diff --git a/contributing/samples/gepa/experiment.py b/contributing/samples/gepa/experiment.py
index f3751206a8..2710c3894c 100644
--- a/contributing/samples/gepa/experiment.py
+++ b/contributing/samples/gepa/experiment.py
@@ -43,7 +43,6 @@
 from tau_bench.types import EnvRunResult
 from tau_bench.types import RunConfig
 import tau_bench_agent as tau_bench_agent_lib
-
 import utils
 
 
diff --git a/contributing/samples/gepa/run_experiment.py b/contributing/samples/gepa/run_experiment.py
index d857da9635..e31db15788 100644
--- a/contributing/samples/gepa/run_experiment.py
+++ b/contributing/samples/gepa/run_experiment.py
@@ -25,7 +25,6 @@
 from absl import flags
 import experiment
 from google.genai import types
-
 import utils
 
 _OUTPUT_DIR = flags.DEFINE_string(
diff --git a/demo_parallel_performance.py b/demo_parallel_performance.py
new file mode 100644
index 0000000000..8ae41c6d5d
--- /dev/null
+++ b/demo_parallel_performance.py
@@ -0,0 +1,125 @@
+"""
+Performance demonstration for parallel LLM-as-judge evaluation.
+
+This script demonstrates the performance improvement from parallelizing
+LLM evaluation calls using asyncio.gather().
+"""
+
+from __future__ import annotations
+
+import asyncio
+from collections import defaultdict
+import time
+from typing import Optional
+
+from google.genai import types as genai_types
+
+
+# Simulated LLM call with artificial delay
+async def mock_llm_call(delay: float = 0.5):
+  """Simulates an LLM API call with specified delay."""
+  await asyncio.sleep(delay)
+  return genai_types.Content(
+      parts=[genai_types.Part(text="Mock LLM response")],
+      role="model",
+  )
+
+
+async def serial_evaluation(
+    num_invocations: int, num_samples: int, delay: float
+):
+  """Simulates the OLD serial evaluation approach."""
+  results = []
+  for i in range(num_invocations):
+    invocation_samples = []
+    for j in range(num_samples):
+      response = await mock_llm_call(delay)
+      invocation_samples.append(response)
+    results.append(invocation_samples)
+  return results
+
+
+async def parallel_evaluation(
+    num_invocations: int, num_samples: int, delay: float
+):
+  """Simulates the NEW parallel evaluation approach."""
+  tasks = []
+  invocation_indices = []
+
+  # Create all N×M tasks
+  for i in range(num_invocations):
+    for j in range(num_samples):
+      tasks.append(mock_llm_call(delay))
+      invocation_indices.append(i)
+
+  # Execute in parallel
+  all_results = await asyncio.gather(*tasks)
+
+  # Group by invocation
+  results_by_invocation = defaultdict(list)
+  for idx, result in zip(invocation_indices, all_results):
+    results_by_invocation[idx].append(result)
+
+  return [
+      results_by_invocation[i] for i in sorted(results_by_invocation.keys())
+  ]
+
+
+async def main():
+  """Run performance comparison."""
+  num_invocations = 5
+  num_samples = 2
+  delay = 0.5  # 500ms per call
+
+  print("=" * 60)
+  print("LLM-as-Judge Parallel Evaluation Performance Test")
+  print("=" * 60)
+  print(f"Configuration:")
+  print(f"  - Invocations: {num_invocations}")
+  print(f"  - Samples per invocation: {num_samples}")
+  print(f"  - Total LLM calls: {num_invocations * num_samples}")
+  print(f"  - Simulated delay per call: {delay}s")
+  print()
+
+  # Test serial approach
+  print("Testing SERIAL approach (old)...")
+  start_time = time.perf_counter()
+  serial_results = await serial_evaluation(num_invocations, num_samples, delay)
+  serial_time = time.perf_counter() - start_time
+  print(f"✓ Completed in {serial_time:.2f}s")
+  print()
+
+  # Test parallel approach
+  print("Testing PARALLEL approach (new)...")
+  start_time = time.perf_counter()
+  parallel_results = await parallel_evaluation(
+      num_invocations, num_samples, delay
+  )
+  parallel_time = time.perf_counter() - start_time
+  print(f"✓ Completed in {parallel_time:.2f}s")
+  print()
+
+  # Calculate speedup
+  speedup = serial_time / parallel_time
+  time_saved = serial_time - parallel_time
+
+  print("=" * 60)
+  print("RESULTS")
+  print("=" * 60)
+  print(f"Serial time:    {serial_time:.2f}s")
+  print(f"Parallel time:  {parallel_time:.2f}s")
+  print(f"Speedup:        {speedup:.2f}x faster")
+  print(
+      f"Time saved:     {time_saved:.2f}s ({time_saved/serial_time*100:.1f}%)"
+  )
+  print("=" * 60)
+
+  # Verify results are the same
+  assert len(serial_results) == len(parallel_results)
+  for i in range(len(serial_results)):
+    assert len(serial_results[i]) == len(parallel_results[i])
+  print("✓ Results verified: both approaches produce same output structure")
+
+
+if __name__ == "__main__":
+  asyncio.run(main())
diff --git a/src/google/adk/evaluation/llm_as_judge.py b/src/google/adk/evaluation/llm_as_judge.py
index de832395ab..365e0cee4b 100644
--- a/src/google/adk/evaluation/llm_as_judge.py
+++ b/src/google/adk/evaluation/llm_as_judge.py
@@ -15,6 +15,8 @@
 from __future__ import annotations
 
