diff --git a/lmdeploy/serve/openai/harmony_utils.py b/lmdeploy/serve/openai/harmony_utils.py
index 25c24749ab..ebd28ebb6f 100644
--- a/lmdeploy/serve/openai/harmony_utils.py
+++ b/lmdeploy/serve/openai/harmony_utils.py
@@ -58,9 +58,14 @@ def parse_streaming(self, tokens: List[int]) -> DeltaMessage:
                                                     index=base_index,
                                                     function=DeltaFunctionCall(name=tool_name, arguments=''))
                 elif delta_text:
+                    # Continuing the same tool call. Ensure we don't duplicate the
+                    # very first delta string in this chunk. Previously we initialized
+                    # with arguments=delta_text and then appended again, causing
+                    # duplicated content like "locationlocation".
                     if delta_tool_call is None:
-                        delta_tool_call = DeltaToolCall(index=base_index,
-                                                        function=DeltaFunctionCall(arguments=delta_text))
+                        # We are in the middle of a tool call carried over from the
+                        # previous chunk. Initialize an empty arguments buffer.
+                        delta_tool_call = DeltaToolCall(index=base_index, function=DeltaFunctionCall(arguments=''))
                     delta_tool_call.function.arguments += delta_text
 
         if delta_tool_call:
diff --git a/tests/test_lmdeploy/test_harmony_gpt_oss_parser.py b/tests/test_lmdeploy/test_harmony_gpt_oss_parser.py
new file mode 100644
index 0000000000..e1e5efeb71
--- /dev/null
+++ b/tests/test_lmdeploy/test_harmony_gpt_oss_parser.py
@@ -0,0 +1,324 @@
+import collections
+import json
+import os
+import sys
+import time
+import types
+from typing import Generator, List
+
+import pytest
+import shortuuid
+
+# Ensure local package is imported (not any site-packages installation)
+REPO_ROOT = os.path.dirname(os.path.dirname(os.path.dirname(__file__)))
+if REPO_ROOT not in sys.path:
+    sys.path.insert(0, REPO_ROOT)
+
+
+def _install_openai_harmony_stub():
+    """Install a minimal stub for `openai_harmony` so the module imports
+    without the real dependency.
+
+    The GptOssChatParser test injects its own dummy parser, so the stub is sufficient.
+    """
+    if 'openai_harmony' in sys.modules:
+        return
+    m = types.ModuleType('openai_harmony')
+
+    class HarmonyEncodingName:
+        HARMONY_GPT_OSS = 'HARMONY_GPT_OSS'
+
+    class Role:
+        ASSISTANT = 'assistant'
+
+    class StreamableParser:  # pragma: no cover - constructor only used
+
+        def __init__(self, encoding, role=None):
+            self.encoding = encoding
+            self.role = role
+
+    def load_harmony_encoding(name):  # pragma: no cover - not used in test
+        return object()
+
+    m.HarmonyEncodingName = HarmonyEncodingName
+    m.Role = Role
+    m.StreamableParser = StreamableParser
+    m.load_harmony_encoding = load_harmony_encoding
+    sys.modules['openai_harmony'] = m
+
+
+TestExpects = collections.namedtuple('TestExpects', 'func_name location')
+
+
+class DummyParser:
+    """A minimal stand-in for Harmony's StreamableParser with channels.
+
+    Control tokens:
+      -1: start functions.get_weather (commentary)
+      -4: start functions.get_time (commentary)
+      -6: start functions.get_weather (again)
+      -9: end current tool call, append to `messages`
+      -2: switch to final (visible) content
+      -3: switch to analysis (reasoning)
+    Other tokens are interpreted as chr(token).
