From 68a91a254395e85496b82659881ece09a5d24f00 Mon Sep 17 00:00:00 2001
From: Rolando Bosch <rbosch@lpci.ai>
Date: Tue, 7 Jul 2026 16:09:21 -0400
Subject: [PATCH 1/2] fix(openai): handle None arguments in legacy
 function_call streaming merge

---
 langfuse/openai.py        |  4 ++-
 tests/unit/test_openai.py | 69 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 72 insertions(+), 1 deletion(-)

diff --git a/langfuse/openai.py b/langfuse/openai.py
index ea9dcd68c..6dcdb3e67 100644
--- a/langfuse/openai.py
+++ b/langfuse/openai.py
@@ -811,7 +811,9 @@ def _extract_streamed_openai_response(resource: Any, chunks: Any) -> Any:
                         curr["name"] = curr["name"] or getattr(
                             tool_call_chunk, "name", None
                         )
-                        curr["arguments"] += getattr(tool_call_chunk, "arguments", "")
+                        curr["arguments"] = (curr.get("arguments") or "") + (
+                            getattr(tool_call_chunk, "arguments", None) or ""
+                        )
 
                 if (
                     delta.get("tool_calls", None) is not None
diff --git a/tests/unit/test_openai.py b/tests/unit/test_openai.py
index 681be4fbf..af9f43e9b 100644
--- a/tests/unit/test_openai.py
+++ b/tests/unit/test_openai.py
@@ -401,6 +401,75 @@ def test_streaming_chat_completion_preserves_tool_calls_after_content():
     assert metadata == {"finish_reason": "tool_calls"}
 
 
+def _make_chat_stream_chunks_with_legacy_function_call_none_arguments():
+    """Mirrors a real OpenAI-compatible-proxy payload shape (see PR #1339,
+    which fixed the identical crash for the newer `tool_calls` array but
+    left this deprecated `function_call` merge branch unpatched): the first
+    streamed delta carries a `function_call` with `arguments=None`, and a
+    later delta appends string arguments to it.
+    """
+    return [
+        SimpleNamespace(
+            model="gpt-4o-mini",
+            choices=[
+                SimpleNamespace(
+                    delta=SimpleNamespace(
+                        role="assistant",
+                        content=None,
+                        function_call=SimpleNamespace(
+                            name="get_weather", arguments=None
+                        ),
+                        tool_calls=None,
+                    ),
+                    finish_reason=None,
+                )
+            ],
+            usage=None,
+        ),
+        SimpleNamespace(
+            model="gpt-4o-mini",
+            choices=[
+                SimpleNamespace(
+                    delta=SimpleNamespace(
+                        role=None,
+                        content=None,
+                        function_call=SimpleNamespace(
+                            name=None, arguments='{"city": "Berlin"}'
+                        ),
+                        tool_calls=None,
+                    ),
+                    finish_reason="function_call",
+                )
+            ],
+            usage=None,
+        ),
+    ]
+
+
+def test_streaming_chat_completion_handles_legacy_function_call_none_arguments():
+    """Regression test: the deprecated `function_call` streaming-merge branch
+    must not crash when the first delta's `arguments` is None (a real
+    OpenAI-compatible-proxy behaviour, analogous to the `tool_calls` bug
+    fixed in PR #1339 but never patched for this legacy branch).
+    """
+    model, completion, usage, metadata = (
+        lf_openai_module._extract_streamed_openai_response(
+            SimpleNamespace(type="chat"),
+            _make_chat_stream_chunks_with_legacy_function_call_none_arguments(),
+        )
+    )
+
+    assert model == "gpt-4o-mini"
+    assert completion == {
+        "role": "assistant",
+        "function_call": {
+            "name": "get_weather",
+            "arguments": '{"city": "Berlin"}',
+        },
+    }
+    assert metadata == {"finish_reason": "function_call"}
+
+
 def test_response_api_output_serializes_openai_parsed_response_objects():
     class ParsedOutput(BaseModel):
         name: str

From 94b33dda2753918cf03b7a1ae5a785d86221cc08 Mon Sep 17 00:00:00 2001
From: Rolando Bosch <rbosch@lpci.ai>
Date: Fri, 10 Jul 2026 21:22:13 -0400
Subject: [PATCH 2/2] test(openai): rebase legacy function_call regression test
 onto #1751 arity

_extract_streamed_openai_response now returns a 5-tuple (adds
service_tier, upstream #1751) instead of 4. Update this test's unpack
to match; the bug assertions (model/completion/metadata content) are
unchanged.
---
 tests/unit/test_openai.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/unit/test_openai.py b/tests/unit/test_openai.py
index af9f43e9b..dff020424 100644
--- a/tests/unit/test_openai.py
+++ b/tests/unit/test_openai.py
@@ -452,7 +452,7 @@ def test_streaming_chat_completion_handles_legacy_function_call_none_arguments()
     OpenAI-compatible-proxy behaviour, analogous to the `tool_calls` bug
     fixed in PR #1339 but never patched for this legacy branch).
     """
-    model, completion, usage, metadata = (
+    model, completion, usage, metadata, _service_tier = (
         lf_openai_module._extract_streamed_openai_response(
             SimpleNamespace(type="chat"),
             _make_chat_stream_chunks_with_legacy_function_call_none_arguments(),