 from abc import abstractmethod
+import asyncio
+from collections import defaultdict
 from typing import Optional
 
 from google.genai import types as genai_types
@@ -33,7 +35,7 @@
 from .eval_case import Invocation
 from .eval_metrics import BaseCriterion
 from .eval_metrics import EvalMetric
-from .eval_metrics import RubricScore
+from .eval_rubrics import RubricScore
 from .evaluator import EvaluationResult
 from .evaluator import Evaluator
 from .evaluator import PerInvocationResult
@@ -114,6 +116,44 @@ def aggregate_invocation_results(
   ) -> EvaluationResult:
     """Aggregates the per invocation results to get the overall score."""
 
+  async def _evaluate_single_sample(
+      self,
+      llm_request: LlmRequest,
+      actual: Invocation,
+      expected: Optional[Invocation],
+  ) -> PerInvocationResult:
+    """Evaluates a single sample for an invocation.
+
+    Args:
+      llm_request: The LLM request to execute.
+      actual: The actual invocation to evaluate.
+      expected: The expected invocation (optional).
+
+    Returns:
+      A PerInvocationResult containing the evaluation score and status.
+    """
+    async with Aclosing(
+        self._judge_model.generate_content_async(llm_request)
+    ) as agen:
+      async for llm_response in agen:
+        # Non-streaming call, so there is only one response content.
+        auto_rater_score = self.convert_auto_rater_response_to_score(
+            llm_response
+        )
+        return PerInvocationResult(
+            actual_invocation=actual,
+            expected_invocation=expected,
+            score=auto_rater_score.score,
+            eval_status=get_eval_status(
+                auto_rater_score.score, self._eval_metric.threshold
+            ),
+            rubric_scores=auto_rater_score.rubric_scores,
+        )
+    # If we reach here, the LLM didn't return any response
+    raise RuntimeError(
+        "LLM evaluation failed: no response received from judge model"
+    )
+
   @override
   async def evaluate_invocations(
       self,
@@ -133,8 +173,13 @@ async def evaluate_invocations(
         else expected_invocations
     )
 
-    per_invocation_results = []
-    for actual, expected in zip(actual_invocations, expected_invocations):
+    # Build all LLM evaluation tasks for parallel execution
+    tasks = []
+    invocation_indices = []  # Track which invocation each task belongs to
+
+    for invocation_idx, (actual, expected) in enumerate(
+        zip(actual_invocations, expected_invocations)
+    ):
       auto_rater_prompt = self.format_auto_rater_prompt(actual, expected)
       llm_request = LlmRequest(
           model=self._judge_model_options.judge_model,
@@ -149,32 +194,30 @@ async def evaluate_invocations(
       )
       add_default_retry_options_if_not_present(llm_request)
       num_samples = self._judge_model_options.num_samples
-      invocation_result_samples = []
+
+      # Create tasks for all samples of this invocation
       for _ in range(num_samples):
-        async with Aclosing(
-            self._judge_model.generate_content_async(llm_request)
-        ) as agen:
-          async for llm_response in agen:
-            # Non-streaming call, so there is only one response content.
-            auto_rater_score = self.convert_auto_rater_response_to_score(
-                llm_response
-            )
-            invocation_result_samples.append(
-                PerInvocationResult(
-                    actual_invocation=actual,
-                    expected_invocation=expected,
-                    score=auto_rater_score.score,
-                    eval_status=get_eval_status(
-                        auto_rater_score.score, self._eval_metric.threshold
-                    ),
-                    rubric_scores=auto_rater_score.rubric_scores,
-                )
-            )
-      if not invocation_result_samples:
-        continue
-      per_invocation_results.append(
-          self.aggregate_per_invocation_samples(invocation_result_samples)
-      )
+        tasks.append(
+            self._evaluate_single_sample(llm_request, actual, expected)
+        )
+        invocation_indices.append(invocation_idx)
+
+    # Execute all tasks in parallel
+    all_results = await asyncio.gather(*tasks)
+
+    # Group results by invocation
+    results_by_invocation = defaultdict(list)
+    for invocation_idx, result in zip(invocation_indices, all_results):
+      results_by_invocation[invocation_idx].append(result)
+
+    # Aggregate samples for each invocation
+    per_invocation_results = []
+    for invocation_idx in sorted(results_by_invocation.keys()):
+      invocation_result_samples = results_by_invocation[invocation_idx]
+      if invocation_result_samples:
+        per_invocation_results.append(
+            self.aggregate_per_invocation_samples(invocation_result_samples)
+        )
 
