diff --git a/.changeset/nice-mugs-mate.md b/.changeset/nice-mugs-mate.md
new file mode 100644
index 000000000000..b6c43b0a4abc
--- /dev/null
+++ b/.changeset/nice-mugs-mate.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: support `exactOptionalPropertyTypes` for optional form schema fields
diff --git a/packages/kit/src/exports/public.d.ts b/packages/kit/src/exports/public.d.ts
index 31b39c82a722..7fb12f663edf 100644
--- a/packages/kit/src/exports/public.d.ts
+++ b/packages/kit/src/exports/public.d.ts
@@ -2090,7 +2090,7 @@ type RecursiveFormFields = RemoteFormFieldContainer<any> & {
 type MaybeArray<T> = T | T[];
 
 export interface RemoteFormInput {
-	[key: string]: MaybeArray<string | number | boolean | File | RemoteFormInput>;
+	[key: string]: MaybeArray<string | number | boolean | File | RemoteFormInput> | undefined;
 }
 
 export interface RemoteFormIssue {
diff --git a/packages/kit/src/runtime/client/client.js b/packages/kit/src/runtime/client/client.js
index a5afd3c03356..525e096f2259 100644
--- a/packages/kit/src/runtime/client/client.js
+++ b/packages/kit/src/runtime/client/client.js
@@ -514,7 +514,14 @@ function persist_state() {
 
 /**
  * @param {string | URL} url
- * @param {{ replaceState?: boolean; noScroll?: boolean; keepFocus?: boolean; invalidateAll?: boolean; invalidate?: Array<string | URL | ((url: URL) => boolean)>; state?: Record<string, any> }} options
+ * @param {{
+ *   replaceState?: boolean | undefined;
+ *   noScroll?: boolean | undefined;
+ *   keepFocus?: boolean | undefined;
+ *   invalidateAll?: boolean | undefined;
+ *   invalidate?: Array<string | URL | ((url: URL) => boolean)> | undefined;
+ *   state?: Record<string, any> | undefined;
+ * }} options
  * @param {number} redirect_count
  * @param {{}} [nav_token]
  */
@@ -697,7 +704,7 @@ async function initialize(result, target, hydrate) {
 					page.status = rendering_error.status;
 					return error;
 				}
-			: undefined
+			: /** @type {never} */ (undefined)
 	});
 
 	// Wait for a microtask in case svelte experimental async is enabled,
@@ -731,11 +738,11 @@ async function initialize(result, target, hydrate) {
  *   url: URL;
  *   params: Record<string, string>;
  *   branch: Array<import('./types.js').BranchNode | undefined>;
- *   errors?: Array<import('types').CSRPageNodeLoader | undefined>;
+ *   errors?: Array<import('types').CSRPageNodeLoader | undefined> | undefined;
  *   status: number;
  *   error: App.Error | null;
  *   route: import('types').CSRRoute | null;
- *   form?: Record<string, any> | null;
+ *   form?: Record<string, any> | null | undefined;
  * }} opts
  */
 async function get_navigation_result_from_branch({
@@ -962,7 +969,7 @@ async function load_node({ loader, parent, url, params, route, server_data_node
 						// implement streaming request bodies and/or the body getter
 						body:
 							resource.method === 'GET' || resource.method === 'HEAD'
-								? undefined
+								? /** @type {never} */ (undefined)
 								: await resource.blob(),
 						cache: resource.cache,
 						credentials: resource.credentials,
@@ -971,7 +978,10 @@ async function load_node({ loader, parent, url, params, route, server_data_node
 						// To keep the two values in sync, we explicitly set the headers to `undefined`.
 						// Also, not sure why, but sometimes 0 is evaluated as truthy so we need to
 						// explicitly compare the headers length to a number here
-						headers: [...resource.headers].length > 0 ? resource?.headers : undefined,
+						headers:
+							[...resource.headers].length > 0
+								? resource?.headers
+								: /** @type {never} */ (undefined),
 						integrity: resource.integrity,
 						keepalive: resource.keepalive,
 						method: resource.method,
@@ -1650,16 +1660,16 @@ function _before_navigate({ url, type, intent, delta, event, scroll }) {
  *     state: Record<string, any>;
  *     scroll: { x: number, y: number };
  *     delta: number;
- *   };
- *   keepfocus?: boolean;
- *   noscroll?: boolean;
- *   replace_state?: boolean;
- *   state?: Record<string, any>;
- *   redirect_count?: number;
- *   nav_token?: {};
- *   accept?: () => void;
- *   block?: () => void;
- *   event?: Event
+ *   } | undefined;
+ *   keepfocus?: boolean | undefined;
+ *   noscroll?: boolean | undefined;
+ *   replace_state?: boolean | undefined;
+ *   state?: Record<string, any> | undefined;
+ *   redirect_count?: number | undefined;
+ *   nav_token?: {} | undefined;
+ *   accept?: (() => void) | undefined;
+ *   block?: (() => void) | undefined;
+ *   event?: Event | undefined;
  * }} opts
  */
 async function navigate({
diff --git a/packages/kit/src/runtime/client/types.d.ts b/packages/kit/src/runtime/client/types.d.ts
index f91124f962c7..768258dd441a 100644
--- a/packages/kit/src/runtime/client/types.d.ts
+++ b/packages/kit/src/runtime/client/types.d.ts
@@ -100,14 +100,14 @@ export type BranchNode = {
 	server: DataNode | null;
 	universal: DataNode | null;
 	data: Record<string, any> | null;
-	slash?: TrailingSlash;
+	slash?: TrailingSlash | undefined;
 };
 
