From f74c8533dc64a214f92c283744f5135b12a6753c Mon Sep 17 00:00:00 2001
From: Charlie Freeman <243496084+cf3901646@users.noreply.github.com>
Date: Mon, 26 Oct 2026 22:47:24 +0800
Subject: [PATCH] fix(streaming): preserve reasoning_details chunks in stream
 wrapper and proxy serializer (#48401)

---
 .../litellm_core_utils/streaming_handler.py   |  1 +
 litellm/proxy/proxy_server.py                 |  2 +
 .../test_reasoning_details_streaming.py       | 75 +++++++++++++++++++
 3 files changed, 78 insertions(+)
 create mode 100644 tests/unit/litellm_core_utils/test_reasoning_details_streaming.py

diff --git a/litellm/litellm_core_utils/streaming_handler.py b/litellm/litellm_core_utils/streaming_handler.py
index 2df470edc59..b4d7a52aeb5 100644
--- a/litellm/litellm_core_utils/streaming_handler.py
+++ b/litellm/litellm_core_utils/streaming_handler.py
@@ -1006,6 +1006,7 @@ class CustomStreamWrapper:
                 and model_response.choices[0].delta.role is not None
             )
             or (getattr(model_response.choices[0].delta, "reasoning_items", None) is not None)
+            or bool(getattr(model_response.choices[0].delta, "reasoning_details", None))
         ):
             return True
         else:
diff --git a/litellm/proxy/proxy_server.py b/litellm/proxy/proxy_server.py
index df948b69a67..c88a900f9cf 100644
--- a/litellm/proxy/proxy_server.py
+++ b/litellm/proxy/proxy_server.py
@@ -10093,6 +10093,8 @@ def _fast_serialize_simple_model_response_stream(
         "thinking_blocks",
         "provider_specific_fields",
         "refusal",
+        "reasoning_items",
+        "reasoning_details",
     )
     if any(getattr(delta, field, None) is not None for field in unsupported_delta_fields):
         return None
diff --git a/tests/unit/litellm_core_utils/test_reasoning_details_streaming.py b/tests/unit/litellm_core_utils/test_reasoning_details_streaming.py
new file mode 100644
index 00000000000..97c6a994bd9
--- /dev/null
+++ b/tests/unit/litellm_core_utils/test_reasoning_details_streaming.py
@@ -0,0 +1,75 @@
+from unittest.mock import MagicMock
+from litellm.litellm_core_utils.streaming_handler import CustomStreamWrapper
+from litellm.types.utils import ModelResponseStream
+
+
+def test_is_chunk_non_empty_with_reasoning_details():
+    """
+    Unit test that chunks carrying only delta.reasoning_details (e.g., OpenRouter
+    Anthropic signatures or encrypted reasoning blobs) are treated as non-empty.
+
+    Regression test for https://github.com/BerriAI/litellm/issues/48401
+    """
+    wrapper = CustomStreamWrapper(
+        completion_stream=iter([]),
+        model="anthropic/claude-sonnet-4.5",
+        custom_llm_provider="openrouter",
+        logging_obj=MagicMock(),
+    )
+
+    chunk_with_signature = {
+        "id": "gen-sig-1",
+        "object": "chat.completion.chunk",
+        "created": 1741037890,
+        "model": "anthropic/claude-sonnet-4.5",
+        "choices": [
+            {
+                "index": 0,
+                "delta": {
+                    "content": None,
+                    "reasoning_details": [
+                        {
+                            "type": "reasoning.text",
+                            "signature": "sig-abc",
+                            "index": 0,
+                            "format": "anthropic-claude-v1",
+                        }
+                    ],
+                },
+                "finish_reason": None,
+            }
+        ],
+    }
+    assert (
+        wrapper.is_chunk_non_empty(
+            completion_obj={},
+            model_response=ModelResponseStream(**chunk_with_signature),
+            response_obj={},
+        )
+        is True
+    )
+
+    chunk_with_empty_reasoning_details = {
+        "id": "gen-sig-2",
+        "object": "chat.completion.chunk",
+        "created": 1741037890,
+        "model": "anthropic/claude-sonnet-4.5",
+        "choices": [
+            {
+                "index": 0,
+                "delta": {
+                    "content": None,
+                    "reasoning_details": [],
+                },
+                "finish_reason": None,
+            }
+        ],
+    }
+    assert (
+        wrapper.is_chunk_non_empty(
+            completion_obj={},
+            model_response=ModelResponseStream(**chunk_with_empty_reasoning_details),
+            response_obj={},
+        )
+        is False
+    )