages/kit/test/apps/basics/test/cross-platform/test.js
+++ b/packages/kit/test/apps/basics/test/cross-platform/test.js
@@ -324,6 +324,7 @@ test.describe('Errors', () => {
 	test('server-side error from load() is an Error', async ({ page }) => {
 		const response = await page.goto('/errors/load-error-server');
 
+		expect(await page.textContent('#root-layout')).toBe('rootlayout');
 		expect(await page.textContent('footer')).toBe('Custom layout');
 		expect(await page.textContent('#message')).toBe(
 			'This is your custom error page saying: "Not found"'

From e4831a5826d1ce4ef114fdc2c281dbd2b87f4992 Mon Sep 17 00:00:00 2001
From: Patrick <Patrick@ShowYou.us>
Date: Wed, 10 Sep 2025 01:34:45 +0200
Subject: [PATCH 02/13] use existing data serializer when a load functions
 throws but drop non-layout nodes

---
 packages/kit/src/runtime/server/page/data_serializer.js | 9 +++++++++
 packages/kit/src/runtime/server/page/index.js           | 4 +++-
 packages/kit/src/runtime/server/page/types.d.ts         | 1 +
 3 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/packages/kit/src/runtime/server/page/data_serializer.js b/packages/kit/src/runtime/server/page/data_serializer.js
index 2f0197ae648b..30b5088a9542 100644
--- a/packages/kit/src/runtime/server/page/data_serializer.js
+++ b/packages/kit/src/runtime/server/page/data_serializer.js
@@ -70,8 +70,13 @@ export function server_data_serializer(event, event_state, options) {
 	}
 
 	const strings = /** @type {string[]} */ ([]);
+	let max_nodes = -1;
 
 	return {
+		set_max_nodes(i) {
+			max_nodes = i;
+		},
+
 		add_node(i, node) {
 			try {
 				if (!node) {
@@ -95,6 +100,10 @@ export function server_data_serializer(event, event_state, options) {
 			const open = `<script${csp.script_needs_nonce ? ` nonce="${csp.nonce}"` : ''}>`;
 			const close = `</script>\n`;
 
+			if (max_nodes > -1) {
+				strings.length = max_nodes;
+			}
+
 			return {
 				data: `[${strings.join(',')}]`,
 				chunks: promise_id > 1 ? iterator.iterate((str) => open + str + close) : null
diff --git a/packages/kit/src/runtime/server/page/index.js b/packages/kit/src/runtime/server/page/index.js
index 9e05ab3a4334..49f7412172e7 100644
--- a/packages/kit/src/runtime/server/page/index.js
+++ b/packages/kit/src/runtime/server/page/index.js
@@ -278,6 +278,8 @@ export async function render_page(
 							let j = i;
 							while (!branch[j]) j -= 1;
 
+							data_serializer.set_max_nodes(j + 1);
+
 							const layouts = compact(branch.slice(0, j + 1));
 							const nodes = new PageNodes(layouts.map((layout) => layout.node));
 
@@ -300,7 +302,7 @@ export async function render_page(
 									server_data: null
 								}),
 								fetched,
-								data_serializer: server_data_serializer(event, event_state, options)
+								data_serializer
 							});
 						}
 					}
diff --git a/packages/kit/src/runtime/server/page/types.d.ts b/packages/kit/src/runtime/server/page/types.d.ts
index bfe975fb37a9..f0d094d122c2 100644
--- a/packages/kit/src/runtime/server/page/types.d.ts
+++ b/packages/kit/src/runtime/server/page/types.d.ts
@@ -45,6 +45,7 @@ export interface Cookie {
 export type ServerDataSerializer = {
 	add_node(i: number, node: ServerDataNode | null): void;
 	get_data(csp: Csp): { data: string; chunks: AsyncIterable<string> | null };
+	set_max_nodes(i: number): void;
 };
 
 export type ServerDataSerializerJson = {

From d2f01bc9589460d38381f9cab2276353a276f5bc Mon Sep 17 00:00:00 2001
From: Patrick <Patrick@ShowYou.us>
Date: Wed, 10 Sep 2025 02:15:30 +0200
Subject: [PATCH 03/13] adjust unrelated tests

---
 .../kit/test/apps/basics/test/client.test.js  |  8 +++--
 .../apps/basics/test/cross-platform/test.js   | 12 +++----
 packages/kit/test/apps/basics/test/test.js    | 32 +++++++++++++------
 3 files changed, 34 insertions(+), 18 deletions(-)

diff --git a/packages/kit/test/apps/basics/test/client.test.js b/packages/kit/test/apps/basics/test/client.test.js
index 44d6a794f171..e948e8586083 100644
--- a/packages/kit/test/apps/basics/test/client.test.js
+++ b/packages/kit/test/apps/basics/test/client.test.js
@@ -102,14 +102,17 @@ test.describe('Load', () => {
 	}) => {
 		await page.goto('/load/server-data-reuse/with-server-load');
 		expect(await page.textContent('pre')).toBe(
-			JSON.stringify({ foo: { bar: 'Custom layout' }, server: true })
+			JSON.stringify({ rootlayout: 'rootlayout', foo: { bar: 'Custom layout' }, server: true })
 		);
 		await app.goto('/load/server-data-reuse/no-load');
-		expect(await page.textContent('pre')).toBe(JSON.stringify({ foo: { bar: 'Custom layout' } }));
+		expect(await page.textContent('pre')).toBe(
+			JSON.stringify({ rootlayout: 'rootlayout', foo: { bar: 'Custom layout' } })
+		);
 
