diff --git a/src/google/adk/evaluation/_eval_set_results_manager_utils.py b/src/google/adk/evaluation/_eval_set_results_manager_utils.py
index 8505e68d..655c9ec3 100644
--- a/src/google/adk/evaluation/_eval_set_results_manager_utils.py
+++ b/src/google/adk/evaluation/_eval_set_results_manager_utils.py
@@ -14,8 +14,11 @@
 
 from __future__ import annotations
 
+import json
 import time
 
+from pydantic import ValidationError
+
 from .eval_result import EvalCaseResult
 from .eval_result import EvalSetResult
 
@@ -42,3 +45,25 @@ def create_eval_set_result(
       creation_timestamp=timestamp,
   )
   return eval_set_result
+
+
+def parse_eval_set_result_json(
+    eval_set_result_json: str | bytes,
+) -> EvalSetResult:
+  """Parses an EvalSetResult from JSON.
+
+  This is backward-compatible with legacy eval set result files that were
+  double-encoded, where the outer JSON is a string containing the inner JSON
+  object.
+  """
+  try:
+    return EvalSetResult.model_validate_json(eval_set_result_json)
+  except (ValidationError, ValueError) as first_error:
+    try:
+      decoded = json.loads(eval_set_result_json)
+    except json.JSONDecodeError:
+      raise first_error
+
+    if isinstance(decoded, str):
+      return EvalSetResult.model_validate_json(decoded)
+    return EvalSetResult.model_validate(decoded)
diff --git a/src/google/adk/evaluation/gcs_eval_set_results_manager.py b/src/google/adk/evaluation/gcs_eval_set_results_manager.py
index 860d932f..05e17d22 100644
--- a/src/google/adk/evaluation/gcs_eval_set_results_manager.py
+++ b/src/google/adk/evaluation/gcs_eval_set_results_manager.py
@@ -22,6 +22,7 @@ from typing_extensions import override
 
 from ..errors.not_found_error import NotFoundError
 from ._eval_set_results_manager_utils import create_eval_set_result
+from ._eval_set_results_manager_utils import parse_eval_set_result_json
 from .eval_result import EvalCaseResult
 from .eval_result import EvalSetResult
 from .eval_set_results_manager import EvalSetResultsManager
@@ -101,7 +102,7 @@ class GcsEvalSetResultsManager(EvalSetResultsManager):
     if not blob.exists():
       raise NotFoundError(f"Eval set result `{eval_set_result_id}` not found.")
     eval_set_result_data = blob.download_as_text()
-    return EvalSetResult.model_validate_json(eval_set_result_data)
+    return parse_eval_set_result_json(eval_set_result_data)
 
   @override
   def list_eval_set_results(self, app_name: str) -> list[str]:
diff --git a/src/google/adk/evaluation/local_eval_set_results_manager.py b/src/google/adk/evaluation/local_eval_set_results_manager.py
index d1e597c9..2eddb772 100644
--- a/src/google/adk/evaluation/local_eval_set_results_manager.py
+++ b/src/google/adk/evaluation/local_eval_set_results_manager.py
@@ -14,7 +14,6 @@
 
 from __future__ import annotations
 
-import json
 import logging
 import os
 
@@ -22,6 +21,7 @@ from typing_extensions import override
 
 from ..errors.not_found_error import NotFoundError
 from ._eval_set_results_manager_utils import create_eval_set_result
+from ._eval_set_results_manager_utils import parse_eval_set_result_json
 from .eval_result import EvalCaseResult
 from .eval_result import EvalSetResult
 from .eval_set_results_manager import EvalSetResultsManager
@@ -54,14 +54,13 @@ class LocalEvalSetResultsManager(EvalSetResultsManager):
     if not os.path.exists(app_eval_history_dir):
       os.makedirs(app_eval_history_dir)
     # Convert to json and write to file.
-    eval_set_result_json = eval_set_result.model_dump_json()
     eval_set_result_file_path = os.path.join(
         app_eval_history_dir,
         eval_set_result.eval_set_result_name + _EVAL_SET_RESULT_FILE_EXTENSION,
     )
     logger.info("Writing eval result to file: %s", eval_set_result_file_path)
     with open(eval_set_result_file_path, "w", encoding="utf-8") as f:
-      f.write(json.dumps(eval_set_result_json, indent=2))
+      f.write(eval_set_result.model_dump_json(indent=2))
 
   @override
   def get_eval_set_result(
@@ -79,8 +78,8 @@ class LocalEvalSetResultsManager(EvalSetResultsManager):
     if not os.path.exists(maybe_eval_result_file_path):
       raise NotFoundError(f"Eval set result `{eval_set_result_id}` not found.")
     with open(maybe_eval_result_file_path, "r", encoding="utf-8") as file:
-      eval_result_data = json.load(file)
-    return EvalSetResult.model_validate_json(eval_result_data)
+      eval_result_data = file.read()
+    return parse_eval_set_result_json(eval_result_data)
 
