diff --git a/tests/entrypoints/openai/test_response_api_with_harmony.py b/tests/entrypoints/openai/test_response_api_with_harmony.py
index 88b3795abe73e..0776f217f44a2 100644
--- a/tests/entrypoints/openai/test_response_api_with_harmony.py
+++ b/tests/entrypoints/openai/test_response_api_with_harmony.py
@@ -318,6 +318,9 @@ async def test_streaming(client: OpenAI, model_name: str, background: bool):
             background=background,
         )
 
+        current_item_id = ""
+        current_content_index = -1
+
         events = []
         current_event_mode = None
         resp_id = None
@@ -329,6 +332,26 @@ async def test_streaming(client: OpenAI, model_name: str, background: bool):
                 current_event_mode = event.type
                 print(f"\n[{event.type}] ", end="", flush=True)
 
+            # verify current_item_id is correct
+            if event.type == "response.output_item.added":
+                assert event.item.id != current_item_id
+                current_item_id = event.item.id
+            elif event.type in [
+                    "response.output_text.delta",
+                    "response.reasoning_text.delta"
+            ]:
+                assert event.item_id == current_item_id
+
+            # verify content_index_id is correct
+            if event.type == "response.content_part.added":
+                assert event.content_index != current_content_index
+                current_content_index = event.content_index
+            elif event.type in [
+                    "response.output_text.delta",
+                    "response.reasoning_text.delta"
+            ]:
+                assert event.content_index == current_content_index
+
             if "text.delta" in event.type:
                 print(event.delta, end="", flush=True)
             elif "reasoning_text.delta" in event.type:
diff --git a/vllm/entrypoints/openai/serving_responses.py b/vllm/entrypoints/openai/serving_responses.py
index 9e285e6e51756..7be5e54208bd4 100644
--- a/vllm/entrypoints/openai/serving_responses.py
+++ b/vllm/entrypoints/openai/serving_responses.py
@@ -1260,9 +1260,9 @@ class OpenAIServingResponses(OpenAIServing):
         _increment_sequence_number_and_return: Callable[[BaseModel],
                                                         BaseModel],
     ) -> AsyncGenerator[BaseModel, None]:
-        current_content_index = 0  # FIXME: this number is never changed
+        current_content_index = -1
         current_output_index = 0
-        current_item_id = ""  # FIXME: this number is never changed
+        current_item_id: str = ""
         sent_output_item_added = False
 
         async for ctx in result_generator:
@@ -1353,6 +1353,7 @@ class OpenAIServingResponses(OpenAIServing):
                         and ctx.parser.current_recipient is None):
                     if not sent_output_item_added:
                         sent_output_item_added = True
+                        current_item_id = f"msg_{random_uuid()}"
                         yield _increment_sequence_number_and_return(
                             openai_responses_types.
                             ResponseOutputItemAddedEvent(
@@ -1368,6 +1369,7 @@ class OpenAIServingResponses(OpenAIServing):
                                     status="in_progress",
                                 ),
                             ))
+                        current_content_index += 1
                         yield _increment_sequence_number_and_return(
                             openai_responses_types.
                             ResponseContentPartAddedEvent(
@@ -1398,6 +1400,7 @@ class OpenAIServingResponses(OpenAIServing):
                       and ctx.parser.current_recipient is None):
                     if not sent_output_item_added:
                         sent_output_item_added = True
+                        current_item_id = f"msg_{random_uuid()}"
                         yield _increment_sequence_number_and_return(
                             openai_responses_types.
                             ResponseOutputItemAddedEvent(
@@ -1412,6 +1415,7 @@ class OpenAIServingResponses(OpenAIServing):
                                     status="in_progress",
                                 ),
                             ))
+                        current_content_index += 1
                         yield _increment_sequence_number_and_return(
                             openai_responses_types.
                             ResponseContentPartAddedEvent(
@@ -1444,6 +1448,7 @@ class OpenAIServingResponses(OpenAIServing):
                       ) and ctx.parser.current_recipient == "python":
                     if not sent_output_item_added:
                         sent_output_item_added = True
+                        current_item_id = f"tool_{random_uuid()}"
                         yield _increment_sequence_number_and_return(
                             openai_responses_types.
                             ResponseOutputItemAddedEvent(
@@ -1516,6 +1521,7 @@ class OpenAIServingResponses(OpenAIServing):
                         raise ValueError(
                             f"Unknown function name: {function_name}")
 
+                    current_item_id = f"tool_{random_uuid()}"
                     yield _increment_sequence_number_and_return(
                         openai_responses_types.ResponseOutputItemAddedEvent(
                             type="response.output_item.added",