fix: detect Gemini thinking chunks in is_chunk_non_empty via original_chunk

When Gemini streams thinking/reasoning chunks, the parsed
reasoning_content lives in response_obj["original_chunk"] — not in the
fresh model_response created by model_response_creator(). The existing
reasoning_content check (line 817-820) only inspects the empty
model_response, so thinking-only chunks are silently dropped.

Add a fallback check that inspects
response_obj["original_chunk"].choices[0].delta.reasoning_content.
Once is_chunk_non_empty returns True, return_processed_chunk_logic
already rebuilds choices from original_chunk.model_dump(), which
preserves reasoning_content — no further changes needed.

Closes #28000

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

litellm/litellm_core_utils/streaming_handler.py

@@ -818,6 +818,16 @@ class CustomStreamWrapper:
                 "reasoning_content" in model_response.choices[0].delta
                 and model_response.choices[0].delta.reasoning_content is not None
             )
+            or (
+                response_obj.get("original_chunk") is not None
+                and len(getattr(response_obj["original_chunk"], "choices", [])) > 0
+                and getattr(
+                    response_obj["original_chunk"].choices[0].delta,
+                    "reasoning_content",
+                    None,
+                )
+                is not None
+            )
             or (model_response.choices[0].delta.provider_specific_fields is not None)
             or (
                 "provider_specific_fields" in model_response

tests/test_litellm/litellm_core_utils/test_streaming_handler.py

@@ -192,6 +192,34 @@ def test_is_chunk_non_empty_with_annotations(
     )
 
 
+def test_is_chunk_non_empty_gemini_thinking_in_original_chunk(
+    initialized_custom_stream_wrapper: CustomStreamWrapper,
+):
+    """Gemini thinking chunks: model_response is a fresh empty object but
+    response_obj['original_chunk'] carries reasoning_content.  The chunk
+    must be considered non-empty so it reaches the caller."""
+    original_chunk = ModelResponseStream(
+        id="gemini-chunk-1",
+        choices=[
+            {
+                "index": 0,
+                "delta": {"content": None, "reasoning_content": "I need to add 2+2"},
+                "finish_reason": None,
+            }
+        ],
+    )
+    # model_response is a fresh empty object (as created by model_response_creator)
+    empty_model_response = ModelResponseStream(
+        id="gemini-chunk-1",
+        choices=[{"index": 0, "delta": {"content": None}, "finish_reason": None}],
+    )
+    assert initialized_custom_stream_wrapper.is_chunk_non_empty(
+        completion_obj={"content": ""},
+        model_response=empty_model_response,
+        response_obj={"original_chunk": original_chunk},
+    )
+
+
 def test_optional_combine_thinking_block_in_choices(
     initialized_custom_stream_wrapper: CustomStreamWrapper,
 ):
@@ -2036,23 +2064,19 @@ async def test_azure_streaming_role_preserved_with_include_usage(sync_mode: bool
             chunks.append(chunk)
 
     # The prompt_filter chunk should be forwarded with choices=[]
-    assert len(chunks[0].choices) == 0, (
-        f"Expected prompt_filter chunk with choices=[], got {len(chunks[0].choices)} choices"
-    )
+    assert (
+        len(chunks[0].choices) == 0
+    ), f"Expected prompt_filter chunk with choices=[], got {len(chunks[0].choices)} choices"
 
     # At least one chunk must have role='assistant' in its delta
     has_role = any(
-        len(c.choices) > 0
-        and getattr(c.choices[0].delta, "role", None) == "assistant"
+        len(c.choices) > 0 and getattr(c.choices[0].delta, "role", None) == "assistant"
         for c in chunks
     )
     assert has_role, (
         "No chunk contained role='assistant' in delta (issue #24221). "
         "Chunk deltas: "
-        + str([
-            c.choices[0].delta if c.choices else "no choices"
-            for c in chunks
-        ])
+        + str([c.choices[0].delta if c.choices else "no choices" for c in chunks])
     )