diff --git a/backend/open_webui/routers/configs.py b/backend/open_webui/routers/configs.py
index fd2ad14602..5512dba5ed 100644
--- a/backend/open_webui/routers/configs.py
+++ b/backend/open_webui/routers/configs.py
@@ -713,16 +713,21 @@ async def get_interface_defaults(user=Depends(get_verified_user)):
     """
     Get global interface defaults for all users.
     Returns empty dict if no defaults are configured.
-    Cached for 10 minutes to reduce redundant fetches.
+
+    Served with a no-store cache policy: this endpoint is fetched on
+    authenticated app load and merged into runtime settings, so caching
+    here would leave users seeing stale defaults for up to the cache TTL
+    after an admin 'save defaults' or 'reset all users' action. The call
+    is lightweight (in-memory config read), so the extra hit is cheap.
     """
     from fastapi.responses import JSONResponse
-    
+
     config = await asyncio.to_thread(get_config)
     defaults = config.get("ui", {}).get("interface_defaults", {})
-    
+
     return JSONResponse(
         content=defaults,
-        headers={"Cache-Control": "private, max-age=600"}
+        headers={"Cache-Control": "no-store"},
     )
 
 
diff --git a/src/lib/components/chat/Settings/Interface.svelte b/src/lib/components/chat/Settings/Interface.svelte
index 6c144db2d6..c1238d28aa 100644
--- a/src/lib/components/chat/Settings/Interface.svelte
+++ b/src/lib/components/chat/Settings/Interface.svelte
@@ -24,6 +24,13 @@
 	// In admin mode, we don't apply CSS side effects like text scale
 	$: isAdminMode = initialSettings !== null;
 
+	// Reading $settings directly mixes the current admin's personal settings
+	// into composition (e.g. ...$settings.title) and render branches (e.g.
+	// {#if !$settings.chatBubble}) when this component is reused by the
+	// defaults modal. activeSettings resolves to initialSettings in admin
+	// mode and $settings in the normal user mode, so neither path leaks.
+	$: activeSettings = isAdminMode ? (initialSettings ?? {}) : $settings;
+
 	let backgroundImageUrl = null;
 	let inputFiles = null;
 	let filesInputElement;
@@ -135,7 +142,7 @@
 	const toggleTitleAutoGenerate = async () => {
 		saveSettings({
 			title: {
-				...$settings.title,
+				...(activeSettings?.title ?? {}),
 				auto: titleAutoGenerate
 			}
 		});
@@ -716,7 +723,7 @@
 				</div>
 			</div>
 
-			{#if !$settings.chatBubble}
+			{#if !chatBubble}
 				<div>
 					<div class=" py-0.5 flex w-full justify-between">
 						<div id="chat-bubble-username-label" class=" self-center text-xs">
diff --git a/src/lib/utils/index.ts b/src/lib/utils/index.ts
index 4d26d1ff4d..52d4190ee1 100644
--- a/src/lib/utils/index.ts
+++ b/src/lib/utils/index.ts
@@ -1883,24 +1883,31 @@ export const deepMerge = (target: any, source: any): any => {
     const result = { ...target };
 
     for (const key in source) {
-        if (Object.prototype.hasOwnProperty.call(source, key)) {
-            const sourceValue = source[key];
-            const targetValue = result[key];
+        if (!Object.prototype.hasOwnProperty.call(source, key)) continue;
 
-            // Recursively merge if both are non-null objects
-            if (
-                sourceValue !== null &&
-                targetValue !== null &&
-                typeof sourceValue === 'object' &&
-                typeof targetValue === 'object' &&
-                !Array.isArray(sourceValue) &&
-                !Array.isArray(targetValue)
-            ) {
-                result[key] = deepMerge(targetValue, sourceValue);
-            } else {
-                // Source value takes precedence (including explicit null)
-                result[key] = sourceValue;
-            }
+        // Sources for this merge include API payloads and localStorage, so
+        // never copy keys that would let a crafted value mutate the
+        // prototype chain or reassign the object constructor.
+        if (key === '__proto__' || key === 'constructor' || key === 'prototype') {
+            continue;
+        }
+
+        const sourceValue = source[key];
+        const targetValue = result[key];
+
+        // Recursively merge if both are non-null objects
+        if (
+            sourceValue !== null &&
+            targetValue !== null &&
+            typeof sourceValue === 'object' &&
+            typeof targetValue === 'object' &&
+            !Array.isArray(sourceValue) &&
+            !Array.isArray(targetValue)
+        ) {
+            result[key] = deepMerge(targetValue, sourceValue);
+        } else {
+            // Source value takes precedence (including explicit null)
+            result[key] = sourceValue;
         }
     }