diff --git a/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormActions/ChatFormActionAdd/ChatFormActionsAdd.svelte b/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormActions/ChatFormActionAdd/ChatFormActionsAdd.svelte
index 54ddcf9b0..6a91bf905 100644
--- a/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormActions/ChatFormActionAdd/ChatFormActionsAdd.svelte
+++ b/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormActions/ChatFormActionAdd/ChatFormActionsAdd.svelte
@@ -1,5 +1,5 @@
 <script lang="ts">
-	import { IsMobile } from '$lib/hooks/is-mobile.svelte';
+	import { isMobile } from '$lib/stores/viewport.svelte';
 	import ChatFormActionAddDropdown from './ChatFormActionAddDropdown.svelte';
 	import ChatFormActionAddSheet from './ChatFormActionAddSheet.svelte';
 	import ChatFormActionAddButton from './ChatFormActionAddButton.svelte';
@@ -31,8 +31,6 @@
 		onMcpSettingsClick,
 		onSystemPromptClick
 	}: Props = $props();
-
-	const isMobile = new IsMobile();
 </script>
 
 {#if isMobile.current}
diff --git a/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormActions/ChatFormActionModels.svelte b/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormActions/ChatFormActionModels.svelte
index 297020605..07f079f5b 100644
--- a/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormActions/ChatFormActionModels.svelte
+++ b/tools/ui/src/lib/components/app/chat/ChatForm/ChatFormActions/ChatFormActionModels.svelte
@@ -3,7 +3,7 @@
 	import { modelsStore, modelOptions, selectedModelId } from '$lib/stores/models.svelte';
 	import { isRouterMode, serverError } from '$lib/stores/server.svelte';
 	import { ModelsSelectorDropdown, ModelsSelectorSheet } from '$lib/components/app';
-	import { IsMobile } from '$lib/hooks/is-mobile.svelte';
+	import { isMobile } from '$lib/stores/viewport.svelte';
 	import { activeMessages } from '$lib/stores/conversations.svelte';
 
 	interface Props {
@@ -152,8 +152,6 @@
 	let selectorModelRef: ModelsSelectorDropdown | ModelsSelectorSheet | undefined =
 		$state(undefined);
 
-	let isMobile = new IsMobile();
-
 	export function open() {
 		selectorModelRef?.open();
 	}
diff --git a/tools/ui/src/lib/components/ui/sidebar/context.svelte.ts b/tools/ui/src/lib/components/ui/sidebar/context.svelte.ts
index 9d49ee1f0..2fa5cc25d 100644
--- a/tools/ui/src/lib/components/ui/sidebar/context.svelte.ts
+++ b/tools/ui/src/lib/components/ui/sidebar/context.svelte.ts
@@ -1,4 +1,4 @@
-import { IsMobile } from '$lib/hooks/is-mobile.svelte.js';
+import { isMobile } from '$lib/stores/viewport.svelte.js';
 import { getContext, setContext } from 'svelte';
 import { SIDEBAR_KEYBOARD_SHORTCUT, SIDEBAR_MIN_WIDTH } from './constants.js';
 
@@ -27,19 +27,17 @@ class SidebarState {
 	sidebarWidth = $state(SIDEBAR_MIN_WIDTH);
 	isResizing = $state(false);
 	setOpen: SidebarStateProps['setOpen'];
-	#isMobile: IsMobile;
 	state = $derived.by(() => (this.open ? 'expanded' : 'collapsed'));
 
 	constructor(props: SidebarStateProps) {
 		this.setOpen = props.setOpen;
-		this.#isMobile = new IsMobile();
 		this.props = props;
 	}
 
 	// Convenience getter for checking if the sidebar is mobile
 	// without this, we would need to use `sidebar.isMobile.current` everywhere
 	get isMobile() {
-		return this.#isMobile.current;
+		return isMobile.current;
 	}
 