 export interface DataNode {
 	type: 'data';
 	data: Record<string, any> | null;
 	uses: Uses;
-	slash?: TrailingSlash;
+	slash?: TrailingSlash | undefined;
 }
 
 export interface NavigationState {
diff --git a/packages/kit/src/types/internal.d.ts b/packages/kit/src/types/internal.d.ts
index c64d0e46fde6..73962192c7bb 100644
--- a/packages/kit/src/types/internal.d.ts
+++ b/packages/kit/src/types/internal.d.ts
@@ -673,8 +673,8 @@ export interface RemoteFormInternals extends BaseRemoteInternals {
 export interface RemotePrerenderInternals extends BaseRemoteInternals {
 	type: 'prerender';
 	has_arg: boolean;
-	dynamic?: boolean;
-	inputs?: RemotePrerenderInputsGenerator;
+	dynamic?: boolean | undefined;
+	inputs?: RemotePrerenderInputsGenerator | undefined;
 }
 
 export type RemoteAnyQueryInternals =
diff --git a/packages/kit/test/types/remote.test.ts b/packages/kit/test/types/remote.test.ts
index 8c44aa4e3d74..780ec6717769 100644
--- a/packages/kit/test/types/remote.test.ts
+++ b/packages/kit/test/types/remote.test.ts
@@ -10,6 +10,9 @@ import {
 	invalid
 } from '@sveltejs/kit';
 
+/* eslint-disable @typescript-eslint/require-await */
+/* eslint-disable @typescript-eslint/no-floating-promises */
+
 const schema: StandardSchemaV1<string> = null as any;
 const schema2: StandardSchemaV1<string, number> = null as any;
 const schema3: StandardSchemaV1<string | undefined, number> = null as any;
@@ -58,6 +61,23 @@ function query_tests() {
 	}
 	void query_with_optional_arg();
 
+	async function query_with_optional_undefined_arg() {
+		const q = query(
+			null as any as StandardSchemaV1<{ a?: string | undefined }>,
+			() => 'Hello world'
+		);
+		// @ts-expect-error
+		void q();
+		void q({});
+		void q({ a: 'hi' });
+		void q({ a: undefined });
+		// @ts-expect-error
+		void q({ a: null });
+		// @ts-expect-error
+		void q(1);
+	}
+	void query_with_optional_undefined_arg();
+
 	async function query_unsafe() {
 		const q = query('unchecked', (a: number) => a);
 		const result: number = await q(1);
@@ -515,6 +535,8 @@ function form_tests() {
 	f6.fields.array[0].array.value();
 	// @ts-expect-error
 	f6.fields.array[0].array.as('text');
+	// @ts-expect-error
+	f6.input!['array[0].prop'] = 123;
 
 	// any
 	const f7 = form(null as any, (data, issue) => {
@@ -540,8 +562,6 @@ function form_tests() {
 	f8.fields.allIssues();
 	// @ts-expect-error
 	f8.fields.x;
-	// @ts-expect-error
-	f6.input!['array[0].prop'] = 123;
 
 	// schema with optional array fields (e.g. Zod's `.default([])` produces an input
 	// type where the property is optional, so its value type is `string[] | undefined`).
@@ -578,6 +598,26 @@ function form_tests() {
 	// @ts-expect-error
 	f_optional_arrays.fields.files.as('text');
 
+	// schema with optional & value-undefined fields. (e.g. Valibot's `.optional()`
+	// produces an input type that accepts `undefined` as value, which under
+	// `exactOptionalPropertyTypes` is treated distinctly from an omitted property.)
+	const f_optional_undefined_prop = form(
+		null as any as StandardSchemaV1<{
+			strings?: string[] | undefined;
+		}>,
+		(data) => {
+			data.strings?.[0] === 'a';
+			return { success: true };
+		}
+	);
+	// `.as()` should be available on optional|undefined fields
+	f_optional_undefined_prop.fields.strings.as('checkbox', 'value');
+	f_optional_undefined_prop.fields.strings.as('select multiple');
+	// indexed access gives back a typed field
+	f_optional_undefined_prop.fields.strings[0].as('text');
+	// @ts-expect-error
+	f_optional_undefined_prop.fields.strings.as('number');
+
 	// doesn't use data
 	const f9 = form(() => Promise.resolve({ success: true }));
 	f9.result?.success === true;
diff --git a/packages/kit/test/types/tsconfig.json b/packages/kit/test/types/tsconfig.json
index 7675e28d941c..2e8a2b2182fb 100644
--- a/packages/kit/test/types/tsconfig.json
+++ b/packages/kit/test/types/tsconfig.json
@@ -1,7 +1,9 @@
 {
 	"extends": "../../tsconfig.json",
 	"compilerOptions": {
-		"noEmit": true
+		"exactOptionalPropertyTypes": true,
+		"noEmit": true,
+		"skipLibCheck": true
 	},
 	"include": ["**/*.test.ts", "../../src/types/*.d.ts"]
 }
diff --git a/packages/kit/types/index.d.ts b/packages/kit/types/index.d.ts
index 1948fe5f4615..8443b401cb69 100644
--- a/packages/kit/types/index.d.ts
+++ b/packages/kit/types/index.d.ts
@@ -2064,7 +2064,7 @@ declare module '@sveltejs/kit' {
 	type MaybeArray<T> = T | T[];
 
 	export interface RemoteFormInput {
-		[key: string]: MaybeArray<string | number | boolean | File | RemoteFormInput>;
+		[key: string]: MaybeArray<string | number | boolean | File | RemoteFormInput> | undefined;
 	}
 
 	export interface RemoteFormIssue {