 		await page.goto('/load/server-data-reuse/with-changing-parent/with-server-load');
 		expect(await page.textContent('pre')).toBe(
 			JSON.stringify({
+				rootlayout: 'rootlayout',
 				foo: { bar: 'Custom layout' },
 				title: '/load/server-data-reuse/with-changing-parent/with-server-load',
 				server: true
@@ -118,6 +121,7 @@ test.describe('Load', () => {
 		await app.goto('/load/server-data-reuse/with-changing-parent/no-load');
 		expect(await page.textContent('pre')).toBe(
 			JSON.stringify({
+				rootlayout: 'rootlayout',
 				foo: { bar: 'Custom layout' },
 				title: '/load/server-data-reuse/with-changing-parent/no-load'
 			})
diff --git a/packages/kit/test/apps/basics/test/cross-platform/test.js b/packages/kit/test/apps/basics/test/cross-platform/test.js
index 8827f50e8cdc..56256bc97187 100644
--- a/packages/kit/test/apps/basics/test/cross-platform/test.js
+++ b/packages/kit/test/apps/basics/test/cross-platform/test.js
@@ -190,26 +190,26 @@ test.describe('Shadowed pages', () => {
 	test('Parent data is present', async ({ page, clicknav }) => {
 		await page.goto('/shadowed/parent');
 		await expect(page.locator('h2')).toHaveText(
-			'Layout data: {"foo":{"bar":"Custom layout"},"layout":"layout"}'
+			'Layout data: {"rootlayout":"rootlayout","foo":{"bar":"Custom layout"},"layout":"layout"}'
 		);
 		await expect(page.locator('p')).toHaveText(
-			'Page data: {"foo":{"bar":"Custom layout"},"layout":"layout","page":"page","data":{"rootlayout":"rootlayout","layout":"layout"}}'
+			'Page data: {"rootlayout":"rootlayout","foo":{"bar":"Custom layout"},"layout":"layout","page":"page","data":{"rootlayout":"rootlayout","layout":"layout"}}'
 		);
 
 		await clicknav('[href="/shadowed/parent?test"]');
 		await expect(page.locator('h2')).toHaveText(
-			'Layout data: {"foo":{"bar":"Custom layout"},"layout":"layout"}'
+			'Layout data: {"rootlayout":"rootlayout","foo":{"bar":"Custom layout"},"layout":"layout"}'
 		);
 		await expect(page.locator('p')).toHaveText(
-			'Page data: {"foo":{"bar":"Custom layout"},"layout":"layout","page":"page","data":{"rootlayout":"rootlayout","layout":"layout"}}'
+			'Page data: {"rootlayout":"rootlayout","foo":{"bar":"Custom layout"},"layout":"layout","page":"page","data":{"rootlayout":"rootlayout","layout":"layout"}}'
 		);
 
 		await clicknav('[href="/shadowed/parent/sub"]');
 		await expect(page.locator('h2')).toHaveText(
-			'Layout data: {"foo":{"bar":"Custom layout"},"layout":"layout"}'
+			'Layout data: {"rootlayout":"rootlayout","foo":{"bar":"Custom layout"},"layout":"layout"}'
 		);
 		await expect(page.locator('p')).toHaveText(
-			'Page data: {"foo":{"bar":"Custom layout"},"layout":"layout","sub":"sub","data":{"rootlayout":"rootlayout","layout":"layout"}}'
+			'Page data: {"rootlayout":"rootlayout","foo":{"bar":"Custom layout"},"layout":"layout","sub":"sub","data":{"rootlayout":"rootlayout","layout":"layout"}}'
 		);
 	});
 
diff --git a/packages/kit/test/apps/basics/test/test.js b/packages/kit/test/apps/basics/test/test.js
index 36c6ed0ad9d8..7659075c7d8e 100644
--- a/packages/kit/test/apps/basics/test/test.js
+++ b/packages/kit/test/apps/basics/test/test.js
@@ -356,6 +356,7 @@ test.describe('Load', () => {
 			expect(await page.textContent('h1')).toBe('message: original + new');
 			expect(await page.textContent('pre')).toBe(
 				JSON.stringify({
+					rootlayout: 'rootlayout',
 					foo: { bar: 'Custom layout' },
 					message: 'original + new',
 					x: 'a',
@@ -370,6 +371,7 @@ test.describe('Load', () => {
 				expect(await page.textContent('h1')).toBe('message: original + new');
 				expect(await page.textContent('pre')).toBe(
 					JSON.stringify({
+						rootlayout: 'rootlayout',
 						foo: { bar: 'Custom layout' },
 						message: 'original + new',
 						x: 'd',
@@ -776,23 +778,23 @@ test.describe('$app/stores', () => {
 		const foo = { bar: 'Custom layout' };
 
