diff --git a/apps/studio/state/ai-assistant-state.test.ts b/apps/studio/state/ai-assistant-state.test.ts
new file mode 100644
index 00000000000..4fb412ffe83
--- /dev/null
+++ b/apps/studio/state/ai-assistant-state.test.ts
@@ -0,0 +1,46 @@
+import { proxy, ref } from 'valtio/vanilla'
+import { describe, expect, it } from 'vitest'
+
+import { sanitizeForCloning } from './ai-assistant-state'
+
+describe('AI assistant chat message sync', () => {
+  // FE-3954: syncing the live array into valtio corrupted it with Proxies, breaking structuredClone in addToolApprovalResponse
+  it('does not corrupt the live SDK message array when synced into valtio state', () => {
+    const state = proxy<{ chats: Record<string, any>; chatInstances: Record<string, any> }>({
+      chats: {},
+      chatInstances: {},
+    })
+    state.chats['chat-1'] = { id: 'chat-1', messages: [] }
+
+    const liveSdkMessages = [
+      {
+        id: 'message-1',
+        role: 'assistant',
+        parts: [
+          { type: 'text', text: "Sure, here's a query" },
+          {
+            type: 'tool-execute_sql',
+            toolCallId: 'tool-1',
+            state: 'approval-requested',
+            approval: { id: 'approval-1' },
+          },
+        ],
+      },
+    ]
+    state.chatInstances['chat-1'] = ref({ messages: liveSdkMessages })
+
+    const chat = state.chats['chat-1']
+    chat.messages = liveSdkMessages.map((message) => sanitizeForCloning(message))
+
+    // mirrors addToolApprovalResponse's own update logic
+    const lastMessage = liveSdkMessages[liveSdkMessages.length - 1]
+    const updatedParts = lastMessage.parts.map((part: any) =>
+      part.state === 'approval-requested' && part.approval?.id === 'approval-1'
+        ? { ...part, state: 'approval-responded', approval: { id: 'approval-1', approved: true } }
+        : part
+    )
+    const replacedMessage = { ...lastMessage, parts: updatedParts }
+
+    expect(() => structuredClone(replacedMessage)).not.toThrow()
+  })
+})
diff --git a/apps/studio/state/ai-assistant-state.tsx b/apps/studio/state/ai-assistant-state.tsx
index c083fa10e52..9f9599b32f9 100644
--- a/apps/studio/state/ai-assistant-state.tsx
+++ b/apps/studio/state/ai-assistant-state.tsx
@@ -152,7 +152,7 @@ async function clearStorage(): Promise<void> {
 
 // Helper function to sanitize objects to ensure they're cloneable
 // Issue due to addToolResult
-function sanitizeForCloning(obj: any): any {
+export function sanitizeForCloning(obj: any): any {
   if (obj === null || obj === undefined) return obj
   if (typeof obj !== 'object') return obj
   return JSON.parse(JSON.stringify(obj))
@@ -329,7 +329,8 @@ function createChatInstance(
         const messages = chatInstance.messages
         const chat = state.chats[options.id]
         if (chat) {
-          chat.messages = messages
+          // Clone first — valtio's proxy() mutates nested properties in place and would corrupt the SDK's live array
+          chat.messages = messages.map((message) => sanitizeForCloning(message))
           chat.updatedAt = new Date()
         }