diff --git a/apps/web/src/lib/components/layout/Layout.svelte b/apps/web/src/lib/components/layout/Layout.svelte
index 88c6226e0..ff192f35a 100644
--- a/apps/web/src/lib/components/layout/Layout.svelte
+++ b/apps/web/src/lib/components/layout/Layout.svelte
@@ -7,6 +7,7 @@ import { loggedInUser } from "#lib/features/auth/user-state.ts";
 import { getPatrons } from "#lib/features/front-page/front-page.remote.ts";
 import { getNotifications } from "#lib/features/notifications/notifications.remote.ts";
 import { toNotificationRows } from "#lib/features/notifications/notifications-utils.ts";
+import { setSidenavCollapsed } from "#lib/features/sidenav/sidenav.remote.ts";
 import { m } from "#lib/paraglide/messages.js";
 import { getLocale } from "#lib/paraglide/runtime.js";
 import { GIT_COMMIT } from "#lib/utils/git-commit.ts";
@@ -100,13 +101,14 @@ function unseenRequestsLabel(count: number) {
 		: m.friends_unseenRequests_other({ count });
 }
 
-function toggleSideNavCollapsed() {
+const sideNavCollapseForm = setSidenavCollapsed.enhance(async ({ submit }) => {
 	sideNavCollapsed = !sideNavCollapsed;
-	void fetch("/sidenav", {
-		method: "POST",
-		body: new URLSearchParams({ collapsed: String(sideNavCollapsed) }),
-	});
-}
+	try {
+		await submit().updates();
+	} catch {
+		// losing the preference cookie is fine, the optimistic state stays
+	}
+});
 
 // mobile hide-on-scroll header, ported from the React useNavOffset
 const MOBILE_BREAKPOINT = 600;
@@ -389,13 +391,17 @@ function formatRelativeDate(timestamp: number) {
 			class="sideNavCollapseButtonContainer"
 			data-testid="sidenav-collapse-button"
 		>
-			<button
-				type="button"
-				class="collapseButton sideNavCollapseButton"
-				onclick={toggleSideNavCollapsed}
-			>
-				<PanelLeft />
-			</button>
+			<form class="sideNavCollapseForm" {...sideNavCollapseForm}>
+				<input
+					{...setSidenavCollapsed.fields.collapsed.as(
+						"hidden",
+						!sideNavCollapsed,
+					)}
+				/>
+				<button class="collapseButton sideNavCollapseButton">
+					<PanelLeft />
+				</button>
+			</form>
 			{#if sideNavCollapsed && showUnseenDot}
 				<NotificationDot />
 			{/if}
@@ -663,6 +669,10 @@ function formatRelativeDate(timestamp: number) {
 		}
 	}
 
+	.sideNavCollapseForm {
+		display: contents;
+	}
+
 	.collapseButton {
 		display: none;
 		align-items: center;
diff --git a/apps/web/src/lib/features/sidenav/sidenav.remote.ts b/apps/web/src/lib/features/sidenav/sidenav.remote.ts
new file mode 100644
index 000000000..e54f94756
--- /dev/null
+++ b/apps/web/src/lib/features/sidenav/sidenav.remote.ts
@@ -0,0 +1,11 @@
+import * as v from "valibot";
+import { form, getRequestEvent } from "$app/server";
+import { writeSidenavCollapsed } from "./sidenav.server.ts";
+
+/** Persists whether the desktop sidenav is collapsed. */
+export const setSidenavCollapsed = form(
+	v.object({ collapsed: v.optional(v.boolean(), false) }),
+	async ({ collapsed }) => {
+		writeSidenavCollapsed(getRequestEvent().cookies, collapsed);
+	},
+);
diff --git a/apps/web/src/routes/sidenav/+server.ts b/apps/web/src/lib/features/sidenav/sidenav.server.ts
similarity index 59%
rename from apps/web/src/routes/sidenav/+server.ts
rename to apps/web/src/lib/features/sidenav/sidenav.server.ts
index c2c08907e..ca1340fbb 100644
--- a/apps/web/src/routes/sidenav/+server.ts
+++ b/apps/web/src/lib/features/sidenav/sidenav.server.ts
@@ -1,19 +1,21 @@
-import { json } from "@sveltejs/kit";
+import type { Cookies } from "@sveltejs/kit";
 import {
 	readSessionCookie,
 	writeSessionCookie,
 } from "#lib/features/auth/session.server.ts";
 import { ServerConfig } from "#lib/server/config.ts";
 import { IS_E2E_TEST_RUN } from "#lib/utils/e2e.ts";
-import type { RequestHandler } from "./$types";
 
 const SIDENAV_COOKIE_NAME = "sidenav";
 const TEN_YEARS_IN_SECONDS = 315_360_000;
 
-export const POST: RequestHandler = async ({ request, cookies }) => {
-	const form = new URLSearchParams(await request.text());
-	const collapsed = form.get("collapsed") === "true";
+/** Reads the desktop sidenav collapsed preference from its cookie. */
+export function readSidenavCollapsed(cookies: Cookies): boolean {
+	return readSessionCookie(cookies.get(SIDENAV_COOKIE_NAME))?.collapsed === true;
+}
 
+/** Persists the desktop sidenav collapsed preference into its cookie. */
+export function writeSidenavCollapsed(cookies: Cookies, collapsed: boolean) {
 	const session = readSessionCookie(cookies.get(SIDENAV_COOKIE_NAME)) ?? {};
 	session.collapsed = collapsed;
 	cookies.set(SIDENAV_COOKIE_NAME, writeSessionCookie(session), {
@@ -23,6 +25,4 @@ export const POST: RequestHandler = async ({ request, cookies }) => {
 		httpOnly: true,
 		maxAge: TEN_YEARS_IN_SECONDS,
 	});
-
-	return json({ success: true });
-};
+}
diff --git a/apps/web/src/routes/+layout.server.ts b/apps/web/src/routes/+layout.server.ts
index 2fcb82e81..1d7a9fcfe 100644
--- a/apps/web/src/routes/+layout.server.ts
+++ b/apps/web/src/routes/+layout.server.ts
@@ -1,15 +1,11 @@
-import { readSessionCookie } from "#lib/features/auth/session.server.ts";
 import { resolveSidebarData } from "#lib/features/sidebar/sidebar.server.ts";
+import { readSidenavCollapsed } from "#lib/features/sidenav/sidenav.server.ts";
 import { getTheme } from "#lib/features/theme/theme.server.ts";
 import type { LayoutServerLoad } from "./$types";
 
-const SIDENAV_COOKIE_NAME = "sidenav";
-
 export const load: LayoutServerLoad = async ({ locals, cookies }) => {
 	const user = locals.user;
 
-	const sidenavSession = readSessionCookie(cookies.get(SIDENAV_COOKIE_NAME));
-
 	return {
 		user: user
 			? {
@@ -28,7 +24,7 @@ export const load: LayoutServerLoad = async ({ locals, cookies }) => {
 					createdAt: user.createdAt,
 				}
 			: undefined,
-		sidenavCollapsed: sidenavSession?.collapsed === true,
+		sidenavCollapsed: readSidenavCollapsed(cookies),
 		sidebar: await resolveSidebarData(user?.id ?? null),
 		theme: getTheme(cookies),
 	};