 		expect(await page.textContent('#store-data')).toBe(
-			JSON.stringify({ foo, name: 'SvelteKit', value: 456, page: 'www' })
+			JSON.stringify({ rootlayout: 'rootlayout', foo, name: 'SvelteKit', value: 456, page: 'www' })
 		);
 
 		await clicknav('a[href="/store/data/zzz"]');
 		expect(await page.textContent('#store-data')).toBe(
-			JSON.stringify({ foo, name: 'SvelteKit', value: 456, page: 'zzz' })
+			JSON.stringify({ rootlayout: 'rootlayout', foo, name: 'SvelteKit', value: 456, page: 'zzz' })
 		);
 
 		await clicknav('a[href="/store/data/xxx"]');
 		expect(await page.textContent('#store-data')).toBe(
-			JSON.stringify({ foo, name: 'SvelteKit', value: 123 })
+			JSON.stringify({ rootlayout: 'rootlayout', foo, name: 'SvelteKit', value: 123 })
 		);
 		expect(await page.textContent('#store-error')).toBe('Params = xxx');
 
 		await clicknav('a[href="/store/data/yyy"]');
 		expect(await page.textContent('#store-data')).toBe(
-			JSON.stringify({ foo, name: 'SvelteKit', value: 123 })
+			JSON.stringify({ rootlayout: 'rootlayout', foo, name: 'SvelteKit', value: 123 })
 		);
 		expect(await page.textContent('#store-error')).toBe('Params = yyy');
 	});
@@ -803,7 +805,12 @@ test.describe('$app/stores', () => {
 		javaScriptEnabled
 	}) => {
 		await page.goto('/store/data/foo?reset=true');
-		const stuff1 = { foo: { bar: 'Custom layout' }, name: 'SvelteKit', value: 123 };
+		const stuff1 = {
+			foo: { bar: 'Custom layout' },
+			name: 'SvelteKit',
+			rootlayout: 'rootlayout',
+			value: 123
+		};
 		const stuff2 = { ...stuff1, foo: true, number: 2 };
 		const stuff3 = { ...stuff2 };
 		await page.goto('/store/data/www');
@@ -900,23 +907,23 @@ test.describe('$app/state', () => {
 		const foo = { bar: 'Custom layout' };
 
 		expect(await page.textContent('#state-data')).toBe(
-			JSON.stringify({ foo, name: 'SvelteKit', value: 456, page: 'www' })
+			'{"rootlayout":"rootlayout","foo":{"bar":"Custom layout"},"name":"SvelteKit","value":456,"page":"www"}'
 		);
 
 		await clicknav('a[href="/state/data/zzz"]');
 		expect(await page.textContent('#state-data')).toBe(
-			JSON.stringify({ foo, name: 'SvelteKit', value: 456, page: 'zzz' })
+			'{"rootlayout":"rootlayout","foo":{"bar":"Custom layout"},"name":"SvelteKit","value":456,"page":"zzz"}'
 		);
 
 		await clicknav('a[href="/state/data/xxx"]');
 		expect(await page.textContent('#state-data')).toBe(
-			JSON.stringify({ foo, name: 'SvelteKit', value: 123 })
+			'{"rootlayout":"rootlayout","foo":{"bar":"Custom layout"},"name":"SvelteKit","value":123}'
 		);
 		expect(await page.textContent('#state-error')).toBe('Params = xxx');
 
 		await clicknav('a[href="/state/data/yyy"]');
 		expect(await page.textContent('#state-data')).toBe(
-			JSON.stringify({ foo, name: 'SvelteKit', value: 123 })
+			'{"rootlayout":"rootlayout","foo":{"bar":"Custom layout"},"name":"SvelteKit","value":123}'
 		);
 		expect(await page.textContent('#state-error')).toBe('Params = yyy');
 	});
@@ -927,7 +934,12 @@ test.describe('$app/state', () => {
 		javaScriptEnabled
 	}) => {
 		await page.goto('/state/data/foo?reset=true');
-		const stuff1 = { foo: { bar: 'Custom layout' }, name: 'SvelteKit', value: 123 };
+		const stuff1 = {
+			foo: { bar: 'Custom layout' },
+			name: 'SvelteKit',
+			rootlayout: 'rootlayout',
+			value: 123
+		};
 		const stuff2 = { ...stuff1, foo: true, number: 2 };
 		const stuff3 = { ...stuff2 };
 		await page.goto('/state/data/www');

From 5bc63aabdc98e400896d62a4ab4a011a05e7a47c Mon Sep 17 00:00:00 2001
From: Patrick <Patrick@ShowYou.us>
Date: Wed, 10 Sep 2025 02:26:25 +0200
Subject: [PATCH 04/13] changeset

---
 .changeset/legal-peas-agree.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/legal-peas-agree.md

diff --git a/.changeset/legal-peas-agree.md b/.changeset/legal-peas-agree.md
new file mode 100644
index 000000000000..224d051b41ab
--- /dev/null
+++ b/.changeset/legal-peas-agree.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: layout load data not serialized on error page

