diff --git a/tests/entrypoints/openai/responses/test_serving_responses.py b/tests/entrypoints/openai/responses/test_serving_responses.py
index 591ba5c42a..acab4cb6e2 100644
--- a/tests/entrypoints/openai/responses/test_serving_responses.py
+++ b/tests/entrypoints/openai/responses/test_serving_responses.py
@@ -67,7 +67,7 @@ from vllm.entrypoints.serve.engine.protocol import ErrorResponse
 from vllm.inputs import tokens_input
 from vllm.logprobs import Logprob as SampleLogprob
 from vllm.outputs import CompletionOutput, RequestOutput
-from vllm.parser.harmony import Segment
+from vllm.parser.harmony import ChunkResult, HarmonyParser, Segment
 from vllm.renderers import TokenizeParams
 from vllm.renderers.online_renderer import (
     OnlineRenderer,
@@ -2129,6 +2129,87 @@ async def test_stream_completed_response_reuses_streamed_items(monkeypatch):
     assert function_call.call_id == "chatcmpl-tool-parser-id"
 
 
+def _harmony_msg(channel: str, text: str, recipient: str | None = None):
+    msg = OpenAIHarmonyMessage.from_role_and_content(Role.ASSISTANT, text)
+    msg = msg.with_channel(channel)
+    return msg.with_recipient(recipient) if recipient else msg
+
+
+def _harmony_segments(msg, *, streamed: bool = True) -> list[Segment]:
+    """Segments the parser yields for one message: a content delta (absent
+    for a zero-delta message), then the completed message."""
+    delta = [Segment(msg.channel, msg.recipient, msg.content[0].text)]
+    completed = Segment(msg.channel, msg.recipient, "", completed_message=msg)
+    return (delta if streamed else []) + [completed]
+
+
+async def _harmony_stream_events(segment_chunks: list[list[Segment]]):
+    serving = _make_serving_instance()
+    serving.use_harmony = True
+    parser = MagicMock(spec=HarmonyParser)
+    parser.process_chunk.side_effect = [
+        ChunkResult(segments=segments, reasoning_token_count=0)
+        for segments in segment_chunks
+    ]
+    context = HarmonyContext([], [], frozenset({"get_weather"}), parser)
+
+    async def result_generator():
+        for token_id in range(len(segment_chunks)):
+            context.append_output(_make_request_output("", [token_id]))
+            yield context
+
+    return [
+        event
+        async for event in serving.responses_stream_generator(
+            request=ResponsesRequest(input="hi", stream=True, store=False),
+            sampling_params=SamplingParams(max_tokens=16),
+            result_generator=result_generator(),
+            context=context,
+            model_name="test-model",
+            tokenizer=MagicMock(),
+            request_metadata=RequestResponseMetadata(request_id="req"),
+        )
+    ]
+
+
+@pytest.mark.asyncio
+async def test_harmony_stream_completed_response_reuses_streamed_ids():
+    """response.completed must carry the id and call_id each item was streamed
+    with, not ones minted when the harmony messages are converted again."""
+    messages = [
+        _harmony_msg("analysis", "think"),
+        _harmony_msg("commentary", '{"city": "Paris"}', "functions.get_weather"),
+        _harmony_msg("final", "Sunny"),
+    ]
+
+    events = await _harmony_stream_events([_harmony_segments(msg) for msg in messages])
+
+    streamed = [e.item for e in events if e.type == "response.output_item.done"]
+    output = events[-1].response.output
+    assert [item.type for item in output] == ["reasoning", "function_call", "message"]
+    assert [item.id for item in output] == [item.id for item in streamed]
+    assert output[1].call_id == streamed[1].call_id
+    assert output[2].content[0].text == "Sunny"
+
+
+@pytest.mark.asyncio
+async def test_harmony_stream_unstreamed_item_keeps_own_id():
+    """An item with no done event (zero-delta) must not take the streamed id
+    of a later item of the same type."""
+    unstreamed = _harmony_msg("analysis", "skipped")
+    reasoning = _harmony_msg("analysis", "think")
+
+    events = await _harmony_stream_events(
+        [_harmony_segments(unstreamed, streamed=False), _harmony_segments(reasoning)]
+    )
+
+    (streamed,) = [e.item for e in events if e.type == "response.output_item.done"]
+    first, second = events[-1].response.output
+    assert first.content[0].text == "skipped"
+    assert first.id != streamed.id
+    assert second.id == streamed.id
+
+
 @pytest.mark.asyncio
 @pytest.mark.parametrize(("parallel_tool_calls", "num_calls"), [(False, 1), (None, 3)])
 async def test_streaming_parallel_tool_calls(
diff --git a/vllm/entrypoints/openai/responses/context.py b/vllm/entrypoints/openai/responses/context.py
index e71af75a0e..a68df6dd4d 100644
--- a/vllm/entrypoints/openai/responses/context.py
+++ b/vllm/entrypoints/openai/responses/context.py
@@ -656,6 +656,11 @@ class HarmonyContext(ConversationContext):
 
         self.last_append_segments: list[Segment] = []
         self.last_append_flush_status: bool = False
+        # Each completed message with the items sent for it in
+        # output_item.done events, so the final response can reuse their ids.
+        self.streamed_items_by_message: list[
+            tuple[Message, list[ResponseOutputItem]]
+        ] = []
 
