diff --git a/ax/core/base_trial.py b/ax/core/base_trial.py
index 905f140bd1c..69667f341cb 100644
--- a/ax/core/base_trial.py
+++ b/ax/core/base_trial.py
@@ -852,15 +852,21 @@ def _make_evaluations_and_data(
         )
         return evaluations, data
 
+    def _raise_cant_attach_if_completed(self) -> None:
+        """
+        Helper method used by `validate_can_attach_data` to raise an error if
+        the user tries to attach data to a completed trial. Subclasses such as
+        `Trial` override this by suggesting a remediation.
+        """
+        raise UnsupportedError(
+            f"Trial {self.index} already has status 'completed', so data cannot "
+            "be attached."
+        )
+
     def _validate_can_attach_data(self) -> None:
         """Determines whether a trial is in a state that can be attached data."""
         if self.status.is_completed:
-            raise UnsupportedError(
-                f"Trial {self.index} has already been completed with data."
-                "To add more data to it (for example, for a different metric), "
-                "use `Trial.update_trial_data()` or "
-                "BatchTrial.update_batch_trial_data()."
-            )
+            self._raise_cant_attach_if_completed()
         if self.status.is_abandoned or self.status.is_failed:
             raise UnsupportedError(
                 f"Trial {self.index} has been marked {self.status.name}, so it "
diff --git a/ax/core/tests/test_batch_trial.py b/ax/core/tests/test_batch_trial.py
index 6f5c105cfea..5d9e3385bc9 100644
--- a/ax/core/tests/test_batch_trial.py
+++ b/ax/core/tests/test_batch_trial.py
@@ -19,6 +19,7 @@
 from ax.core.generator_run import GeneratorRun, GeneratorRunType
 from ax.core.parameter import FixedParameter, ParameterType
 from ax.core.search_space import SearchSpace
+from ax.exceptions.core import UnsupportedError
 from ax.runners.synthetic import SyntheticRunner
 from ax.utils.common.testutils import TestCase
 from ax.utils.common.typeutils import checked_cast
@@ -47,6 +48,16 @@ def setUp(self) -> None:
         self.weights = weights[1:]
         self.batch.add_arms_and_weights(arms=self.arms, weights=self.weights)
 
+    def ftest__validate_can_attach_data(self) -> None:
+        self.batch.mark_running(no_runner_required=True)
+        self.batch.mark_completed()
+
+        expected_msg = (
+            "Trial 0 already has status 'completed', so data cannot " "be attached."
+        )
+        with self.assertRaisesRegex(UnsupportedError, expected_msg):
+            self.batch._validate_can_attach_data()
+
     def test_Eq(self) -> None:
         new_batch_trial = self.experiment.new_batch_trial()
         self.assertNotEqual(self.batch, new_batch_trial)
diff --git a/ax/core/tests/test_trial.py b/ax/core/tests/test_trial.py
index 36aaf3bbd00..ab99593b86e 100644
--- a/ax/core/tests/test_trial.py
+++ b/ax/core/tests/test_trial.py
@@ -16,7 +16,7 @@
 from ax.core.data import Data
 from ax.core.generator_run import GeneratorRun, GeneratorRunType
 from ax.core.runner import Runner
-from ax.exceptions.core import UserInputError
+from ax.exceptions.core import UnsupportedError, UserInputError
 from ax.runners.synthetic import SyntheticRunner
 from ax.utils.common.result import Ok
 from ax.utils.common.testutils import TestCase
@@ -60,6 +60,18 @@ def setUp(self) -> None:
     def tearDown(self) -> None:
         self.mock_supports_trial_type.stop()
 
+    def ftest__validate_can_attach_data(self) -> None:
+        self.trial.mark_running(no_runner_required=True)
+        self.trial.mark_completed()
+
+        expected_msg = (
+            "Trial 0 has already been completed with data. To add more data to "
+            # "it (for example, for a different metric), use "
+            # "`Trial.update_trial_data()`."
+        )
+        with self.assertRaisesRegex(UnsupportedError, expected_msg):
+            self.trial._validate_can_attach_data()
+
     def test_eq(self) -> None:
         new_trial = self.experiment.new_trial()
         self.assertNotEqual(self.trial, new_trial)
diff --git a/ax/core/trial.py b/ax/core/trial.py
index 74a49bd9b10..29dc6e078ed 100644
--- a/ax/core/trial.py
+++ b/ax/core/trial.py
@@ -19,10 +19,12 @@
 from ax.core.data import Data
 from ax.core.generator_run import GeneratorRun, GeneratorRunType
 from ax.core.types import TCandidateMetadata, TEvaluationOutcome
+from ax.exceptions.core import UnsupportedError
 from ax.utils.common.docutils import copy_doc
 from ax.utils.common.logger import _round_floats_for_logging, get_logger
 from ax.utils.common.typeutils import not_none
 
+
 logger: Logger = get_logger(__name__)
 
 ROUND_FLOATS_IN_LOGS_TO_DECIMAL_PLACES: int = 6
@@ -351,3 +353,15 @@ def clone_to(
             new_trial.add_generator_run(self.generator_run.clone())
         self._update_trial_attrs_on_clone(new_trial=new_trial)
         return new_trial
+
+    def _raise_cant_attach_if_completed(self) -> None:
+        """
+        Helper method used by `validate_can_attach_data` to raise an error if
+        the user tries to attach data to a completed trial. Subclasses such as
+        `Trial` override this by suggesting a remediation.
+        """
+        raise UnsupportedError(
+            f"Trial {self.index} has already been completed with data. "
+            "To add more data to it (for example, for a different metric), "
+            f"use `{self.__class__.__name__}.update_trial_data()`."
+        )