From cc65f34a143676ddd2212119ddd779f392e431c4 Mon Sep 17 00:00:00 2001
From: Patrick <Patrick@ShowYou.us>
Date: Wed, 10 Sep 2025 02:36:22 +0200
Subject: [PATCH 05/13] remove unused var

---
 packages/kit/test/apps/basics/test/test.js | 2 --
 1 file changed, 2 deletions(-)

diff --git a/packages/kit/test/apps/basics/test/test.js b/packages/kit/test/apps/basics/test/test.js
index 7659075c7d8e..92a5e6a1ea95 100644
--- a/packages/kit/test/apps/basics/test/test.js
+++ b/packages/kit/test/apps/basics/test/test.js
@@ -904,8 +904,6 @@ test.describe('$app/state', () => {
 	test('page state contains data', async ({ page, clicknav }) => {
 		await page.goto('/state/data/www');
 
-		const foo = { bar: 'Custom layout' };
-
 		expect(await page.textContent('#state-data')).toBe(
 			'{"rootlayout":"rootlayout","foo":{"bar":"Custom layout"},"name":"SvelteKit","value":456,"page":"www"}'
 		);

From e7504191d50211b3c64e2486eb1381408d8c31a8 Mon Sep 17 00:00:00 2001
From: Patrick <Patrick@ShowYou.us>
Date: Wed, 10 Sep 2025 04:19:44 +0200
Subject: [PATCH 06/13] Revert "adjust unrelated tests"

This reverts commit d2f01bc9589460d38381f9cab2276353a276f5bc.
---
 .../kit/test/apps/basics/test/client.test.js  |  8 ++---
 .../apps/basics/test/cross-platform/test.js   | 12 +++----
 packages/kit/test/apps/basics/test/test.js    | 32 ++++++-------------
 3 files changed, 18 insertions(+), 34 deletions(-)

diff --git a/packages/kit/test/apps/basics/test/client.test.js b/packages/kit/test/apps/basics/test/client.test.js
index e948e8586083..44d6a794f171 100644
--- a/packages/kit/test/apps/basics/test/client.test.js
+++ b/packages/kit/test/apps/basics/test/client.test.js
@@ -102,17 +102,14 @@ test.describe('Load', () => {
 	}) => {
 		await page.goto('/load/server-data-reuse/with-server-load');
 		expect(await page.textContent('pre')).toBe(
-			JSON.stringify({ rootlayout: 'rootlayout', foo: { bar: 'Custom layout' }, server: true })
+			JSON.stringify({ foo: { bar: 'Custom layout' }, server: true })
 		);
 		await app.goto('/load/server-data-reuse/no-load');
-		expect(await page.textContent('pre')).toBe(
-			JSON.stringify({ rootlayout: 'rootlayout', foo: { bar: 'Custom layout' } })
-		);
+		expect(await page.textContent('pre')).toBe(JSON.stringify({ foo: { bar: 'Custom layout' } }));
 
 		await page.goto('/load/server-data-reuse/with-changing-parent/with-server-load');
 		expect(await page.textContent('pre')).toBe(
 			JSON.stringify({
-				rootlayout: 'rootlayout',
 				foo: { bar: 'Custom layout' },
 				title: '/load/server-data-reuse/with-changing-parent/with-server-load',
 				server: true
@@ -121,7 +118,6 @@ test.describe('Load', () => {
 		await app.goto('/load/server-data-reuse/with-changing-parent/no-load');
 		expect(await page.textContent('pre')).toBe(
 			JSON.stringify({
-				rootlayout: 'rootlayout',
 				foo: { bar: 'Custom layout' },
 				title: '/load/server-data-reuse/with-changing-parent/no-load'
 			})
diff --git a/packages/kit/test/apps/basics/test/cross-platform/test.js b/packages/kit/test/apps/basics/test/cross-platform/test.js
index 56256bc97187..8827f50e8cdc 100644
--- a/packages/kit/test/apps/basics/test/cross-platform/test.js
+++ b/packages/kit/test/apps/basics/test/cross-platform/test.js
@@ -190,26 +190,26 @@ test.describe('Shadowed pages', () => {
 	test('Parent data is present', async ({ page, clicknav }) => {
 		await page.goto('/shadowed/parent');
 		await expect(page.locator('h2')).toHaveText(
-			'Layout data: {"rootlayout":"rootlayout","foo":{"bar":"Custom layout"},"layout":"layout"}'
+			'Layout data: {"foo":{"bar":"Custom layout"},"layout":"layout"}'
 		);
 		await expect(page.locator('p')).toHaveText(
-			'Page data: {"rootlayout":"rootlayout","foo":{"bar":"Custom layout"},"layout":"layout","page":"page","data":{"rootlayout":"rootlayout","layout":"layout"}}'
+			'Page data: {"foo":{"bar":"Custom layout"},"layout":"layout","page":"page","data":{"rootlayout":"rootlayout","layout":"layout"}}'
 		);
 