+    """
+
+    class _Msg:
+
+        def __init__(self, channel, recipient):
+            self.channel = channel
+            self.recipient = recipient
+
+    def __init__(self):
+        self.current_channel = None
+        self.current_recipient = None
+        self.last_content_delta = ''
+        self.messages = []
+
+    def process(self, token):
+        if token == -1:
+            self.current_channel = 'commentary'
+            self.current_recipient = 'functions.get_weather'
+            self.last_content_delta = ''
+            return
+        if token == -4:
+            self.current_channel = 'commentary'
+            self.current_recipient = 'functions.get_time'
+            self.last_content_delta = ''
+            return
+        if token == -6:
+            self.current_channel = 'commentary'
+            self.current_recipient = 'functions.get_weather'
+            self.last_content_delta = ''
+            return
+        if token == -9:
+            if self.current_channel == 'commentary' and self.current_recipient and self.current_recipient.startswith(
+                    'functions.'):
+                self.messages.append(self._Msg(self.current_channel, self.current_recipient))
+            # reset recipient to signal end of current tool call
+            self.current_recipient = None
+            self.current_channel = None
+            self.last_content_delta = ''
+            return
+        if token == -2:
+            self.current_channel = 'final'
+            self.current_recipient = None
+            self.last_content_delta = ''
+            return
+        if token == -3:
+            self.current_channel = 'analysis'
+            self.current_recipient = None
+            self.last_content_delta = ''
+            return
+        # regular character token
+        self.last_content_delta = chr(token)
+
+
+def _chat_completion_v1(request, token_chunks: List[List[int]]):
+    from lmdeploy.serve.openai.harmony_utils import GptOssChatParser
+    from lmdeploy.serve.openai.protocol import (ChatCompletionResponse, ChatCompletionResponseChoice,
+                                                ChatCompletionResponseStreamChoice, ChatCompletionStreamResponse,
+                                                UsageInfo)
+
+    request_id = f'chat-{shortuuid.random()}'
+    created_time = int(time.time())
+    model_name = request.model
+
+    parser = GptOssChatParser()
+    parser.parser = DummyParser()
+
+    if request.stream:
+
+        def completion_stream_generator() -> Generator['ChatCompletionStreamResponse', None, None]:
+            finish_reason = 'stop'
+            for chunk in token_chunks:
+                delta_message = parser.parse_streaming(chunk)
+                choice_data = ChatCompletionResponseStreamChoice(index=0,
+                                                                 delta=delta_message,
+                                                                 finish_reason=finish_reason,
+                                                                 logprobs=None)
+                response = ChatCompletionStreamResponse(id=request_id,
+                                                        created=created_time,
+                                                        model=model_name,
+                                                        choices=[choice_data],
+                                                        usage=None)
+                yield response
+
+        return completion_stream_generator()
+
+    # Non-stream path: parse all tokens at once using parse_full
+    tokens: List[int] = []
+    for c in token_chunks:
+        tokens.extend(c)
+    message = parser.parse_full(tokens)
+    finish_reason = 'tool_calls' if message.tool_calls else 'stop'
+    choice_data = ChatCompletionResponseChoice(index=0, message=message, finish_reason=finish_reason)
+    return ChatCompletionResponse(id=request_id,
+                                  created=created_time,
+                                  model=model_name,
+                                  choices=[choice_data],
+                                  usage=UsageInfo())
+
+
+def _stream_parse(request, token_chunks: List[List[int]]):
+    from lmdeploy.serve.openai.protocol import DeltaMessage
+
+    content = ''
+    reasoning_content = ''
+    tool_calls_by_index = {}
+
+    for i, stream_resp in enumerate(_chat_completion_v1(request, token_chunks)):
+        delta_message: DeltaMessage = stream_resp.choices[0].delta
+        if delta_message.content:
+            content += delta_message.content
+        if delta_message.reasoning_content:
+            reasoning_content += delta_message.reasoning_content
+        if delta_message.tool_calls:
+            for c in delta_message.tool_calls:
+                idx = c.index
+                existing_call = tool_calls_by_index.get(idx, None)
+                if not existing_call:
+                    tool_calls_by_index[idx] = c
+                    continue
+                if c.function.name:
+                    existing_call.function.name = c.function.name
+                if c.function.arguments:
+                    existing_call.function.arguments = existing_call.function.arguments or ''
+                    existing_call.function.arguments += c.function.arguments
+    # sorted list for stable order
+    tool_calls = [tool_calls_by_index[i] for i in sorted(tool_calls_by_index.keys())]
+    return content, reasoning_content, tool_calls
+
+
+def _t(s: str) -> List[int]:
+    return [ord(c) for c in s]
+
+
