diff --git a/libs/partners/openai/langchain_openai/chat_models/base.py b/libs/partners/openai/langchain_openai/chat_models/base.py
index 56d9db2b7e2..790867cae53 100644
--- a/libs/partners/openai/langchain_openai/chat_models/base.py
+++ b/libs/partners/openai/langchain_openai/chat_models/base.py
@@ -826,14 +826,15 @@ class BaseChatOpenAI(BaseChatModel):
             (Defaults to 1)
         """
         model = values.get("model_name") or values.get("model") or ""
+        model_lower = model.lower()
 
         # For o1 models, set temperature=1 if not provided
-        if model.startswith("o1") and "temperature" not in values:
+        if model_lower.startswith("o1") and "temperature" not in values:
             values["temperature"] = 1
 
         # For gpt-5 models, handle temperature restrictions
         # Note that gpt-5-chat models do support temperature
-        if model.startswith("gpt-5") and "chat" not in model:
+        if model_lower.startswith("gpt-5") and "chat" not in model_lower:
             temperature = values.get("temperature")
             if temperature is not None and temperature != 1:
                 # For gpt-5 (non-chat), only temperature=1 is supported
@@ -1668,15 +1669,13 @@ class BaseChatOpenAI(BaseChatModel):
             model = self.tiktoken_model_name
         else:
             model = self.model_name
+
         try:
             encoding = tiktoken.encoding_for_model(model)
         except KeyError:
+            model_lower = model.lower()
             encoder = "cl100k_base"
-            if (
-                self.model_name.startswith("gpt-4o")
-                or self.model_name.startswith("gpt-4.1")
-                or self.model_name.startswith("gpt-5")
-            ):
+            if model_lower.startswith(("gpt-4o", "gpt-4.1", "gpt-5")):
                 encoder = "o200k_base"
             encoding = tiktoken.get_encoding(encoder)
         return model, encoding
diff --git a/libs/partners/openai/tests/unit_tests/chat_models/test_base.py b/libs/partners/openai/tests/unit_tests/chat_models/test_base.py
index b09a8b93b6e..8911290dc6f 100644
--- a/libs/partners/openai/tests/unit_tests/chat_models/test_base.py
+++ b/libs/partners/openai/tests/unit_tests/chat_models/test_base.py
@@ -3026,3 +3026,33 @@ def test_gpt_5_temperature(use_responses_api: bool) -> None:
     messages = [HumanMessage(content="Hello")]
     payload = llm._get_request_payload(messages)
     assert payload["temperature"] == 0.5  # gpt-5-chat is exception
+
+
+@pytest.mark.parametrize("use_responses_api", [False, True])
+@pytest.mark.parametrize(
+    "model_name",
+    [
+        "GPT-5-NANO",
+        "GPT-5-2025-01-01",
+        "Gpt-5-Turbo",
+        "gPt-5-mini",
+    ],
+)
+def test_gpt_5_temperature_case_insensitive(
+    use_responses_api: bool, model_name: str
+) -> None:
+    llm = ChatOpenAI(
+        model=model_name, temperature=0.5, use_responses_api=use_responses_api
+    )
+
+    messages = [HumanMessage(content="Hello")]
+    payload = llm._get_request_payload(messages)
+    assert "temperature" not in payload
+
+    for chat_model in ["GPT-5-CHAT", "Gpt-5-Chat", "gpt-5-chat"]:
+        llm = ChatOpenAI(
+            model=chat_model, temperature=0.7, use_responses_api=use_responses_api
+        )
+        messages = [HumanMessage(content="Hello")]
+        payload = llm._get_request_payload(messages)
+        assert payload["temperature"] == 0.7