 	// Event handler to apply to the `<svelte:window>`
diff --git a/tools/ui/src/lib/hooks/is-mobile.svelte.ts b/tools/ui/src/lib/hooks/is-mobile.svelte.ts
deleted file mode 100644
index 6454fc5b5..000000000
--- a/tools/ui/src/lib/hooks/is-mobile.svelte.ts
+++ /dev/null
@@ -1,8 +0,0 @@
-import { DEFAULT_MOBILE_BREAKPOINT } from '$lib/constants';
-import { MediaQuery } from 'svelte/reactivity';
-
-export class IsMobile extends MediaQuery {
-	constructor(breakpoint: number = DEFAULT_MOBILE_BREAKPOINT) {
-		super(`max-width: ${breakpoint - 1}px`);
-	}
-}
diff --git a/tools/ui/src/lib/stores/settings.svelte.ts b/tools/ui/src/lib/stores/settings.svelte.ts
index 58eea1ee6..8d3c711b8 100644
--- a/tools/ui/src/lib/stores/settings.svelte.ts
+++ b/tools/ui/src/lib/stores/settings.svelte.ts
@@ -41,8 +41,7 @@ import {
 	SETTINGS_KEYS,
 	USER_OVERRIDES_LOCALSTORAGE_KEY
 } from '$lib/constants';
-
-import { IsMobile } from '$lib/hooks/is-mobile.svelte';
+import { isMobile } from '$lib/stores/viewport.svelte';
 import { ParameterSyncService } from '$lib/services/parameter-sync.service';
 import { serverStore } from '$lib/stores/server.svelte';
 import {
@@ -132,7 +131,7 @@ class SettingsStore {
 
 			// Default sendOnEnter to false on mobile when the user has no saved preference
 			if (!(SETTINGS_KEYS.SEND_ON_ENTER in savedVal)) {
-				if (new IsMobile().current) {
+				if (isMobile.current) {
 					this.config[SETTINGS_KEYS.SEND_ON_ENTER] = false;
 				}
 			}
diff --git a/tools/ui/src/lib/stores/viewport.svelte.ts b/tools/ui/src/lib/stores/viewport.svelte.ts
new file mode 100644
index 000000000..dac241a01
--- /dev/null
+++ b/tools/ui/src/lib/stores/viewport.svelte.ts
@@ -0,0 +1,9 @@
+import { browser } from '$app/environment';
+import { DEFAULT_MOBILE_BREAKPOINT } from '$lib/constants/viewport';
+import { MediaQuery } from 'svelte/reactivity';
+
+export const viewport = $state({
+	width: browser ? window.innerWidth : 0
+});
+
+export const isMobile = new MediaQuery(`max-width: ${DEFAULT_MOBILE_BREAKPOINT - 1}px`);
diff --git a/tools/ui/src/routes/+layout.svelte b/tools/ui/src/routes/+layout.svelte
index 0610b07ae..2f1f52497 100644
--- a/tools/ui/src/routes/+layout.svelte
+++ b/tools/ui/src/routes/+layout.svelte
@@ -26,18 +26,18 @@
 	import { modelsStore } from '$lib/stores/models.svelte';
 	import { mcpStore } from '$lib/stores/mcp.svelte';
 	import { TOOLTIP_DELAY_DURATION } from '$lib/constants';
-	import { IsMobile } from '$lib/hooks/is-mobile.svelte';
 	import { useKeyboardShortcuts } from '$lib/hooks/use-keyboard-shortcuts.svelte';
 	import { useSettingsNavigation } from '$lib/hooks/use-settings-navigation.svelte';
 	import { conversations } from '$lib/stores/conversations.svelte';
+	import { isMobile } from '$lib/stores/viewport.svelte';
 
 	let { children } = $props();
 	let alwaysShowSidebarOnDesktop = $derived(config().alwaysShowSidebarOnDesktop);
-	let isMobile = new IsMobile();
 	let isDesktop = $derived(!isMobile.current);
 	let sidebarOpen = $state(false);
 	let mounted = $state(false);
 	let innerHeight = $state<number | undefined>();
+	let innerWidth = $state(browser ? window.innerWidth : 0);
 
 	let chatSidebar:
 		| {
@@ -278,4 +278,4 @@
 	</Sidebar.Provider>
 </Tooltip.Provider>
 
-<svelte:window onkeydown={handleKeydown} bind:innerHeight />
+<svelte:window onkeydown={handleKeydown} bind:innerHeight bind:innerWidth />