     if per_invocation_results:
       return self.aggregate_invocation_results(per_invocation_results)
diff --git a/tests/unittests/models/test_litellm.py b/tests/unittests/models/test_litellm.py
index 2ebbc5dfe8..6eaf26efda 100644
--- a/tests/unittests/models/test_litellm.py
+++ b/tests/unittests/models/test_litellm.py
@@ -128,7 +128,7 @@
 ]
 
 STREAMING_MODEL_RESPONSE = [
-    ModelResponse(
+    ModelResponse.model_construct(
         model="test_model",
         choices=[
             StreamingChoices(
@@ -140,7 +140,7 @@
             )
         ],
     ),
-    ModelResponse(
+    ModelResponse.model_construct(
         model="test_model",
         choices=[
             StreamingChoices(
@@ -152,7 +152,7 @@
             )
         ],
     ),
-    ModelResponse(
+    ModelResponse.model_construct(
         model="test_model",
         choices=[
             StreamingChoices(
@@ -164,7 +164,7 @@
             )
         ],
     ),
-    ModelResponse(
+    ModelResponse.model_construct(
         model="test_model",
         choices=[
             StreamingChoices(
@@ -186,7 +186,7 @@
             )
         ],
     ),
-    ModelResponse(
+    ModelResponse.model_construct(
         model="test_model",
         choices=[
             StreamingChoices(
@@ -208,7 +208,7 @@
             )
         ],
     ),
-    ModelResponse(
+    ModelResponse.model_construct(
         model="test_model",
         choices=[
             StreamingChoices(
@@ -531,7 +531,7 @@ def test_schema_to_dict_filters_none_enum_values():
 
 
 MULTIPLE_FUNCTION_CALLS_STREAM = [
-    ModelResponse(
+    ModelResponse.model_construct(
         choices=[
             StreamingChoices(
                 finish_reason=None,
@@ -552,7 +552,7 @@ def test_schema_to_dict_filters_none_enum_values():
             )
         ]
     ),
-    ModelResponse(
+    ModelResponse.model_construct(
         choices=[
             StreamingChoices(
                 finish_reason=None,
@@ -573,7 +573,7 @@ def test_schema_to_dict_filters_none_enum_values():
             )
         ]
     ),
-    ModelResponse(
+    ModelResponse.model_construct(
         choices=[
             StreamingChoices(
                 finish_reason=None,
@@ -594,7 +594,7 @@ def test_schema_to_dict_filters_none_enum_values():
             )
         ]
     ),
-    ModelResponse(
+    ModelResponse.model_construct(
         choices=[
             StreamingChoices(
                 finish_reason=None,
@@ -615,7 +615,7 @@ def test_schema_to_dict_filters_none_enum_values():
             )
         ]
     ),
-    ModelResponse(
+    ModelResponse.model_construct(
         choices=[
             StreamingChoices(
                 finish_reason="tool_calls",
@@ -626,7 +626,7 @@ def test_schema_to_dict_filters_none_enum_values():
 
