diff --git a/src/google/adk/evaluation/base_eval_service.py b/src/google/adk/evaluation/base_eval_service.py
index 34c5fe2fe5..71d304e640 100644
--- a/src/google/adk/evaluation/base_eval_service.py
+++ b/src/google/adk/evaluation/base_eval_service.py
@@ -45,6 +45,7 @@ class EvaluateConfig(BaseModel):
 
   parallelism: int = Field(
       default=4,
+      ge=1,
       description="""Number of parallel evaluations to run during an Eval. Few
 factors to consider while changing this value:
 
@@ -71,6 +72,7 @@ class InferenceConfig(BaseModel):
 
   parallelism: int = Field(
       default=4,
+      ge=1,
       description="""Number of parallel inferences to run during an Eval. Few
 factors to consider while changing this value:
 
diff --git a/tests/unittests/evaluation/test_base_eval_service.py b/tests/unittests/evaluation/test_base_eval_service.py
new file mode 100644
index 0000000000..f87104d086
--- /dev/null
+++ b/tests/unittests/evaluation/test_base_eval_service.py
@@ -0,0 +1,41 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from __future__ import annotations
+
+from google.adk.evaluation.base_eval_service import EvaluateConfig
+from google.adk.evaluation.base_eval_service import InferenceConfig
+from google.adk.evaluation.eval_metrics import EvalMetric
+from pydantic import ValidationError
+import pytest
+
+
+def test_evaluate_config_rejects_zero_parallelism():
+  """parallelism=0 must be rejected.
+
+  It is forwarded unclamped to asyncio.Semaphore(value=parallelism), where a
+  zero value hangs every acquire() indefinitely instead of raising -- a
+  degenerate value that fails silently rather than loudly.
+  """
+  with pytest.raises(ValidationError):
+    EvaluateConfig(
+        eval_metrics=[EvalMetric(metric_name="response_match_score")],
+        parallelism=0,
+    )
+
+
+def test_inference_config_rejects_zero_parallelism():
+  """Same defect as EvaluateConfig.parallelism, on the inference side."""
+  with pytest.raises(ValidationError):
+    InferenceConfig(parallelism=0)