   @override
   def list_eval_set_results(self, app_name: str) -> list[str]:
diff --git a/tests/unittests/evaluation/test_gcs_eval_set_results_manager.py b/tests/unittests/evaluation/test_gcs_eval_set_results_manager.py
index 7fd0bb97..ab04ace1 100644
--- a/tests/unittests/evaluation/test_gcs_eval_set_results_manager.py
+++ b/tests/unittests/evaluation/test_gcs_eval_set_results_manager.py
@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import json
+
 from google.adk.errors.not_found_error import NotFoundError
 from google.adk.evaluation._eval_set_results_manager_utils import _sanitize_eval_set_result_name
 from google.adk.evaluation._eval_set_results_manager_utils import create_eval_set_result
@@ -165,6 +167,33 @@ class TestGcsEvalSetResultsManager:
     )
     assert retrieved_eval_set_result == eval_set_result
 
+  def test_get_eval_set_result_double_encoded_legacy(
+      self, gcs_eval_set_results_manager, mocker
+  ):
+    mocker.patch("time.time", return_value=12345678)
+    app_name = "test_app"
+    eval_set_id = "test_eval_set"
+    eval_case_results = _get_test_eval_case_results()
+    eval_set_result = create_eval_set_result(
+        app_name, eval_set_id, eval_case_results
+    )
+
+    blob_name = gcs_eval_set_results_manager._get_eval_set_result_blob_name(
+        app_name, eval_set_result.eval_set_result_id
+    )
+    blob = gcs_eval_set_results_manager.bucket.blob(blob_name)
+    double_encoded_json = json.dumps(eval_set_result.model_dump_json())
+    blob.upload_from_string(
+        double_encoded_json, content_type="application/json"
+    )
+
+    retrieved_eval_set_result = (
+        gcs_eval_set_results_manager.get_eval_set_result(
+            app_name, eval_set_result.eval_set_result_id
+        )
+    )
+    assert retrieved_eval_set_result == eval_set_result
+
   def test_list_eval_set_results(self, gcs_eval_set_results_manager, mocker):
     mocker.patch("time.time", return_value=123)
     app_name = "test_app"
diff --git a/tests/unittests/evaluation/test_local_eval_set_results_manager.py b/tests/unittests/evaluation/test_local_eval_set_results_manager.py
index 45500d71..2bec0b64 100644
--- a/tests/unittests/evaluation/test_local_eval_set_results_manager.py
+++ b/tests/unittests/evaluation/test_local_eval_set_results_manager.py
@@ -85,11 +85,12 @@ class TestLocalEvalSetResultsManager:
     )
     assert os.path.exists(expected_file_path)
     with open(expected_file_path, "r") as f:
-      actual_eval_set_result_json = json.load(f)
+      actual_eval_set_result_data = json.load(f)
 
-    # need to convert eval_set_result to json
-    expected_eval_set_result_json = self.eval_set_result.model_dump_json()
-    assert expected_eval_set_result_json == actual_eval_set_result_json
+    # Verify the file contains a proper JSON object (not double-encoded)
+    # Use mode='json' to serialize enums to their values for comparison
+    expected_eval_set_result_data = self.eval_set_result.model_dump(mode="json")
+    assert expected_eval_set_result_data == actual_eval_set_result_data
 
   def test_get_eval_set_result(self, mocker):
     mock_time = mocker.patch("time.time")
@@ -102,6 +103,24 @@ class TestLocalEvalSetResultsManager:
     )
     assert retrieved_result == self.eval_set_result
 
+  def test_get_eval_set_result_double_encoded_legacy(self):
+    eval_history_dir = os.path.join(
+        self.agents_dir, self.app_name, _ADK_EVAL_HISTORY_DIR
+    )
+    os.makedirs(eval_history_dir, exist_ok=True)
+    eval_set_result_file_path = os.path.join(
+        eval_history_dir,
+        self.eval_set_result_name + _EVAL_SET_RESULT_FILE_EXTENSION,
+    )
+    double_encoded_json = json.dumps(self.eval_set_result.model_dump_json())
+    with open(eval_set_result_file_path, "w", encoding="utf-8") as f:
+      f.write(double_encoded_json)
+
+    retrieved_result = self.manager.get_eval_set_result(
+        self.app_name, self.eval_set_result_name
+    )
+    assert retrieved_result == self.eval_set_result
+
   def test_get_eval_set_result_not_found(self, mocker):
     mock_time = mocker.patch("time.time")
     mock_time.return_value = self.timestamp