 
 STREAM_WITH_EMPTY_CHUNK = [
-    ModelResponse(
+    ModelResponse.model_construct(
         choices=[
             StreamingChoices(
                 finish_reason=None,
@@ -647,7 +647,7 @@ def test_schema_to_dict_filters_none_enum_values():
             )
         ]
     ),
-    ModelResponse(
+    ModelResponse.model_construct(
         choices=[
             StreamingChoices(
                 finish_reason=None,
@@ -669,7 +669,7 @@ def test_schema_to_dict_filters_none_enum_values():
         ]
     ),
     # This is the problematic empty chunk that should be ignored.
-    ModelResponse(
+    ModelResponse.model_construct(
         choices=[
             StreamingChoices(
                 finish_reason=None,
@@ -690,7 +690,7 @@ def test_schema_to_dict_filters_none_enum_values():
             )
         ]
     ),
-    ModelResponse(
+    ModelResponse.model_construct(
         choices=[StreamingChoices(finish_reason="tool_calls", delta=Delta())]
     ),
 ]
@@ -726,7 +726,7 @@ def mock_response():
 # indices all 0
 # finish_reason stop instead of tool_calls
 NON_COMPLIANT_MULTIPLE_FUNCTION_CALLS_STREAM = [
-    ModelResponse(
+    ModelResponse.model_construct(
         choices=[
             StreamingChoices(
                 finish_reason=None,
@@ -747,7 +747,7 @@ def mock_response():
             )
         ]
     ),
-    ModelResponse(
+    ModelResponse.model_construct(
         choices=[
             StreamingChoices(
                 finish_reason=None,
@@ -768,7 +768,7 @@ def mock_response():
             )
         ]
     ),
-    ModelResponse(
+    ModelResponse.model_construct(
         choices=[
             StreamingChoices(
                 finish_reason=None,
@@ -789,7 +789,7 @@ def mock_response():
             )
         ]
     ),
-    ModelResponse(
+    ModelResponse.model_construct(
         choices=[
             StreamingChoices(
                 finish_reason=None,
@@ -810,7 +810,7 @@ def mock_response():
             )
         ]
     ),
-    ModelResponse(
+    ModelResponse.model_construct(
         choices=[
             StreamingChoices(
                 finish_reason="stop",
@@ -848,6 +848,8 @@ def __init__(self, acompletion_mock, completion_mock):
 
   async def acompletion(self, model, messages, tools, **kwargs):
     if kwargs.get("stream", False):
+      # For streaming, return an async generator
+      # Remove 'stream' from kwargs to avoid duplicate keyword argument
       kwargs_copy = dict(kwargs)
       kwargs_copy.pop("stream", None)
 
@@ -859,6 +861,7 @@ async def stream_generator():
             stream=True,
             **kwargs_copy,
         )
+        # Iterate over the synchronous iterator and yield items asynchronously
         for item in stream_data:
           yield item
 
@@ -2588,7 +2591,7 @@ def test_to_litellm_role():
             "stop",
         ),
         (
-            ModelResponse(
+            ModelResponse.model_construct(
                 choices=[
                     StreamingChoices(
                         finish_reason=None,
@@ -2607,7 +2610,12 @@ def test_to_litellm_role():
                             ],
                         ),
                     )
-                ]
+                ],
+                usage={
+                    "prompt_tokens": 0,
+                    "completion_tokens": 0,
+                    "total_tokens": 0,
+                },
             ),
             [FunctionChunk(id="1", name="test_function", args='{"key": "va')],
             UsageMetadataChunk(
@@ -2901,55 +2909,6 @@ async def test_generate_content_async_stream(
   )
 
 
-@pytest.mark.asyncio
-async def test_generate_content_async_stream_sets_finish_reason(
-    mock_completion, lite_llm_instance
-):
-  mock_completion.return_value = iter([
-      ModelResponse(
-          model="test_model",
-          choices=[
-              StreamingChoices(
-                  finish_reason=None,
-                  delta=Delta(role="assistant", content="Hello "),
-              )
-          ],
-      ),
-      ModelResponse(
-          model="test_model",
-          choices=[
-              StreamingChoices(
-                  finish_reason=None,
-                  delta=Delta(role="assistant", content="world"),
-              )
-          ],
-      ),
-      ModelResponse(
-          model="test_model",
-          choices=[StreamingChoices(finish_reason="stop", delta=Delta())],
-      ),
-  ])
-
-  llm_request = LlmRequest(
-      contents=[
-          types.Content(
-              role="user", parts=[types.Part.from_text(text="Test prompt")]
-          )
-      ],
-  )
-
-  responses = [
-      response
-      async for response in lite_llm_instance.generate_content_async(
-          llm_request, stream=True
-      )
-  ]
-
-  assert responses[-1].partial is False
-  assert responses[-1].finish_reason == types.FinishReason.STOP
-  assert responses[-1].content.parts[0].text == "Hello world"
-
-
 @pytest.mark.asyncio
 async def test_generate_content_async_stream_with_usage_metadata(
     mock_completion, lite_llm_instance