diff --git a/litellm/litellm_core_utils/streaming_chunk_builder_utils.py b/litellm/litellm_core_utils/streaming_chunk_builder_utils.py
index c87357122ce..118a2282a16 100644
--- a/litellm/litellm_core_utils/streaming_chunk_builder_utils.py
+++ b/litellm/litellm_core_utils/streaming_chunk_builder_utils.py
@@ -1535,8 +1535,8 @@ def _streamed_vllm_choice_fields(choices: Iterable[object]) -> Mapping[str, obje
     return MappingProxyType({field: streamed_values[field] for field in VLLM_CHOICE_FIELDS if field in present_fields})
 
 
-def _delta_reasoning_items(delta: object) -> Sequence[object]:
-    items: Final = _field_of(delta, "reasoning_items")
+def _delta_reasoning_items(delta: object, field: str = "reasoning_items") -> Sequence[object]:
+    items: Final = _field_of(delta, field)
     return cast(Sequence[object], items) if isinstance(items, list) else ()  # cast-ok: items forwarded verbatim
 
 
@@ -1547,8 +1547,8 @@ def _chunk_first_delta(chunk: object) -> object:
     return _field_of(cast(Sequence[object], choices)[0], "delta")  # cast-ok: untyped chunk
 
 
-def _chunk_reasoning_items(chunk: object) -> Sequence[object]:
-    return _delta_reasoning_items(_chunk_first_delta(chunk))
+def _chunk_reasoning_items(chunk: object, field: str = "reasoning_items") -> Sequence[object]:
+    return _delta_reasoning_items(_chunk_first_delta(chunk), field)
 
 
 def _chunk_refusal(chunk: object) -> str:
@@ -1561,6 +1561,7 @@ _NON_TEXT_DELTA_FIELDS: Final = (
     "function_call",
     "reasoning_content",
     "reasoning_items",
+    "reasoning_details",
     "thinking_blocks",
     "annotations",
     "audio",
@@ -1789,9 +1790,16 @@ def _assembled_message_fields(processor: ChunkProcessor, chunks: Sequence) -> It
     )
     citation_fields: Final = {"citations": _joined_streamed_citations(streamed_citations)} if streamed_citations else {}
     provider_annotation_fields: Final = {"annotations": list(provider_annotations)} if provider_annotations else {}
+    # Keep every streamed reasoning_details entry in arrival order, as the
+    # non-streaming response does. Do not merge by index: some providers
+    # send whole entries that all have index 0.
+    reasoning_details: Final = tuple(
+        chain.from_iterable(_chunk_reasoning_items(chunk, "reasoning_details") for chunk in typed_chunks)
+    )
+    reasoning_detail_fields: Final = {"reasoning_details": list(reasoning_details)} if reasoning_details else {}
     combined_provider_fields: Final = {
         key: value
-        for fields in (provider_annotation_fields, citation_fields, *provider_field_dicts)
+        for fields in (provider_annotation_fields, citation_fields, *provider_field_dicts, reasoning_detail_fields)
         for key, value in fields.items()
         if key != "citation"
     }
diff --git a/tests/unit/litellm_core_utils/test_streaming_chunk_builder_utils.py b/tests/unit/litellm_core_utils/test_streaming_chunk_builder_utils.py
index 527c83d5760..9dca75caa23 100644
--- a/tests/unit/litellm_core_utils/test_streaming_chunk_builder_utils.py
+++ b/tests/unit/litellm_core_utils/test_streaming_chunk_builder_utils.py
@@ -1233,6 +1233,34 @@ def test_stream_chunk_builder_tolerates_trailing_chunk_without_choices():
     assert response.choices[0].message.content == "Hello world"
 
 
+def test_stream_chunk_builder_keeps_reasoning_details_from_every_chunk():
+    """A streamed `reasoning_details` list must rebuild into the same
+    `provider_specific_fields["reasoning_details"]` that a non-streaming
+    response carries: every entry, in arrival order, not merged by `index`
+    (Vercel AI Gateway sends whole entries that all have `index: 0`)."""
+    text_entry: Final = {"type": "reasoning.text", "text": "step one", "index": 0}
+    encrypted_entry: Final = {"type": "reasoning.encrypted", "data": "ENC-TWO", "index": 0}
+    chunks: Final = [
+        ModelResponseStream(
+            model="m",
+            choices=[
+                StreamingChoices(index=0, delta=Delta(role="assistant", content="Hi", reasoning_details=[text_entry]))
+            ],
+        ),
+        ModelResponseStream(
+            model="m", choices=[StreamingChoices(index=0, delta=Delta(reasoning_details=[encrypted_entry]))]
+        ),
+        ModelResponseStream(model="m", choices=[StreamingChoices(index=0, delta=Delta(content=" there"))]),
+    ]
+
+    response = stream_chunk_builder(chunks=chunks)
+
+    assert response is not None
+    message = response.choices[0].message
+    assert message.content == "Hi there"
+    assert message.provider_specific_fields == {"reasoning_details": [text_entry, encrypted_entry]}
+
+
 def test_anthropic_speed_and_geo_survive_stream_assembly():
     """Anthropic prices fast mode and non-global regions with a multiplier read off
     ``usage.speed`` / ``usage.inference_geo``. Dropping them while reassembling a stream