         # Turn tracking - replaces multiple individual tracking variables
         self.current_turn_metrics = TurnMetrics()
diff --git a/vllm/entrypoints/openai/responses/serving.py b/vllm/entrypoints/openai/responses/serving.py
index bd876e4916..3a417ca7a5 100644
--- a/vllm/entrypoints/openai/responses/serving.py
+++ b/vllm/entrypoints/openai/responses/serving.py
@@ -12,6 +12,7 @@ from typing import Any, Final, cast
 from fastapi import Request
 from openai.types.responses import (
     ResponseOutputItem,
+    ResponseOutputItemDoneEvent,
     ResponseOutputMessage,
     ResponseOutputText,
     ResponseStatus,
@@ -68,6 +69,7 @@ from vllm.entrypoints.openai.responses.utils import (
     build_response_output_items,
     extract_function_tool_names,
     extract_tool_types,
+    reuse_streamed_item_ids,
 )
 from vllm.entrypoints.serve.engine.protocol import ErrorResponse
 from vllm.entrypoints.serve.utils.api_utils import get_max_tokens
@@ -781,17 +783,21 @@ class OpenAIServingResponses(GenerateBaseServing):
             assert isinstance(context, HarmonyContext)
             output = []
             harmony_msgs = context.messages[context.num_init_messages :]
-            if harmony_msgs:
-                fn_names = context.function_tool_names
-                for msg in harmony_msgs[:-1]:
-                    output.extend(harmony_to_response_output(msg, fn_names))
-                output.extend(
-                    harmony_to_response_output(
-                        harmony_msgs[-1],
-                        fn_names,
-                        incomplete=context.last_append_flush_status,
-                    )
+            # Streamed messages are a subsequence of harmony_msgs, in order.
+            streamed = iter(context.streamed_items_by_message)
+            next_streamed = next(streamed, None)
+            for i, msg in enumerate(harmony_msgs):
+                items = harmony_to_response_output(
+                    msg,
+                    context.function_tool_names,
+                    incomplete=(
+                        i == len(harmony_msgs) - 1 and context.last_append_flush_status
+                    ),
                 )
+                if next_streamed is not None and next_streamed[0] is msg:
+                    reuse_streamed_item_ids(items, next_streamed[1])
+                    next_streamed = next(streamed, None)
+                output.extend(items)
 
             if request.enable_response_messages:
                 input_messages = context.messages[: context.num_init_messages]
@@ -1291,16 +1297,24 @@ class OpenAIServingResponses(GenerateBaseServing):
                         yield _increment_sequence_number_and_return(event)
 
                 elif completed_message := segment.completed_message:
+                    done_items: list[ResponseOutputItem] = []
                     # TODO: Fix browser emitted as MCP calls
                     for event in emit_previous_item_done_events(
                         completed_message, state, ctx.function_tool_names
                     ):
+                        if isinstance(event, ResponseOutputItemDoneEvent):
+                            done_items.append(event.item)
                         yield _increment_sequence_number_and_return(event)
 
                     for event in emit_tool_action_events(
                         completed_message, state, self.tool_server
                     ):
+                        if isinstance(event, ResponseOutputItemDoneEvent):
+                            done_items.append(event.item)
                         yield _increment_sequence_number_and_return(event)
+                    ctx.streamed_items_by_message.append(
+                        (completed_message, done_items)
+                    )
                     state.reset_for_new_item()
 
     async def responses_stream_generator(
diff --git a/vllm/entrypoints/openai/responses/streaming_events.py b/vllm/entrypoints/openai/responses/streaming_events.py
index b4731dc687..2f5b151a1b 100644
--- a/vllm/entrypoints/openai/responses/streaming_events.py
+++ b/vllm/entrypoints/openai/responses/streaming_events.py
@@ -202,7 +202,7 @@ def emit_reasoning_delta_events(
     events: list[StreamingResponsesResponse] = []
     if not state.sent_output_item_added:
         state.sent_output_item_added = True
-        state.current_item_id = f"msg_{random_uuid()}"
+        state.current_item_id = f"rs_{random_uuid()}"
         events.append(
             ResponseOutputItemAddedEvent(
                 type="response.output_item.added",
diff --git a/vllm/entrypoints/openai/responses/utils.py b/vllm/entrypoints/openai/responses/utils.py
index 3caff4b056..6c8507840f 100644
--- a/vllm/entrypoints/openai/responses/utils.py
+++ b/vllm/entrypoints/openai/responses/utils.py
@@ -112,6 +112,28 @@ def build_response_output_items(
     return outputs
 
 
+def reuse_streamed_item_ids(
+    items: list[ResponseOutputItem],
+    streamed_items: list[ResponseOutputItem],
+) -> None:
+    """Give rebuilt output items the ids already streamed for the same message.
+
+    Items are paired by type in order; an item without a streamed counterpart
+    keeps its own id.
+    """
+    remaining = list(streamed_items)
+    for item in items:
+        for i, streamed in enumerate(remaining):
+            if streamed.type != item.type:
+                continue
+            item.id = streamed.id
+            if isinstance(item, ResponseFunctionToolCall):
+                assert isinstance(streamed, ResponseFunctionToolCall)
+                item.call_id = streamed.call_id
+            del remaining[i]
+            break
+
+
 def should_continue_final_message(
     request_input: str | list[ResponseInputOutputItem],
 ) -> bool: