diff --git a/sagemaker-train/src/sagemaker/train/common_utils/finetune_utils.py b/sagemaker-train/src/sagemaker/train/common_utils/finetune_utils.py
index 9048204cc6..8e4734f01b 100644
--- a/sagemaker-train/src/sagemaker/train/common_utils/finetune_utils.py
+++ b/sagemaker-train/src/sagemaker/train/common_utils/finetune_utils.py
@@ -12,6 +12,9 @@
 from sagemaker.core.helper.session_helper import Session
 from sagemaker.core.s3.utils import resolve_s3_uri_placeholders
 from sagemaker.train.common_utils.recipe_utils import _get_hub_content_metadata
+# Single source of truth for Lambda-ARN detection, shared with the reward verifier
+# so both code paths agree on what counts as a Lambda ARN.
+from sagemaker.train.common_utils.rlvr_reward_verifier import LAMBDA_ARN_REGEX
 from sagemaker.train.common import TrainingType, CustomizationTechnique, JOB_TYPE, FineTuningOptions
 from sagemaker.core.shapes import ServerlessJobConfig, Channel, DataSource, ModelPackageConfig, MlflowConfig
 from sagemaker.core.training.configs import HyperPodCompute, TrainingJobCompute
diff --git a/sagemaker-train/tests/unit/train/common_utils/test_finetune_utils.py b/sagemaker-train/tests/unit/train/common_utils/test_finetune_utils.py
index ead56fa65d..c69cbd07d7 100644
--- a/sagemaker-train/tests/unit/train/common_utils/test_finetune_utils.py
+++ b/sagemaker-train/tests/unit/train/common_utils/test_finetune_utils.py
@@ -16,6 +16,7 @@
     _get_default_s3_output_path,
     _extract_dataset_source,
     _extract_evaluator_arn,
+    _is_lambda_arn,
     _resolve_model_name,
     _resolve_model_package_arn,
     _get_fine_tuning_options_and_model_arn,
@@ -1332,3 +1333,33 @@ def test_missing_hyperpod_cli_raises_runtime_error(
                 fu.get_hyperpod_recipe_path(
                     "nova-lite", "SFT", "LORA", session, job_name="myjob"
                 )
+
+
+class TestIsLambdaArn:
+    """Regression coverage for _is_lambda_arn (the LAMBDA_ARN_REGEX constant
+    was previously undefined, raising NameError at call time)."""
+
+    def test_valid_lambda_arn(self):
+        assert _is_lambda_arn(
+            "arn:aws:lambda:us-west-2:123456789012:function:my-reward-fn"
+        ) is True
+
+    def test_valid_lambda_arn_aws_partition_variants(self):
+        assert _is_lambda_arn(
+            "arn:aws-us-gov:lambda:us-gov-west-1:123456789012:function:fn"
+        ) is True
+
+    def test_evaluator_hub_content_arn_is_not_lambda(self):
+        assert _is_lambda_arn(
+            "arn:aws:sagemaker:us-west-2:123456789012:hub-content/"
+            "SageMakerPublicHub/JsonDoc/my-evaluator/1.0"
+        ) is False
+
+    def test_arbitrary_string_is_not_lambda(self):
+        assert _is_lambda_arn("not-an-arn") is False
+
+    def test_uses_shared_regex_from_reward_verifier(self):
+        # Both call sites must agree on what a Lambda ARN is; assert they
+        # share the same compiled pattern rather than diverging copies.
+        from sagemaker.train.common_utils import rlvr_reward_verifier
+        assert fu.LAMBDA_ARN_REGEX is rlvr_reward_verifier.LAMBDA_ARN_REGEX