diff --git a/sagemaker-core/src/sagemaker/core/shapes/shapes.py b/sagemaker-core/src/sagemaker/core/shapes/shapes.py
index 5da1b83f50..ae67d598f4 100644
--- a/sagemaker-core/src/sagemaker/core/shapes/shapes.py
+++ b/sagemaker-core/src/sagemaker/core/shapes/shapes.py
@@ -10125,7 +10125,7 @@ class DataCaptureConfigSummary(Base):
     capture_status: StrPipeVar
     current_sampling_percentage: int
     destination_s3_uri: StrPipeVar
-    kms_key_id: StrPipeVar
+    kms_key_id: Optional[StrPipeVar] = Unassigned()
 
 
 class DebugRuleEvaluationStatus(Base):
diff --git a/sagemaker-core/src/sagemaker/core/tools/constants.py b/sagemaker-core/src/sagemaker/core/tools/constants.py
index 0768664920..6c1917dc6a 100644
--- a/sagemaker-core/src/sagemaker/core/tools/constants.py
+++ b/sagemaker-core/src/sagemaker/core/tools/constants.py
@@ -126,6 +126,9 @@
     "ModelPackageSecurityConfig": ["KmsKeyId"],
     # S3Uri is optional when ModelDataSource references escrow-managed artifacts (RMP).
     "S3ModelDataSource": ["S3Uri"],
+    # DescribeEndpoint omits DataCaptureConfig.KmsKeyId when data capture is enabled
+    # without a customer-managed KMS key (S3 default encryption is used instead).
+    "DataCaptureConfigSummary": ["KmsKeyId"],
 }
 
 # Members where the generated primitive type should be replaced with a PipelineVariable
diff --git a/sagemaker-core/tests/unit/generated/test_shapes.py b/sagemaker-core/tests/unit/generated/test_shapes.py
index ffe5eef5e0..cfe118d651 100644
--- a/sagemaker-core/tests/unit/generated/test_shapes.py
+++ b/sagemaker-core/tests/unit/generated/test_shapes.py
@@ -1,10 +1,13 @@
 import ast
+import datetime
 import unittest
+from unittest.mock import MagicMock, patch
 
 from pydantic import BaseModel, ValidationError
 
 import os
-from sagemaker.core.shapes import Base, AdditionalS3DataSource
+from sagemaker.core.resources import Base as ResourceBase, Endpoint
+from sagemaker.core.shapes import Base, AdditionalS3DataSource, DataCaptureConfigSummary
 from sagemaker.core.utils.utils import Unassigned
 