 		await clicknav('[href="/shadowed/parent?test"]');
 		await expect(page.locator('h2')).toHaveText(
-			'Layout data: {"rootlayout":"rootlayout","foo":{"bar":"Custom layout"},"layout":"layout"}'
+			'Layout data: {"foo":{"bar":"Custom layout"},"layout":"layout"}'
 		);
 		await expect(page.locator('p')).toHaveText(
-			'Page data: {"rootlayout":"rootlayout","foo":{"bar":"Custom layout"},"layout":"layout","page":"page","data":{"rootlayout":"rootlayout","layout":"layout"}}'
+			'Page data: {"foo":{"bar":"Custom layout"},"layout":"layout","page":"page","data":{"rootlayout":"rootlayout","layout":"layout"}}'
 		);
 
 		await clicknav('[href="/shadowed/parent/sub"]');
 		await expect(page.locator('h2')).toHaveText(
-			'Layout data: {"rootlayout":"rootlayout","foo":{"bar":"Custom layout"},"layout":"layout"}'
+			'Layout data: {"foo":{"bar":"Custom layout"},"layout":"layout"}'
 		);
 		await expect(page.locator('p')).toHaveText(
-			'Page data: {"rootlayout":"rootlayout","foo":{"bar":"Custom layout"},"layout":"layout","sub":"sub","data":{"rootlayout":"rootlayout","layout":"layout"}}'
+			'Page data: {"foo":{"bar":"Custom layout"},"layout":"layout","sub":"sub","data":{"rootlayout":"rootlayout","layout":"layout"}}'
 		);
 	});
 
diff --git a/packages/kit/test/apps/basics/test/test.js b/packages/kit/test/apps/basics/test/test.js
index 92a5e6a1ea95..7d17574ac091 100644
--- a/packages/kit/test/apps/basics/test/test.js
+++ b/packages/kit/test/apps/basics/test/test.js
@@ -356,7 +356,6 @@ test.describe('Load', () => {
 			expect(await page.textContent('h1')).toBe('message: original + new');
 			expect(await page.textContent('pre')).toBe(
 				JSON.stringify({
-					rootlayout: 'rootlayout',
 					foo: { bar: 'Custom layout' },
 					message: 'original + new',
 					x: 'a',
@@ -371,7 +370,6 @@ test.describe('Load', () => {
 				expect(await page.textContent('h1')).toBe('message: original + new');
 				expect(await page.textContent('pre')).toBe(
 					JSON.stringify({
-						rootlayout: 'rootlayout',
 						foo: { bar: 'Custom layout' },
 						message: 'original + new',
 						x: 'd',
@@ -778,23 +776,23 @@ test.describe('$app/stores', () => {
 		const foo = { bar: 'Custom layout' };
 
 		expect(await page.textContent('#store-data')).toBe(
-			JSON.stringify({ rootlayout: 'rootlayout', foo, name: 'SvelteKit', value: 456, page: 'www' })
+			JSON.stringify({ foo, name: 'SvelteKit', value: 456, page: 'www' })
 		);
 
 		await clicknav('a[href="/store/data/zzz"]');
 		expect(await page.textContent('#store-data')).toBe(
-			JSON.stringify({ rootlayout: 'rootlayout', foo, name: 'SvelteKit', value: 456, page: 'zzz' })
+			JSON.stringify({ foo, name: 'SvelteKit', value: 456, page: 'zzz' })
 		);
 
 		await clicknav('a[href="/store/data/xxx"]');
 		expect(await page.textContent('#store-data')).toBe(
-			JSON.stringify({ rootlayout: 'rootlayout', foo, name: 'SvelteKit', value: 123 })
+			JSON.stringify({ foo, name: 'SvelteKit', value: 123 })
 		);
 		expect(await page.textContent('#store-error')).toBe('Params = xxx');
 
 		await clicknav('a[href="/store/data/yyy"]');
 		expect(await page.textContent('#store-data')).toBe(
-			JSON.stringify({ rootlayout: 'rootlayout', foo, name: 'SvelteKit', value: 123 })
+			JSON.stringify({ foo, name: 'SvelteKit', value: 123 })
 		);
 		expect(await page.textContent('#store-error')).toBe('Params = yyy');
 	});
@@ -805,12 +803,7 @@ test.describe('$app/stores', () => {
 		javaScriptEnabled
 	}) => {
 		await page.goto('/store/data/foo?reset=true');
-		const stuff1 = {
-			foo: { bar: 'Custom layout' },
-			name: 'SvelteKit',
-			rootlayout: 'rootlayout',
-			value: 123
-		};
+		const stuff1 = { foo: { bar: 'Custom layout' }, name: 'SvelteKit', value: 123 };
 		const stuff2 = { ...stuff1, foo: true, number: 2 };
 		const stuff3 = { ...stuff2 };
 		await page.goto('/store/data/www');
@@ -905,23 +898,23 @@ test.describe('$app/state', () => {
 		await page.goto('/state/data/www');
 
 		expect(await page.textContent('#state-data')).toBe(
-			'{"rootlayout":"rootlayout","foo":{"bar":"Custom layout"},"name":"SvelteKit","value":456,"page":"www"}'
+			JSON.stringify({ foo, name: 'SvelteKit', value: 456, page: 'www' })
 		);
 
 		await clicknav('a[href="/state/data/zzz"]');
 		expect(await page.textContent('#state-data')).toBe(
-			'{"rootlayout":"rootlayout","foo":{"bar":"Custom layout"},"name":"SvelteKit","value":456,"page":"zzz"}'
+			JSON.stringify({ foo, name: 'SvelteKit', value: 456, page: 'zzz' })
 		);
 
 		await clicknav('a[href="/state/data/xxx"]');
 		expect(await page.textContent('#state-data')).toBe(
-			'{"rootlayout":"rootlayout","foo":{"bar":"Custom layout"},"name":"SvelteKit","value":123}'
+			JSON.stringify({ foo, name: 'SvelteKit', value: 123 })
 		);
 		expect(await page.textContent('#state-error')).toBe('Params = xxx');
 
 		await clicknav('a[href="/state/data/yyy"]');
 		expect(await page.textContent('#state-data')).toBe(
-			'{"rootlayout":"rootlayout","foo":{"bar":"Custom layout"},"name":"SvelteKit","value":123}'
+			JSON.stringify({ foo, name: 'SvelteKit', value: 123 })
 		);
 		expect(await page.textContent('#state-error')).toBe('Params = yyy');
 	});
@@ -932,12 +925,7 @@ test.describe('$app/state', () => {
 		javaScriptEnabled
 	}) => {
 		await page.goto('/state/data/foo?reset=true');
-		const stuff1 = {
-			foo: { bar: 'Custom layout' },
-			name: 'SvelteKit',
-			rootlayout: 'rootlayout',
-			value: 123
-		};
+		const stuff1 = { foo: { bar: 'Custom layout' }, name: 'SvelteKit', value: 123 };
 		const stuff2 = { ...stuff1, foo: true, number: 2 };
 		const stuff3 = { ...stuff2 };
 		await page.goto('/state/data/www');

From dd8e92e40d902c753ade31f6a0db6199408b0e44 Mon Sep 17 00:00:00 2001
From: Patrick <Patrick@ShowYou.us>
Date: Wed, 10 Sep 2025 04:21:26 +0200
Subject: [PATCH 07/13] Revert "test"

This reverts commit e1ba14a439a0a44564d4c07113ca2fb439beb845.
---
 packages/kit/test/apps/basics/src/routes/+layout.js       | 3 +--
 packages/kit/test/apps/basics/src/routes/+layout.svelte   | 1 -
 packages/kit/test/apps/basics/test/cross-platform/test.js | 1 -
 3 files changed, 1 insertion(+), 4 deletions(-)

diff --git a/packages/kit/test/apps/basics/src/routes/+layout.js b/packages/kit/test/apps/basics/src/routes/+layout.js
index 547834859adf..113e4a079bd0 100644
--- a/packages/kit/test/apps/basics/src/routes/+layout.js
+++ b/packages/kit/test/apps/basics/src/routes/+layout.js
@@ -1,8 +1,7 @@
 /** @type {import('./$types').LayoutLoad} */
-export async function load({ data }) {
+export async function load() {
 	// Do NOT make this load function depend on something which would cause it to rerun
 	return {
-		...data,
 		foo: {
 			bar: 'Custom layout'
 		}
diff --git a/packages/kit/test/apps/basics/src/routes/+layout.svelte b/packages/kit/test/apps/basics/src/routes/+layout.svelte
index e4d1e773b1e9..969229a5f678 100644
--- a/packages/kit/test/apps/basics/src/routes/+layout.svelte
+++ b/packages/kit/test/apps/basics/src/routes/+layout.svelte
@@ -9,7 +9,6 @@
 
 <slot />
 
-<div id="root-layout">{data.rootlayout}</div>
 <footer>{data.foo.bar}</footer>
 
 <style>
diff --git a/packages/kit/test/apps/basics/test/cross-platform/test.js b/packages/kit/test/apps/basics/test/cross-platform/test.js
index 8827f50e8cdc..7de5222bece0 100644
--- a/packages/kit/test/apps/basics/test/cross-platform/test.js
+++ b/packages/kit/test/apps/basics/test/cross-platform/test.js
@@ -324,7 +324,6 @@ test.describe('Errors', () => {
 	test('server-side error from load() is an Error', async ({ page }) => {
 		const response = await page.goto('/errors/load-error-server');
 
-		expect(await page.textContent('#root-layout')).toBe('rootlayout');
 		expect(await page.textContent('footer')).toBe('Custom layout');
 		expect(await page.textContent('#message')).toBe(
 			'This is your custom error page saying: "Not found"'

From 39eaabdcf90bd93ea95656d507354d2e033f3973 Mon Sep 17 00:00:00 2001
From: Patrick <Patrick@ShowYou.us>
Date: Wed, 10 Sep 2025 04:21:51 +0200
Subject: [PATCH 08/13] Revert "remove unused var"

This reverts commit cc65f34a143676ddd2212119ddd779f392e431c4.
---
 packages/kit/test/apps/basics/test/test.js | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/packages/kit/test/apps/basics/test/test.js b/packages/kit/test/apps/basics/test/test.js
index 7d17574ac091..36c6ed0ad9d8 100644
--- a/packages/kit/test/apps/basics/test/test.js
+++ b/packages/kit/test/apps/basics/test/test.js
@@ -897,6 +897,8 @@ test.describe('$app/state', () => {
 	test('page state contains data', async ({ page, clicknav }) => {
 		await page.goto('/state/data/www');
 
+		const foo = { bar: 'Custom layout' };
+
 		expect(await page.textContent('#state-data')).toBe(
 			JSON.stringify({ foo, name: 'SvelteKit', value: 456, page: 'www' })
 		);

From 7c25f525159c00cf5d7471cc2dc41e6d1ae2d8d3 Mon Sep 17 00:00:00 2001
From: Patrick <Patrick@ShowYou.us>
Date: Wed, 10 Sep 2025 04:23:32 +0200
Subject: [PATCH 09/13] better test without needing to change others

---
 .../errors/load-error-server/layout-data/+error.svelte    | 0
 .../load-error-server/layout-data/+layout.server.js       | 6 ++++++
 .../errors/load-error-server/layout-data/+layout.svelte   | 8 ++++++++
 .../routes/errors/load-error-server/layout-data/+page.js  | 6 ++++++
 .../errors/load-error-server/layout-data/+page.svelte     | 5 +++++
 5 files changed, 25 insertions(+)
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+error.svelte
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+layout.server.js
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+layout.svelte
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+page.js
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+page.svelte

diff --git a/packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+error.svelte b/packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+error.svelte
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+layout.server.js b/packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+layout.server.js
new file mode 100644
index 000000000000..f1d54f24d996
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+layout.server.js
@@ -0,0 +1,6 @@
+/** @type {import('./$types').LayoutServerLoad} */
+export function load() {
+	return {
+		answer: 42
+	};
+}
diff --git a/packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+layout.svelte b/packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+layout.svelte
new file mode 100644
index 000000000000..1030328152c1
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+layout.svelte
@@ -0,0 +1,8 @@
+<script lang="ts">
+	/** @type {import('./$types').LayoutData} */
+	export let data;
+</script>
+
+<slot />
+
+<div id="error-layout-data">{data.answer}</div>
diff --git a/packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+page.js b/packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+page.js
new file mode 100644
index 000000000000..0b245b4eb003
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+page.js
@@ -0,0 +1,6 @@
+import { error } from '@sveltejs/kit';
+
+/** @type {import('@sveltejs/kit').Load} */
+export async function load() {
+	error(404, 'Not found');
+}
diff --git a/packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+page.svelte b/packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+page.svelte
new file mode 100644
index 000000000000..a75b7f3ea78a
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+page.svelte
@@ -0,0 +1,5 @@
+
+	test('server-side error from load() still has layout data', async ({ page }) => {
+		await page.goto('/errors/load-error-server/layout-data');
+		expect(await page.textContent('#error-layout-data')).toBe('42');
+	});

From d0f6a59d1911ac20fdbe35cea98775137ca3ef35 Mon Sep 17 00:00:00 2001
From: Patrick <Patrick@ShowYou.us>
Date: Wed, 10 Sep 2025 04:26:42 +0200
Subject: [PATCH 10/13] move test to correct file

---
 .../routes/errors/load-error-server/layout-data/+page.svelte | 5 -----
 packages/kit/test/apps/basics/test/cross-platform/test.js    | 5 +++++
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+page.svelte b/packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+page.svelte
index a75b7f3ea78a..e69de29bb2d1 100644
--- a/packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+page.svelte
+++ b/packages/kit/test/apps/basics/src/routes/errors/load-error-server/layout-data/+page.svelte
@@ -1,5 +0,0 @@
-
-	test('server-side error from load() still has layout data', async ({ page }) => {
-		await page.goto('/errors/load-error-server/layout-data');
-		expect(await page.textContent('#error-layout-data')).toBe('42');
-	});
diff --git a/packages/kit/test/apps/basics/test/cross-platform/test.js b/packages/kit/test/apps/basics/test/cross-platform/test.js
index 7de5222bece0..d9bd0a929348 100644
--- a/packages/kit/test/apps/basics/test/cross-platform/test.js
+++ b/packages/kit/test/apps/basics/test/cross-platform/test.js
@@ -331,6 +331,11 @@ test.describe('Errors', () => {
 		expect(/** @type {Response} */ (response).status()).toBe(555);
 	});
 
+	test('server-side error from load() still has layout data', async ({ page }) => {
+		await page.goto('/errors/load-error-server/layout-data');
+		expect(await page.textContent('#error-layout-data')).toBe('42');
+	});
+
 	test('error in endpoint', async ({ page, read_errors }) => {
 		const res = await page.goto('/errors/endpoint');
 

From 9c1131b05a7a7b9afb0f41d0df183de2d2f9606a Mon Sep 17 00:00:00 2001
From: Patrick <Patrick@ShowYou.us>
Date: Wed, 10 Sep 2025 15:47:36 +0200
Subject: [PATCH 11/13] don't resolve promises for nodes > max_nodes