+# Basic: single function call split across two chunks (bug repro scenario)
+TOKENS_SINGLE_CALL_TWO_CHUNKS = [
+    [-1] + _t('{"location": "Paris'),
+    _t(', France"}'),
+]
+
+# Multiple calls with indices and different function names
+TOKENS_TWO_CALLS_DIFFERENT_FUNCS = [
+    [-1] + _t('{"location": "Berlin"}') + [-9] + [-4] + _t('{"city": "New'),
+    _t(' York"}') + [-9],
+]
+
+# Interleaved channels: analysis, tool call, final content
+TOKENS_INTERLEAVED = [
+    [-3] + _t('Thinking about the weather. ') + [-1] + _t('{"location": "Par'),
+    _t('is, France"}') + [-9] + [-2] + _t('Fetching the weather now.'),
+]
+
+# Two calls, same function name, indices increment
+TOKENS_TWO_CALLS_SAME_FUNC = [
+    [-1] + _t('{"location": "Tokyo"}') + [-9],
+    [-6] + _t('{"location": "Ky'),
+    _t('oto"}') + [-9],
+]
+
+
+@pytest.mark.parametrize(('token_chunks', 'expects'), [
+    (TOKENS_SINGLE_CALL_TWO_CHUNKS, [TestExpects('get_weather', 'Paris, France')]),
+])
+def test_parser_stream_basic(token_chunks: List[List[int]], expects: List[TestExpects]):
+    from lmdeploy.serve.openai.protocol import ChatCompletionRequest
+
+    _install_openai_harmony_stub()
+    request = ChatCompletionRequest(model='gpt-oss', messages=[], stream=True)
+    content, reasoning_content, tool_calls = _stream_parse(request, token_chunks)
+
+    assert len(tool_calls) == len(expects)
+    for parsed_call, expected_call in zip(tool_calls, expects):
+        assert parsed_call.function.name == expected_call.func_name
+        args = json.loads(parsed_call.function.arguments)
+        assert args['location'] == expected_call.location
+    assert content.strip() == ''
+    assert (reasoning_content or '').strip() == ''
+
+
+def test_parser_stream_multiple_calls_indices():
+    from lmdeploy.serve.openai.protocol import ChatCompletionRequest
+
+    _install_openai_harmony_stub()
+    request = ChatCompletionRequest(model='gpt-oss', messages=[], stream=True)
+    content, reasoning_content, tool_calls = _stream_parse(request, TOKENS_TWO_CALLS_DIFFERENT_FUNCS)
+
+    assert len(tool_calls) == 2
+    # tool_calls sorted by index ensures stable order
+    tc0, tc1 = tool_calls
+    assert tc0.index == 0 and tc1.index == 1
+    assert tc0.function.name == 'get_weather'
+    assert json.loads(tc0.function.arguments)['location'] == 'Berlin'
+    assert tc1.function.name == 'get_time'
+    assert json.loads(tc1.function.arguments)['city'] == 'New York'
+    assert (content or '').strip() == ''
+    assert (reasoning_content or '').strip() == ''
+
+
+def test_parser_stream_interleaved_channels():
+    from lmdeploy.serve.openai.protocol import ChatCompletionRequest
+
+    _install_openai_harmony_stub()
+    request = ChatCompletionRequest(model='gpt-oss', messages=[], stream=True)
+    content, reasoning_content, tool_calls = _stream_parse(request, TOKENS_INTERLEAVED)
+
+    assert json.loads(tool_calls[0].function.arguments)['location'] == 'Paris, France'
+    assert reasoning_content == 'Thinking about the weather. '
+    assert content == 'Fetching the weather now.'
+
+
+@pytest.mark.parametrize(('token_chunks', 'expects'), [
+    (TOKENS_TWO_CALLS_SAME_FUNC, [TestExpects('get_weather', 'Tokyo'),
+                                  TestExpects('get_weather', 'Kyoto')]),
+])
+def test_parser_stream_two_calls_same_func(token_chunks: List[List[int]], expects: List[TestExpects]):
+    from lmdeploy.serve.openai.protocol import ChatCompletionRequest
+
+    _install_openai_harmony_stub()
+    request = ChatCompletionRequest(model='gpt-oss', messages=[], stream=True)
+    _, _, tool_calls = _stream_parse(request, token_chunks)
+
+    assert len(tool_calls) == len(expects)
+    for parsed_call, expected_call in zip(tool_calls, expects):
+        assert parsed_call.function.name == expected_call.func_name
+        args = json.loads(parsed_call.function.arguments)
+        assert args['location'] == expected_call.location
+
+
+def test_open_tool_call_no_args():
+    from lmdeploy.serve.openai.protocol import ChatCompletionRequest
+
+    _install_openai_harmony_stub()
+    request = ChatCompletionRequest(model='gpt-oss', messages=[], stream=True)
+    content, reasoning_content, tool_calls = _stream_parse(request, [[-1]])
+
+    assert len(tool_calls) == 1
+    assert tool_calls[0].function.name == 'get_weather'
+    assert (tool_calls[0].function.arguments or '') == ''
+    assert (content or '') == ''
+    assert (reasoning_content or '') == ''
+
+
+@pytest.mark.parametrize(('token_chunks', 'expects'), [
+    (TOKENS_SINGLE_CALL_TWO_CHUNKS, [TestExpects('get_weather', 'Paris, France')]),
+    (TOKENS_TWO_CALLS_SAME_FUNC, [TestExpects('get_weather', 'Tokyo'),
+                                  TestExpects('get_weather', 'Kyoto')]),
+])
+def test_parser_nonstream(token_chunks: List[List[int]], expects: List[TestExpects]):
+    from lmdeploy.serve.openai.protocol import ChatCompletionRequest
+
+    _install_openai_harmony_stub()
+    resp = _chat_completion_v1(ChatCompletionRequest(model='gpt-oss', messages=[], stream=False), token_chunks)
+
+    assert len(resp.choices) == 1
+    first_message = resp.choices[0].message
+    assert first_message.content is None
+    assert (first_message.reasoning_content or '') == ''
+    assert len(first_message.tool_calls) == len(expects)
+    for parsed_call, expected_call in zip(first_message.tool_calls, expects):
+        assert parsed_call.function.name == expected_call.func_name
+        args = json.loads(parsed_call.function.arguments)
+        assert args['location'] == expected_call.location