diff --git a/litellm/llms/openai/chat/gpt_reasoning_transformation.py b/litellm/llms/openai/chat/gpt_reasoning_transformation.py
index 0311f6c6590..03dcfdb726e 100644
--- a/litellm/llms/openai/chat/gpt_reasoning_transformation.py
+++ b/litellm/llms/openai/chat/gpt_reasoning_transformation.py
@@ -78,7 +78,7 @@ def unsupported_reasoning_effort_message(param: str, level: str, model: str) ->
 
 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 "gpt-5-chat" not in normalized
+    return any(marker in normalized 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 39d7c9da2ce..b941af27b76 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
@@ -87,6 +87,10 @@ NON_GPT5_MODELS = [
     "o1",
     "o3",
     "o3-mini",
+    "gpt-5-gateway/claude-3-5-sonnet",
+    "gpt-5-team/llama-3.3-70b",
+    "gpt-6-proxy/mistral-large",
+    "my-gpt-5-org/gemini-2.0-flash",
 ]