diff --git a/src/openai/lib/streaming/_assistants.py b/src/openai/lib/streaming/_assistants.py
index 6efb3ca3f1..7347c2bd9a 100644
--- a/src/openai/lib/streaming/_assistants.py
+++ b/src/openai/lib/streaming/_assistants.py
@@ -980,13 +980,24 @@ def accumulate_event(
 def accumulate_delta(acc: dict[object, object], delta: dict[object, object]) -> dict[object, object]:
     for key, delta_value in delta.items():
         if key not in acc:
-            acc[key] = delta_value
-            continue
+            # delta lists of dicts can contain multiple entries with the same
+            # `index`, so we initialise to [] and let the merge loop handle it
+            if is_list(delta_value) and delta_value and is_dict(delta_value[0]):
+                acc_delta_value: list[object] = []
+                acc[key] = acc_delta_value
+            else:
+                acc[key] = delta_value
+                continue
 
         acc_value = acc[key]
         if acc_value is None:
-            acc[key] = delta_value
-            continue
+            if is_list(delta_value) and delta_value and is_dict(delta_value[0]):
+                acc_delta_value: list[object] = []  # type: ignore[no-redef]
+                acc_value = acc_delta_value
+                acc[key] = acc_value
+            else:
+                acc[key] = delta_value
+                continue
 
         # the `index` property is used in arrays of objects so it should
         # not be accumulated like other values e.g.
@@ -1007,7 +1018,7 @@ def accumulate_delta(acc: dict[object, object], delta: dict[object, object]) ->
         elif is_list(acc_value) and is_list(delta_value):
             # for lists of non-dictionary items we'll only ever get new entries
             # in the array, existing entries will never be changed
-            if all(isinstance(x, (str, int, float)) for x in acc_value):
+            if acc_value and all(isinstance(x, (str, int, float)) for x in acc_value):
                 acc_value.extend(delta_value)
                 continue
 
diff --git a/src/openai/lib/streaming/_deltas.py b/src/openai/lib/streaming/_deltas.py
index a5e1317612..f00b1fe5e6 100644
--- a/src/openai/lib/streaming/_deltas.py
+++ b/src/openai/lib/streaming/_deltas.py
@@ -6,13 +6,24 @@
 def accumulate_delta(acc: dict[object, object], delta: dict[object, object]) -> dict[object, object]:
     for key, delta_value in delta.items():
         if key not in acc:
-            acc[key] = delta_value
-            continue
+            # delta lists of dicts can contain multiple entries with the same
+            # `index`, so we initialise to [] and let the merge loop handle it
+            if is_list(delta_value) and delta_value and is_dict(delta_value[0]):
+                acc_delta_value: list[object] = []
+                acc[key] = acc_delta_value
+            else:
+                acc[key] = delta_value
+                continue
 
         acc_value = acc[key]
         if acc_value is None:
-            acc[key] = delta_value
-            continue
+            if is_list(delta_value) and delta_value and is_dict(delta_value[0]):
+                acc_delta_value: list[object] = []  # type: ignore[no-redef]
+                acc_value = acc_delta_value
+                acc[key] = acc_value
+            else:
+                acc[key] = delta_value
+                continue
 
         # the `index` property is used in arrays of objects so it should
         # not be accumulated like other values e.g.
@@ -33,7 +44,7 @@ def accumulate_delta(acc: dict[object, object], delta: dict[object, object]) ->
         elif is_list(acc_value) and is_list(delta_value):
             # for lists of non-dictionary items we'll only ever get new entries
             # in the array, existing entries will never be changed
-            if all(isinstance(x, (str, int, float)) for x in acc_value):
+            if acc_value and all(isinstance(x, (str, int, float)) for x in acc_value):
                 acc_value.extend(delta_value)
                 continue
 
diff --git a/src/openai/lib/streaming/chat/_completions.py b/src/openai/lib/streaming/chat/_completions.py
index 5f072cafbd..2e677276e3 100644
--- a/src/openai/lib/streaming/chat/_completions.py
+++ b/src/openai/lib/streaming/chat/_completions.py
@@ -415,7 +415,7 @@ def _accumulate_chunk(self, chunk: ChatCompletionChunk) -> ParsedChatCompletionS
                         type_=ParsedChoiceSnapshot,
                         value={
                             **choice.model_dump(exclude_unset=True, exclude={"delta"}),
-                            "message": choice.delta.to_dict(),
+                            "message": accumulate_delta({}, cast("dict[object, object]", choice.delta.to_dict())),
                         },
                     ),
                 )
@@ -744,7 +744,7 @@ def _convert_initial_chunk_into_snapshot(chunk: ChatCompletionChunk) -> ParsedCh
     for choice in chunk.choices:
         choices[choice.index] = {
             **choice.model_dump(exclude_unset=True, exclude={"delta"}),
-            "message": choice.delta.to_dict(),
+            "message": accumulate_delta({}, cast("dict[object, object]", choice.delta.to_dict())),
         }
 
     return cast(
diff --git a/tests/lib/test_deltas.py b/tests/lib/test_deltas.py
new file mode 100644
index 0000000000..f3caa70632
--- /dev/null
+++ b/tests/lib/test_deltas.py
@@ -0,0 +1,41 @@
+from __future__ import annotations
+
+from typing import Any, cast
+
+from openai.lib.streaming._deltas import accumulate_delta
+
+
+def test_duplicate_indexes_in_unseeded_slot_are_merged() -> None:
+    acc: dict[object, object] = {}
+    accumulate_delta(
+        acc,
+        {
+            "tool_calls": [
+                {"index": 0, "id": "call_abc", "function": {"name": "get_weather"}, "type": "function"},
+                {"index": 0, "function": {"arguments": '{"city"'}},
+                {"index": 0, "function": {"arguments": ': "Reykjavik"}'}},
+            ],
+        },
+    )
+    tool_calls = cast("list[Any]", acc["tool_calls"])
+    assert len(tool_calls) == 1
+    assert tool_calls[0]["id"] == "call_abc"
+    assert tool_calls[0]["function"]["name"] == "get_weather"
+    assert tool_calls[0]["function"]["arguments"] == '{"city": "Reykjavik"}'
+
+
+def test_unique_indexes_preserved() -> None:
+    acc: dict[object, object] = {}
+    accumulate_delta(
+        acc,
+        {
+            "tool_calls": [
+                {"index": 0, "id": "call_1", "function": {"name": "fn_a"}, "type": "function"},
+                {"index": 1, "id": "call_2", "function": {"name": "fn_b"}, "type": "function"},
+            ],
+        },
+    )
+    tool_calls = cast("list[Any]", acc["tool_calls"])
+    assert len(tool_calls) == 2
+    assert tool_calls[0]["id"] == "call_1"
+    assert tool_calls[1]["id"] == "call_2"