 # Use the installed package location
@@ -58,3 +61,54 @@ def _fetch_number_of_classes_in_file_not_inheriting_a_class(
                     if not any(base_class.id == base_class_name for base_class in node.bases):
                         count = count + 1
         return count
+
+
+class TestDataCaptureConfigSummaryOptionalKmsKeyId(unittest.TestCase):
+    """DescribeEndpoint omits DataCaptureConfig.KmsKeyId when data capture is enabled
+    without a customer-managed KMS key (issue #5738)."""
+
+    _DESCRIBE_ENDPOINT_RESPONSE = {
+        "EndpointName": "my-endpoint",
+        "EndpointArn": "arn:aws:sagemaker:us-west-2:111122223333:endpoint/my-endpoint",
+        "EndpointConfigName": "my-endpoint-config",
+        "EndpointStatus": "InService",
+        "CreationTime": datetime.datetime(2026, 1, 1),
+        "LastModifiedTime": datetime.datetime(2026, 1, 1),
+        "DataCaptureConfig": {
+            "EnableCapture": True,
+            "CaptureStatus": "Started",
+            "CurrentSamplingPercentage": 100,
+            "DestinationS3Uri": "s3://my-bucket/data-capture",
+        },
+    }
+
+    def test_shape_validates_without_kms_key_id(self):
+        summary = DataCaptureConfigSummary(
+            enable_capture=True,
+            capture_status="Started",
+            current_sampling_percentage=100,
+            destination_s3_uri="s3://my-bucket/data-capture",
+        )
+        assert isinstance(summary.kms_key_id, Unassigned)
+
+    def test_shape_accepts_kms_key_id(self):
+        summary = DataCaptureConfigSummary(
+            enable_capture=True,
+            capture_status="Started",
+            current_sampling_percentage=100,
+            destination_s3_uri="s3://my-bucket/data-capture",
+            kms_key_id="my-kms-key",
+        )
+        assert summary.kms_key_id == "my-kms-key"
+
+    def test_endpoint_get_without_kms_key_id(self):
+        client = MagicMock()
+        client.describe_endpoint.return_value = self._DESCRIBE_ENDPOINT_RESPONSE
+        # Endpoint.get() resolves its client via resources.Base.get_sagemaker_client.
+        with patch.object(ResourceBase, "get_sagemaker_client", return_value=client):
+            endpoint = Endpoint.get("my-endpoint")
+
+        client.describe_endpoint.assert_called_once_with(EndpointName="my-endpoint")
+        assert endpoint.data_capture_config.enable_capture is True
+        assert endpoint.data_capture_config.destination_s3_uri == "s3://my-bucket/data-capture"
+        assert isinstance(endpoint.data_capture_config.kms_key_id, Unassigned)
diff --git a/sagemaker-core/tests/unit/tools/test_shapes_extractor.py b/sagemaker-core/tests/unit/tools/test_shapes_extractor.py
index 4230042e0d..84cebacee7 100644
--- a/sagemaker-core/tests/unit/tools/test_shapes_extractor.py
+++ b/sagemaker-core/tests/unit/tools/test_shapes_extractor.py
@@ -435,3 +435,60 @@ def test_override_is_targeted_not_blanket(self, extractor):
         members = extractor.generate_shape_members("UnrelatedConfig")
         assert "IntPipeVar" not in members["instance_count"]
         assert "int" in members["instance_count"]
+
+
+class TestDataCaptureConfigSummaryRequiredToOptionalOverride:
+    """The service model marks DataCaptureConfigSummary.KmsKeyId as required, but
+    DescribeEndpoint omits it when data capture has no customer-managed KMS key
+    (issue #5738). REQUIRED_TO_OPTIONAL_OVERRIDES must keep codegen emitting it as
+    Optional, otherwise Endpoint.get() fails validation on such endpoints."""
+
+    _STRING = {"type": "string"}
+    _MODEL = {
+        "EnableCapture": {"type": "boolean"},
+        "CaptureStatus": _STRING,
+        "SamplingPercentage": {"type": "integer"},
+        "DestinationS3Uri": _STRING,
+        "KmsKeyId": _STRING,
+        "DataCaptureConfigSummary": {
+            "type": "structure",
+            "required": [
+                "EnableCapture",
+                "CaptureStatus",
+                "CurrentSamplingPercentage",
+                "DestinationS3Uri",
+                "KmsKeyId",
+            ],
+            "members": {
+                "EnableCapture": {"shape": "EnableCapture"},
+                "CaptureStatus": {"shape": "CaptureStatus"},
+                "CurrentSamplingPercentage": {"shape": "SamplingPercentage"},
+                "DestinationS3Uri": {"shape": "DestinationS3Uri"},
+                "KmsKeyId": {"shape": "KmsKeyId"},
+            },
+        },
+    }
+
+    @pytest.fixture
+    def extractor(self, tmp_path):
+        # Constructing the extractor regenerates shape_dag.py; point that write
+        # at a temp file so the unit test leaves the checked-in file alone.
+        with (
+            patch("sagemaker.core.tools.shapes_extractor.reformat_file_with_black"),
+            patch(
+                "sagemaker.core.tools.shapes_extractor.SHAPE_DAG_FILE_PATH",
+                str(tmp_path / "shape_dag.py"),
+            ),
+        ):
+            return ShapesExtractor(combined_shapes=self._MODEL)
+
+    def test_kms_key_id_generated_as_optional(self, extractor):
+        members = extractor.generate_shape_members("DataCaptureConfigSummary")
+        assert members["kms_key_id"] == "Optional[StrPipeVar] = Unassigned()"
+
+    def test_other_members_stay_required(self, extractor):
+        members = extractor.generate_shape_members("DataCaptureConfigSummary")
+        assert members["enable_capture"] == "bool"
+        assert members["capture_status"] == "StrPipeVar"
+        assert members["current_sampling_percentage"] == "int"
+        assert members["destination_s3_uri"] == "StrPipeVar"