From 462ff24185d46c33e50f94e94f439d721ff92671 Mon Sep 17 00:00:00 2001
From: mo <mosafariuk@gmail.com>
Date: Tue, 27 Oct 2026 11:53:11 +0000
Subject: [PATCH] fix(openai): don't consume iterator arguments before calling
 OpenAI

OpenAI accepts any iterable for messages. Langfuse read a generator while extracting the prompt and then passed the exhausted generator to OpenAI, so the request went out with messages=[] and the API returned a 400. One-shot iterators in the call kwargs are now turned into lists before they are read.
---
 langfuse/openai.py        |  8 +++++++-
 tests/unit/test_openai.py | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+), 1 deletion(-)

diff --git a/langfuse/openai.py b/langfuse/openai.py
index 32637569b..789afe87b 100644
--- a/langfuse/openai.py
+++ b/langfuse/openai.py
@@ -27,6 +27,7 @@
 import os
 import types
 from collections import defaultdict
+from collections.abc import Iterator
 from dataclasses import dataclass
 from datetime import datetime
 from inspect import isawaitable, isclass
@@ -311,7 +312,12 @@ def __init__(
         self.args["trace_id"] = trace_id
         self.args["parent_observation_id"] = parent_observation_id
 
-        self.kwargs = kwargs
+        # Materialize one-shot iterators (e.g. a generator of messages) so that
+        # reading them for tracing does not leave OpenAI with an empty argument.
+        self.kwargs = {
+            key: list(value) if isinstance(value, Iterator) else value
+            for key, value in kwargs.items()
+        }
 
     def get_langfuse_args(self) -> Any:
         return {**self.args, **self.kwargs}
diff --git a/tests/unit/test_openai.py b/tests/unit/test_openai.py
index a2a4f7f4f..f531ea8c4 100644
--- a/tests/unit/test_openai.py
+++ b/tests/unit/test_openai.py
@@ -372,6 +372,43 @@ def test_chat_completion_exports_generation_span(
     }
 
 
+def test_chat_completion_does_not_consume_messages_iterator(
+    langfuse_memory_client, get_span, json_attr
+):
+    openai_client = lf_openai.OpenAI(api_key="test")
+    response = ChatCompletion.model_validate(
+        {
+            "id": "chatcmpl-1",
+            "object": "chat.completion",
+            "created": 1,
+            "model": "gpt-4o-mini",
+            "choices": [
+                {
+                    "index": 0,
+                    "message": {"role": "assistant", "content": "hi"},
+                    "finish_reason": "stop",
+                }
+            ],
+        }
+    )
+    messages = [{"role": "user", "content": "hello"}]
+
+    with patch.object(
+        openai_client.chat.completions, "_post", return_value=response
+    ) as post:
+        openai_client.chat.completions.create(
+            name="unit-openai-iterator",
+            model="gpt-4o-mini",
+            messages=(message for message in messages),
+        )
+
+    assert post.call_args.kwargs["body"]["messages"] == messages
+
+    langfuse_memory_client.flush()
+    span = get_span("unit-openai-iterator")
+    assert json_attr(span, LangfuseOtelSpanAttributes.OBSERVATION_INPUT) == messages
+
+
 def test_chat_completion_with_none_choices_does_not_crash(
     langfuse_memory_client, get_span, json_attr
 ):