diff --git a/sagemaker-serve/src/sagemaker/serve/model_builder.py b/sagemaker-serve/src/sagemaker/serve/model_builder.py
index 10a29fa5f8..4086784a9f 100644
--- a/sagemaker-serve/src/sagemaker/serve/model_builder.py
+++ b/sagemaker-serve/src/sagemaker/serve/model_builder.py
@@ -4247,25 +4247,20 @@ def _deploy_core_endpoint(self, **kwargs):
                 explainer_config_dict = explainer_config._to_request_dict()
 
             if update_endpoint:
+                # Use a unique config name: the model name is reused across updates, and
+                # endpoint config names cannot be reused.
                 endpoint_config_name = self.sagemaker_session.create_endpoint_config(
-                    name=self.model_name,
-                    model_name=self.model_name,
-                    initial_instance_count=initial_instance_count,
-                    instance_type=instance_type,
-                    accelerator_type=accelerator_type,
+                    name=unique_name_from_base(self.model_name),
+                    production_variants=[production_variant],
                     tags=tags,
                     kms_key=kms_key,
                     data_capture_config_dict=data_capture_config_dict,
-                    volume_size=volume_size,
-                    model_data_download_timeout=model_data_download_timeout,
-                    container_startup_health_check_timeout=container_startup_health_check_timeout,
-                    explainer_config_dict=explainer_config_dict,
                     async_inference_config_dict=async_inference_config_dict,
-                    serverless_inference_config_dict=serverless_inference_config_dict,
-                    routing_config=routing_config,
-                    inference_ami_version=inference_ami_version,
+                    explainer_config_dict=explainer_config_dict,
+                )
+                self.sagemaker_session.update_endpoint(
+                    self.endpoint_name, endpoint_config_name, wait=wait
                 )
-                self.sagemaker_session.update_endpoint(self.endpoint_name, endpoint_config_name)
             else:
                 self.sagemaker_session.endpoint_from_production_variants(
                     name=self.endpoint_name,
diff --git a/sagemaker-serve/tests/unit/test_model_builder_deploy.py b/sagemaker-serve/tests/unit/test_model_builder_deploy.py
index d255f42b1e..4be42a5954 100644
--- a/sagemaker-serve/tests/unit/test_model_builder_deploy.py
+++ b/sagemaker-serve/tests/unit/test_model_builder_deploy.py
@@ -3,6 +3,7 @@
 Focuses on increasing coverage for deploy-related functionality.
 """
 
+import inspect
 import unittest
 from unittest.mock import Mock, patch
 
@@ -10,6 +11,7 @@
 from sagemaker.serve.utils.types import ModelServer
 from sagemaker.serve.mode.function_pointers import Mode
 from sagemaker.core.resources import Model, Endpoint
+from sagemaker.core.helper.session_helper import Session
 from sagemaker.core.enums import EndpointType
 from sagemaker.core.inference_config import (
     ResourceRequirements,
@@ -299,6 +301,41 @@ def test_deploy_core_endpoint_invalid_async_config_raises_error(self):
 
         self.assertIn("AsyncInferenceConfig object", str(context.exception))
 
+    @patch("sagemaker.serve.model_builder.Endpoint.get")
+    def test_deploy_core_endpoint_update_endpoint_creates_config_from_variants(
+        self, mock_endpoint_get
+    ):
+        """update_endpoint=True must call Session.create_endpoint_config with its real signature."""
+        builder = ModelBuilder(
+            model=Mock(),
+            role_arn="arn:aws:iam::123456789012:role/TestRole",
+            sagemaker_session=self.mock_session,
+        )
+        builder.built_model = Mock()
+        builder.model_name = "demo-model"
+        self.mock_session.create_endpoint_config.return_value = "demo-model-config"
+
+        builder._deploy_core_endpoint(
+            endpoint_name="demo-endpoint",
+            instance_type="ml.g5.xlarge",
+            initial_instance_count=1,
+            wait=False,
+            update_endpoint=True,
+        )
+
+        call_kwargs = self.mock_session.create_endpoint_config.call_args.kwargs
+        inspect.signature(Session.create_endpoint_config).bind(None, **call_kwargs)
+        self.assertTrue(call_kwargs["name"].startswith("demo-model"))
+        self.assertNotEqual(call_kwargs["name"], "demo-model")
+        [variant] = call_kwargs["production_variants"]
+        self.assertEqual(variant["ModelName"], "demo-model")
+        self.assertEqual(variant["InstanceType"], "ml.g5.xlarge")
+        self.assertEqual(variant["InitialInstanceCount"], 1)
+        self.mock_session.update_endpoint.assert_called_once_with(
+            "demo-endpoint", "demo-model-config", wait=False
+        )
+        self.mock_session.endpoint_from_production_variants.assert_not_called()
+
     def test_deploy_core_endpoint_invalid_serverless_config_raises_error(self):
         """Test _deploy_core_endpoint raises error for invalid serverless config."""
         builder = ModelBuilder(