From 265bf871bcb85ef09466b59d1cd93b02d2a0f79e Mon Sep 17 00:00:00 2001
From: daniel-meismer-zocdoc <daniel@zocdoc.com>
Date: Wed, 15 Jul 2026 16:03:07 -0400
Subject: [PATCH] fix(responses): accept empty function argument deltas
 (#33341)

---
 .../transformation.py                         |  2 +-
 ...responses_transformation_transformation.py | 21 +++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/litellm/completion_extras/litellm_responses_transformation/transformation.py b/litellm/completion_extras/litellm_responses_transformation/transformation.py
index aecb2552b53..349fe9903ef 100644
--- a/litellm/completion_extras/litellm_responses_transformation/transformation.py
+++ b/litellm/completion_extras/litellm_responses_transformation/transformation.py
@@ -1196,7 +1196,7 @@ class OpenAiResponsesToChatCompletionStreamIterator(BaseModelResponseIterator):
                 )
         elif event_type == "response.function_call_arguments.delta":
             content_part: Optional[str] = parsed_chunk.get("delta", None)
-            if content_part:
+            if isinstance(content_part, str):
                 tool_call_index = parsed_chunk.get("output_index", 0)
                 return ModelResponseStream(
                     choices=[
diff --git a/tests/test_litellm/completion_extras/test_litellm_responses_transformation_transformation.py b/tests/test_litellm/completion_extras/test_litellm_responses_transformation_transformation.py
index 05bdc40112c..b432e6fd979 100644
--- a/tests/test_litellm/completion_extras/test_litellm_responses_transformation_transformation.py
+++ b/tests/test_litellm/completion_extras/test_litellm_responses_transformation_transformation.py
@@ -257,3 +257,24 @@ def test_translate_responses_chunk_passthrough_chat_completion_chunk():
 
     assert result.choices[0].delta.content == "Hi! How can I help?"
     assert result.choices[0].finish_reason is None
+
+
+def test_translate_responses_chunk_accepts_empty_function_argument_delta():
+    from litellm.completion_extras.litellm_responses_transformation.transformation import (
+        OpenAiResponsesToChatCompletionStreamIterator,
+    )
+
+    result = OpenAiResponsesToChatCompletionStreamIterator.translate_responses_chunk_to_openai_stream(
+        {
+            "type": "response.function_call_arguments.delta",
+            "item_id": "fc_test",
+            "output_index": 0,
+            "delta": "",
+            "sequence_number": 3,
+        }
+    )
+
+    tool_calls = result.choices[0].delta.tool_calls
+    assert tool_calls is not None
+    assert tool_calls[0].function is not None
+    assert tool_calls[0].function.arguments == ""