diff --git a/chatkit/server.py b/chatkit/server.py
index 9d82b64..c27a6af 100644
--- a/chatkit/server.py
+++ b/chatkit/server.py
@@ -697,7 +697,7 @@ async def _process_events(
             with agents_sdk_user_agent_override():
                 async for event in stream():
                     if isinstance(event, ThreadItemAddedEvent):
-                        pending_items[event.item.id] = event.item
+                        pending_items[event.item.id] = event.item.model_copy(deep=True)
 
                     match event:
                         case ThreadItemDoneEvent():
diff --git a/tests/test_chatkit_server.py b/tests/test_chatkit_server.py
index d7ac167..deeccb5 100644
--- a/tests/test_chatkit_server.py
+++ b/tests/test_chatkit_server.py
@@ -73,11 +73,15 @@
     ThreadUpdatedEvent,
     ThreadUpdateParams,
     ToolChoice,
+    ThoughtTask,
     UserMessageInput,
     UserMessageItem,
     UserMessageTextContent,
     WidgetItem,
     WidgetRootUpdated,
+    Workflow,
+    WorkflowItem,
+    WorkflowTaskAdded,
 )
 from chatkit.widgets import Card, Text
 from tests._types import RequestContext
@@ -345,6 +349,58 @@ def generate_item_id(
             )
 
 
+async def test_workflow_updates_not_applied_twice():
+    async def responder(
+        thread: ThreadMetadata, input: UserMessageItem | None, context: Any
+    ) -> AsyncIterator[ThreadStreamEvent]:
+        workflow_item = WorkflowItem(
+            id="wf_1",
+            created_at=datetime.now(),
+            workflow=Workflow(type="reasoning", tasks=[]),
+            thread_id=thread.id,
+        )
+        yield ThreadItemAddedEvent(item=workflow_item)
+
+        task = ThoughtTask(content="First thought")
+        # Simulate responders that mutate the workflow item before emitting an update
+        workflow_item.workflow.tasks.append(task)
+        yield ThreadItemUpdatedEvent(
+            item_id=workflow_item.id,
+            update=WorkflowTaskAdded(task=task, task_index=0),
+        )
+
+        yield ThreadItemDoneEvent(item=workflow_item)
+
+    with make_server(responder) as server:
+        events = await server.process_streaming(
+            ThreadsCreateReq(
+                params=ThreadCreateParams(
+                    input=UserMessageInput(
+                        content=[UserMessageTextContent(text="Hello")],
+                        attachments=[],
+                        inference_options=InferenceOptions(),
+                    )
+                )
+            )
+        )
+
+        thread = next(event.thread for event in events if event.type == "thread.created")
+        workflow_done = next(
+            event
+            for event in events
+            if event.type == "thread.item.done" and event.item.type == "workflow"
+        )
+
+        assert len(workflow_done.item.workflow.tasks) == 1
+        assert workflow_done.item.workflow.tasks[0].content == "First thought"
+
+        stored_workflow = await server.store.load_item(
+            thread.id, "wf_1", DEFAULT_CONTEXT
+        )
+        assert len(stored_workflow.workflow.tasks) == 1
+        assert stored_workflow.workflow.tasks[0].content == "First thought"
+
+
 async def test_flows_context_to_responder():
     responder_context = None
     add_feedback_context = None