---
 .../runtime/server/page/data_serializer.js    | 93 ++++++++++---------
 1 file changed, 50 insertions(+), 43 deletions(-)

diff --git a/packages/kit/src/runtime/server/page/data_serializer.js b/packages/kit/src/runtime/server/page/data_serializer.js
index 30b5088a9542..c0e4a7cede4f 100644
--- a/packages/kit/src/runtime/server/page/data_serializer.js
+++ b/packages/kit/src/runtime/server/page/data_serializer.js
@@ -17,60 +17,67 @@ import {
  */
 export function server_data_serializer(event, event_state, options) {
 	let promise_id = 1;
+	let max_nodes = -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);
+	/** @param {number} index */
+	function get_replacer(index) {
+		/** @param {any} thing */
+		return 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 ${event.route.id}`)
+								);
+								data = undefined;
+								str = devalue.uneval([, error], replacer);
+							}
+
+							if (index >= max_nodes) {
+								return '';
+							}
+
+							return `${global}.resolve(${id}, ${str.includes('app.decode') ? `(app) => ${str}` : `() => ${str}`})`;
 						}
+					);
 
-						return `${global}.resolve(${id}, ${str.includes('app.decode') ? `(app) => ${str}` : `() => ${str}`})`;
-					}
-				);
+				iterator.add(promise);
 
-			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}', ${devalue.uneval(encoded, replacer)})`;
+				return `${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[]} */ ([]);
-	let max_nodes = -1;
 
 	return {
 		set_max_nodes(i) {
@@ -88,7 +95,7 @@ export function server_data_serializer(event, event_state, options) {
 				const payload = { type: 'data', data: node.data, uses: serialize_uses(node) };
 				if (node.slash) payload.slash = node.slash;
 
-				strings[i] = devalue.uneval(payload, replacer);
+				strings[i] = devalue.uneval(payload, get_replacer(i));
 			} catch (e) {
 				// @ts-expect-error
 				e.path = e.path.slice(1);

From fb40c6265cbd7fe43357ffdab1645779676cacce Mon Sep 17 00:00:00 2001
From: Patrick <Patrick@ShowYou.us>
Date: Wed, 10 Sep 2025 15:53:47 +0200
Subject: [PATCH 12/13] ups

---
 packages/kit/src/runtime/server/page/data_serializer.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/kit/src/runtime/server/page/data_serializer.js b/packages/kit/src/runtime/server/page/data_serializer.js
index c0e4a7cede4f..52eaa11fb8b4 100644
--- a/packages/kit/src/runtime/server/page/data_serializer.js
+++ b/packages/kit/src/runtime/server/page/data_serializer.js
@@ -55,7 +55,7 @@ export function server_data_serializer(event, event_state, options) {
 								str = devalue.uneval([, error], replacer);
 							}
 
-							if (index >= max_nodes) {
+							if (max_nodes > -1 && index >= max_nodes) {
 								return '';
 							}
 

From 7331cba4177672c10936e3ea1d944fcac5795575 Mon Sep 17 00:00:00 2001
From: Patrick <Patrick@ShowYou.us>
Date: Wed, 10 Sep 2025 16:12:33 +0200
Subject: [PATCH 13/13] move stuff around

---
 .../runtime/server/page/data_serializer.js    | 26 +++++++++++--------
 1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/packages/kit/src/runtime/server/page/data_serializer.js b/packages/kit/src/runtime/server/page/data_serializer.js
index 52eaa11fb8b4..8f630c5efe23 100644
--- a/packages/kit/src/runtime/server/page/data_serializer.js
+++ b/packages/kit/src/runtime/server/page/data_serializer.js
@@ -55,11 +55,10 @@ export function server_data_serializer(event, event_state, options) {
 								str = devalue.uneval([, error], replacer);
 							}
 
-							if (max_nodes > -1 && index >= max_nodes) {
-								return '';
-							}
-
-							return `${global}.resolve(${id}, ${str.includes('app.decode') ? `(app) => ${str}` : `() => ${str}`})`;
+							return {
+								index,
+								str: `${global}.resolve(${id}, ${str.includes('app.decode') ? `(app) => ${str}` : `() => ${str}`})`
+							};
 						}
 					);
 
@@ -107,13 +106,18 @@ export function server_data_serializer(event, event_state, options) {
 			const open = `<script${csp.script_needs_nonce ? ` nonce="${csp.nonce}"` : ''}>`;
 			const close = `</script>\n`;
 
-			if (max_nodes > -1) {
-				strings.length = max_nodes;
-			}
-
 			return {
-				data: `[${strings.join(',')}]`,
-				chunks: promise_id > 1 ? iterator.iterate((str) => open + str + close) : null
+				data: `[${(max_nodes > -1 ? strings.slice(0, max_nodes) : strings).join(',')}]`,
+				chunks:
+					promise_id > 1
+						? iterator.iterate(({ index, str }) => {
+								if (max_nodes > -1 && index >= max_nodes) {
+									return '';
+								}
+
+								return open + str + close;
+							})
+						: null
 			};
 		}
 	};