diff --git a/litellm/completion_extras/litellm_responses_transformation/transformation.py b/litellm/completion_extras/litellm_responses_transformation/transformation.py
index 0974830324f..e546a0dbb02 100644
--- a/litellm/completion_extras/litellm_responses_transformation/transformation.py
+++ b/litellm/completion_extras/litellm_responses_transformation/transformation.py
@@ -296,8 +296,12 @@ class LiteLLMResponsesTransformationHandler(CompletionTransformationBridge):
                 request_data["instructions"] = instructions
             elif key == "stream_options" and isinstance(value, dict):
                 request_data["stream_options"] = value.get("include_obfuscation")
-            elif key == "user" and isinstance(value, str) and len(value) <= 64:
-                request_data["user"] = value
+            elif key == "user" and isinstance(value, str):
+                # OpenAI API requires user param to be max 64 chars - truncate if longer
+                if len(value) <= 64:
+                    request_data["user"] = value
+                else:
+                    request_data["user"] = value[:64]
             else:
                 request_data[key] = value
 
diff --git a/litellm/litellm_core_utils/exception_mapping_utils.py b/litellm/litellm_core_utils/exception_mapping_utils.py
index 03fbdd463dd..dde44cced36 100644
--- a/litellm/litellm_core_utils/exception_mapping_utils.py
+++ b/litellm/litellm_core_utils/exception_mapping_utils.py
@@ -70,6 +70,11 @@ class ExceptionCheckers:
         Check if an error string indicates a context window exceeded error.
         """
         _error_str_lowercase = error_str.lower()
+        # Exclude param validation errors (e.g. OpenAI "user" param max 64 chars)
+        if "string_above_max_length" in _error_str_lowercase:
+            return False
+        if "invalid 'user'" in _error_str_lowercase and "string too long" in _error_str_lowercase:
+            return False
         known_exception_substrings = [
             "exceed context limit",
             "this model's maximum context length is",
diff --git a/tests/llm_translation/test_openai.py b/tests/llm_translation/test_openai.py
index 6a2ad406788..eff11c9cee3 100644
--- a/tests/llm_translation/test_openai.py
+++ b/tests/llm_translation/test_openai.py
@@ -519,7 +519,7 @@ async def test_openai_codex_stream(sync_mode):
     from litellm.main import stream_chunk_builder
 
     kwargs = {
-        "model": "openai/codex-mini-latest",
+        "model": "openai/gpt-5-codex-mini",
         "messages": [{"role": "user", "content": "Hey!"}],
         "stream": True,
     }
@@ -549,16 +549,16 @@ async def test_openai_codex(sync_mode):
     router = Router(
         model_list=[
             {
-                "model_name": "openai-codex-mini-latest",
+                "model_name": "openai-gpt-5-codex-mini",
                 "litellm_params": {
-                    "model": "openai/codex-mini-latest",
+                    "model": "openai/gpt-5-codex-mini",
                 },
             }
         ]
     )
 
     kwargs = {
-        "model": "openai-codex-mini-latest",
+        "model": "openai-gpt-5-codex-mini",
         "messages": [{"role": "user", "content": "Hey!"}],
     }
 
diff --git a/tests/test_litellm/litellm_core_utils/test_exception_mapping_utils.py b/tests/test_litellm/litellm_core_utils/test_exception_mapping_utils.py
index c7ad18cfb0c..beb978584cb 100644
--- a/tests/test_litellm/litellm_core_utils/test_exception_mapping_utils.py
+++ b/tests/test_litellm/litellm_core_utils/test_exception_mapping_utils.py
@@ -77,6 +77,15 @@ context_window_test_cases = [
     ("Rate limit reached for requests.", False),
     ("The context is large, but acceptable.", False),
     ("", False),  # Empty string
+    # OpenAI user param length validation - not a context window error
+    (
+        "Invalid 'user': string too long. Expected a string with maximum length 64, but got a string with length 123 instead.",
+        False,
+    ),
+    (
+        '{"error": {"message": "Invalid \'user\': string too long.", "code": "string_above_max_length"}}',
+        False,
+    ),
 ]