diff --git a/langfuse/openai.py b/langfuse/openai.py
index 43e00b54a..73b85d965 100644
--- a/langfuse/openai.py
+++ b/langfuse/openai.py
@@ -842,19 +842,34 @@ def _extract_streamed_openai_response(resource: Any, chunks: Any) -> Any:
 
                     for raw_tool_call in delta.get("tool_calls", []):
                         index = _get_attr_or_item(raw_tool_call, "index", None)
+                        tool_call_id = _get_attr_or_item(raw_tool_call, "id", None)
+                        tool_call_type = _get_attr_or_item(raw_tool_call, "type", None)
+                        tool_call_chunk = _get_attr_or_item(
+                            raw_tool_call, "function", None
+                        )
 
                         if not isinstance(index, int):
-                            index = len(curr) - 1 if curr else 0
+                            # Some OpenAI-compatible APIs (e.g. Gemini) omit the index
+                            # and send each parallel call whole in its own chunk: a new
+                            # id, or a name without an id, starts a call; anything else
+                            # continues the latest one.
+                            starts_call = not curr or (
+                                (
+                                    tool_call_id is not None
+                                    and tool_call_id != curr[-1].get("id")
+                                )
+                                or (
+                                    tool_call_id is None
+                                    and _get_attr_or_item(tool_call_chunk, "name", None)
+                                    is not None
+                                )
+                            )
+                            index = len(curr) if starts_call else len(curr) - 1
 
                         while len(curr) <= index:
                             curr.append({"function": {"name": "", "arguments": ""}})
 
                         current_tool_call = curr[index]
-                        tool_call_id = _get_attr_or_item(raw_tool_call, "id", None)
-                        tool_call_type = _get_attr_or_item(raw_tool_call, "type", None)
-                        tool_call_chunk = _get_attr_or_item(
-                            raw_tool_call, "function", None
-                        )
 
                         if tool_call_id is not None:
                             current_tool_call["id"] = tool_call_id
diff --git a/tests/unit/test_openai.py b/tests/unit/test_openai.py
index b688fc6f1..01f31e61a 100644
--- a/tests/unit/test_openai.py
+++ b/tests/unit/test_openai.py
@@ -401,6 +401,103 @@ def test_streaming_chat_completion_preserves_tool_calls_after_content():
     assert metadata == {"finish_reason": "tool_calls"}
 
 
+def _tool_call_chunk(*, id=None, name=None, arguments=None, finish_reason=None):
+    # Gemini's OpenAI-compatible API leaves tool_calls[].index unset
+    return SimpleNamespace(
+        model="gemini-3.5-flash",
+        choices=[
+            SimpleNamespace(
+                delta=SimpleNamespace(
+                    role=None,
+                    content=None,
+                    function_call=None,
+                    tool_calls=[
+                        SimpleNamespace(
+                            index=None,
+                            id=id,
+                            type="function" if id else None,
+                            function=SimpleNamespace(name=name, arguments=arguments),
+                        )
+                    ],
+                ),
+                finish_reason=finish_reason,
+            )
+        ],
+        usage=None,
+    )
+
+
+def test_streaming_chat_completion_keeps_parallel_tool_calls_without_index_apart():
+    _model, completion, _usage, _metadata, _service_tier = (
+        lf_openai_module._extract_streamed_openai_response(
+            SimpleNamespace(type="chat"),
+            [
+                _tool_call_chunk(
+                    id="call_paris",
+                    name="get_weather",
+                    arguments='{"city": "Paris"}',
+                ),
+                _tool_call_chunk(
+                    id="call_tokyo",
+                    name="get_weather",
+                    arguments='{"city": "Tokyo"}',
+                    finish_reason="stop",
+                ),
+            ],
+        )
+    )
+
+    assert [
+        (call["id"], call["function"]["arguments"]) for call in completion["tool_calls"]
+    ] == [
+        ("call_paris", '{"city": "Paris"}'),
+        ("call_tokyo", '{"city": "Tokyo"}'),
+    ]
+
+
+def test_streaming_chat_completion_keeps_tool_calls_without_index_or_id_apart():
+    _model, completion, _usage, _metadata, _service_tier = (
+        lf_openai_module._extract_streamed_openai_response(
+            SimpleNamespace(type="chat"),
+            [
+                _tool_call_chunk(name="get_weather", arguments='{"city": "Paris"}'),
+                _tool_call_chunk(name="get_weather", arguments='{"city": "Tokyo"}'),
+            ],
+        )
+    )
+
+    assert [
+        (call["function"]["name"], call["function"]["arguments"])
+        for call in completion["tool_calls"]
+    ] == [
+        ("get_weather", '{"city": "Paris"}'),
+        ("get_weather", '{"city": "Tokyo"}'),
+    ]
+
+
+def test_streaming_chat_completion_merges_fragments_of_one_tool_call_without_index():
+    _model, completion, _usage, _metadata, _service_tier = (
+        lf_openai_module._extract_streamed_openai_response(
+            SimpleNamespace(type="chat"),
+            [
+                _tool_call_chunk(
+                    id="call_paris", name="get_weather", arguments='{"city"'
+                ),
+                _tool_call_chunk(arguments=': "Paris"}'),
+                _tool_call_chunk(id="call_paris", arguments=""),
+            ],
+        )
+    )
+
+    assert completion["tool_calls"] == [
+        {
+            "id": "call_paris",
+            "type": "function",
+            "function": {"name": "get_weather", "arguments": '{"city": "Paris"}'},
+        }
+    ]
+
+
 def test_response_api_output_serializes_openai_parsed_response_objects():
     class ParsedOutput(BaseModel):
         name: str