diff --git a/.changeset/immutable-tool-call-updates.md b/.changeset/immutable-tool-call-updates.md
new file mode 100644
index 000000000..42b2702bb
--- /dev/null
+++ b/.changeset/immutable-tool-call-updates.md
@@ -0,0 +1,18 @@
+---
+'@tanstack/ai': patch
+---
+
+fix(ai): produce new object references in tool-call message updaters
+
+`updateToolCallApproval`, `updateToolCallState`, `updateToolCallWithOutput`,
+and `updateToolCallApprovalResponse` previously mutated the found tool-call
+part in-place (`toolCallPart.state = ...`) after spreading the parts array.
+The shallow `[...msg.parts]` copy created a new array but preserved the
+original object references, so frameworks that rely on reference identity
+for change detection (Svelte 5 proxies, Vue 3 reactivity, etc.) could not
+observe the updates.
+
+Each function now replaces the part at its index with a spread copy
+(`parts[index] = { ...toolCallPart, ...changes }`), producing a fresh
+object on every update. This aligns with the pattern already used by
+`updateToolCallPart`, `updateTextPart`, and `updateThinkingPart`.
diff --git a/packages/typescript/ai/src/activities/chat/stream/message-updaters.ts b/packages/typescript/ai/src/activities/chat/stream/message-updaters.ts
index 80b94d59a..57ffbe2e6 100644
--- a/packages/typescript/ai/src/activities/chat/stream/message-updaters.ts
+++ b/packages/typescript/ai/src/activities/chat/stream/message-updaters.ts
@@ -150,10 +150,14 @@ export function updateToolCallApproval(
     )
 
     if (toolCallPart) {
-      toolCallPart.state = 'approval-requested'
-      toolCallPart.approval = {
-        id: approvalId,
-        needsApproval: true,
+      const index = parts.indexOf(toolCallPart)
+      parts[index] = {
+        ...toolCallPart,
+        state: 'approval-requested',
+        approval: {
+          id: approvalId,
+          needsApproval: true,
+        },
       }
     }
 
@@ -181,7 +185,8 @@ export function updateToolCallState(
     )
 
     if (toolCallPart) {
-      toolCallPart.state = state
+      const index = parts.indexOf(toolCallPart)
+      parts[index] = { ...toolCallPart, state }
     }
 
     return { ...msg, parts }
@@ -206,11 +211,11 @@ export function updateToolCallWithOutput(
     )
 
     if (toolCallPart) {
-      toolCallPart.output = errorText ? { error: errorText } : output
-      if (state) {
-        toolCallPart.state = state
-      } else {
-        toolCallPart.state = 'input-complete'
+      const index = parts.indexOf(toolCallPart)
+      parts[index] = {
+        ...toolCallPart,
+        output: errorText ? { error: errorText } : output,
+        state: state ?? 'input-complete',
       }
     }
 
@@ -235,8 +240,12 @@ export function updateToolCallApprovalResponse(
     )
 
     if (toolCallPart && toolCallPart.approval) {
-      toolCallPart.approval.approved = approved
-      toolCallPart.state = 'approval-responded'
+      const index = parts.indexOf(toolCallPart)
+      parts[index] = {
+        ...toolCallPart,
+        approval: { ...toolCallPart.approval, approved },
+        state: 'approval-responded',
+      }
     }
 
     return { ...msg, parts }
diff --git a/packages/typescript/ai/tests/message-updaters.test.ts b/packages/typescript/ai/tests/message-updaters.test.ts
index 5de1a031e..23344b0c4 100644
--- a/packages/typescript/ai/tests/message-updaters.test.ts
+++ b/packages/typescript/ai/tests/message-updaters.test.ts
@@ -864,5 +864,101 @@ describe('message-updaters', () => {
       expect(result[0]?.parts).not.toBe(originalParts)
       expect(messages[0]?.parts).toBe(originalParts)
     })
+
+    it('updateToolCallApproval should not mutate the original tool-call part', () => {
+      const originalPart: ToolCallPart = {
+        type: 'tool-call',
+        id: 'call-1',
+        name: 'deleteFile',
+        arguments: '{"path":"/tmp/file"}',
+        state: 'input-complete',
+      }
+      const messages = [
+        createMessage('msg-1', 'assistant', [originalPart]),
+      ]
+
+      const result = updateToolCallApproval(messages, 'msg-1', 'call-1', 'approval-1')
+
+      // Original part must be unchanged
+      expect(originalPart.state).toBe('input-complete')
+      expect(originalPart.approval).toBeUndefined()
+
+      // Result must have new values
+      const resultPart = result[0]?.parts[0] as ToolCallPart
+      expect(resultPart).not.toBe(originalPart)
+      expect(resultPart.state).toBe('approval-requested')
+      expect(resultPart.approval).toEqual({ id: 'approval-1', needsApproval: true })
+    })
+
+    it('updateToolCallState should not mutate the original tool-call part', () => {
+      const originalPart: ToolCallPart = {
+        type: 'tool-call',
+        id: 'call-1',
+        name: 'getWeather',
+        arguments: '{}',
+        state: 'input-streaming',
+      }
+      const messages = [
+        createMessage('msg-1', 'assistant', [originalPart]),
+      ]
+
+      const result = updateToolCallState(messages, 'msg-1', 'call-1', 'input-complete')
+
+      expect(originalPart.state).toBe('input-streaming')
+
+      const resultPart = result[0]?.parts[0] as ToolCallPart
+      expect(resultPart).not.toBe(originalPart)
+      expect(resultPart.state).toBe('input-complete')
+    })
+
+    it('updateToolCallWithOutput should not mutate the original tool-call part', () => {
+      const originalPart: ToolCallPart = {
+        type: 'tool-call',
+        id: 'call-1',
+        name: 'getWeather',
+        arguments: '{}',
+        state: 'input-complete',
+      }
+      const messages = [
+        createMessage('msg-1', 'assistant', [originalPart]),
+      ]
+      const output = { temperature: 20 }
+
+      const result = updateToolCallWithOutput(messages, 'call-1', output)
+
+      expect(originalPart.output).toBeUndefined()
+      expect(originalPart.state).toBe('input-complete')
+
+      const resultPart = result[0]?.parts[0] as ToolCallPart
+      expect(resultPart).not.toBe(originalPart)
+      expect(resultPart.output).toEqual(output)
+    })
+
+    it('updateToolCallApprovalResponse should not mutate the original tool-call part', () => {
+      const originalApproval = { id: 'approval-1', needsApproval: true as const }
+      const originalPart: ToolCallPart = {
+        type: 'tool-call',
+        id: 'call-1',
+        name: 'deleteFile',
+        arguments: '{}',
+        state: 'approval-requested',
+        approval: originalApproval,
+      }
+      const messages = [
+        createMessage('msg-1', 'assistant', [originalPart]),
+      ]
+
+      const result = updateToolCallApprovalResponse(messages, 'approval-1', true)
+
+      // Original part and approval must be unchanged
+      expect(originalPart.state).toBe('approval-requested')
+      expect(originalApproval.approved).toBeUndefined()
+
+      const resultPart = result[0]?.parts[0] as ToolCallPart
+      expect(resultPart).not.toBe(originalPart)
+      expect(resultPart.approval).not.toBe(originalApproval)
+      expect(resultPart.state).toBe('approval-responded')
+      expect(resultPart.approval?.approved).toBe(true)
+    })
   })
 })