From b248b1c7dc12c194c4e1e176b9432391ef5ae5ab Mon Sep 17 00:00:00 2001
From: "devin-ai-integration[bot]"
 <158243242+devin-ai-integration[bot]@users.noreply.github.com>
Date: Sat, 26 Sep 2026 16:09:09 -0700
Subject: [PATCH] fix(openai): exclude fine-tuned and custom gpt-5-chat aliases
 from gpt-5 reasoning path (#43185)

* fix(openai): exclude fine-tuned and custom gpt-5-chat aliases from gpt-5 reasoning path

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* test(openai): keep gpt-5-chat alias regression test diff minimal

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* test(openai): cover temperature pass-through for gpt-5-chat aliases

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* test(openai): annotate locals and wrap long lines in gpt-5-chat alias test

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

---------

Co-authored-by: mateo <mateo@berri.ai>
Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
---
 .../llms/openai/chat/gpt_5_transformation.py  |  2 +-
 .../llms/openai/test_is_model_gpt_5_model.py  | 34 +++++++++++++++++--
 2 files changed, 32 insertions(+), 4 deletions(-)

diff --git a/litellm/llms/openai/chat/gpt_5_transformation.py b/litellm/llms/openai/chat/gpt_5_transformation.py
index d0e5ff01e71..bf6b52225f2 100644
--- a/litellm/llms/openai/chat/gpt_5_transformation.py
+++ b/litellm/llms/openai/chat/gpt_5_transformation.py
@@ -69,7 +69,7 @@ GPT_REASONING_SERIES_MARKERS: Final = ("gpt-5", "gpt-6")
 
 def is_gpt_reasoning_series_name(model: str) -> bool:
     normalized: Final = model.split("/")[-1]
-    return any(marker in model for marker in GPT_REASONING_SERIES_MARKERS) and not normalized.startswith("gpt-5-chat")
+    return any(marker in model for marker in GPT_REASONING_SERIES_MARKERS) and "gpt-5-chat" not in normalized
 
 
 class OpenAIGPT5Config(OpenAIGPTConfig):
diff --git a/tests/unit/llms/openai/test_is_model_gpt_5_model.py b/tests/unit/llms/openai/test_is_model_gpt_5_model.py
index 0bb8425d95e..f6fef92fc6a 100644
--- a/tests/unit/llms/openai/test_is_model_gpt_5_model.py
+++ b/tests/unit/llms/openai/test_is_model_gpt_5_model.py
@@ -26,14 +26,18 @@ There are two distinct families:
   ``gpt-5.3-chat``, …) — ARE GPT-5 reasoning models and must stay on the GPT-5
   path.
 
-The fix uses a prefix check (``startswith("gpt-5-chat")``) on the normalised model
-name instead of a substring check, which correctly distinguishes the two families.
+The fix uses a substring check for ``gpt-5-chat`` on the normalised model
+name (not a prefix check), which correctly distinguishes the two families.
 """
 
+from typing import Final
+
 import pytest
 
-from litellm.llms.openai.chat.gpt_5_transformation import OpenAIGPT5Config
+import litellm
 from litellm.llms.azure.chat.gpt_5_transformation import AzureOpenAIGPT5Config
+from litellm.llms.openai.chat.gpt_5_transformation import OpenAIGPT5Config
+from litellm.llms.openai.responses.transformation import OpenAIResponsesAPIConfig
 
 # ---------------------------------------------------------------------------
 # Parametrized fixtures
@@ -73,6 +77,9 @@ NON_GPT5_MODELS = [
     "gpt-5-chat",  # gpt-5-chat family — regular chat path
     "gpt-5-chat-latest",  # gpt-5-chat family with alias suffix
     "gpt-5-chat-2025-08-07",  # gpt-5-chat family with date suffix
+    "ft:gpt-5-chat-latest:org:abc",
+    "my-custom-gpt-5-chat",
+    "openai/ft:gpt-5-chat-latest:org:abc",
     "gpt-4",
     "gpt-4o",
     "gpt-4-turbo",
@@ -117,6 +124,27 @@ class TestOpenAIGPT5ConfigIsModelGpt5Model:
                 model
             ), f"Expected '{model}' (gpt-5-chat family) NOT to be on the GPT-5 path"
 
+    def test_responses_api_gpt5_chat_aliases_are_not_gpt5(self):
+        for model in ["ft:gpt-5-chat-latest:org:abc", "openai/my-custom-gpt-5-chat"]:
+            assert not OpenAIResponsesAPIConfig._is_gpt_5_model(
+                model
+            ), f"Expected Responses API '{model}' NOT to be on the GPT-5 path"
+
+    @pytest.mark.parametrize("model", ["ft:gpt-5-chat-latest:org:abc", "my-custom-gpt-5-chat"])
+    def test_gpt5_chat_aliases_keep_non_default_temperature(self, model: str):
+        chat_params: Final = litellm.get_optional_params(
+            model=model, custom_llm_provider="openai", temperature=0.7
+        )
+        responses_params: Final = OpenAIResponsesAPIConfig().map_openai_params(
+            response_api_optional_params={"temperature": 0.7}, model=model, drop_params=False
+        )
+        assert chat_params["temperature"] == 0.7, (
+            f"chat completions dropped or rejected temperature for '{model}'"
+        )
+        assert responses_params["temperature"] == 0.7, (
+            f"responses dropped or rejected temperature for '{model}'"
+        )
+
 
 # Models that are gpt-5.4 or newer. main.py gates the automatic switch to the
 # /v1/responses bridge (when reasoning_effort is set and tools are passed) on