diff --git a/src/google/adk/evaluation/eval_case.py b/src/google/adk/evaluation/eval_case.py
index 4902b3561..1885e0eb3 100644
--- a/src/google/adk/evaluation/eval_case.py
+++ b/src/google/adk/evaluation/eval_case.py
@@ -31,12 +31,25 @@ class EvalBaseModel(BaseModel):
   )
 
 
+class ToolUseWithResponse(EvalBaseModel):
+  """Represents a tool use with its response."""
+
+  function_call: genai_types.FunctionCall
+  """The function call."""
+
+  function_response: Optional[genai_types.FunctionResponse] = None
+  """The function response."""
+
+
 class IntermediateData(EvalBaseModel):
   """Container for intermediate data that an agent would generate as it responds with a final answer."""
 
   tool_uses: list[genai_types.FunctionCall] = []
   """Tool use trajectory in chronological order."""
 
+  tool_uses_with_responses: list[ToolUseWithResponse] = []
+  """Tool use trajectory with responses in chronological order."""
+
   intermediate_responses: list[tuple[str, list[genai_types.Part]]] = []
   """Intermediate responses generated by sub-agents to convey progress or status
   in a multi-agent system, distinct from the final response.
diff --git a/src/google/adk/evaluation/evaluation_generator.py b/src/google/adk/evaluation/evaluation_generator.py
index 1359967bc..5901c8628 100644
--- a/src/google/adk/evaluation/evaluation_generator.py
+++ b/src/google/adk/evaluation/evaluation_generator.py
@@ -32,6 +32,7 @@
 from .eval_case import IntermediateData
 from .eval_case import Invocation
 from .eval_case import SessionInput
+from .eval_case import ToolUseWithResponse
 from .eval_set import EvalSet
 
 
@@ -180,6 +181,7 @@ async def _generate_inferences_from_root_agent(
       final_response = None
       user_content = invocation.user_content
       tool_uses = []
+      tool_uses_with_responses_by_id = {}
       invocation_id = ""
 
       async for event in runner.run_async(
@@ -191,16 +193,29 @@ async def _generate_inferences_from_root_agent(
 
         if event.is_final_response() and event.content and event.content.parts:
           final_response = event.content
-        elif event.get_function_calls():
-          for call in event.get_function_calls():
-            tool_uses.append(call)
+        else:
+          if event.get_function_calls():
+            for call in event.get_function_calls():
+              tool_uses.append(call)
+              tool_uses_with_responses_by_id[call.id] = ToolUseWithResponse(
+                  function_call=call
+              )
+          elif event.get_function_responses():
+            for response in event.get_function_responses():
+              if response.id in tool_uses_with_responses_by_id:
+                tool_uses_with_responses_by_id[
+                    response.id
+                ].function_response = response
 
       response_invocations.append(
           Invocation(
               invocation_id=invocation_id,
               user_content=user_content,
               final_response=final_response,
-              intermediate_data=IntermediateData(tool_uses=tool_uses),
+              intermediate_data=IntermediateData(
+                  tool_uses=tool_uses,
+                  tool_uses_with_responses=tool_uses_with_responses_by_id.values(),
+              ),
           )
       )
 
diff --git a/src/google/adk/evaluation/hallucinations_v1.py b/src/google/adk/evaluation/hallucinations_v1.py
new file mode 100644
index 000000000..7722ee7b3
--- /dev/null
+++ b/src/google/adk/evaluation/hallucinations_v1.py
@@ -0,0 +1,280 @@
+# Copyright 2025 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
+
+import json
+import logging
+import re
+from typing import Optional
+
+from google.genai import types as genai_types
+from typing_extensions import override
+
+from ..models.llm_response import LlmResponse
+from ..utils.feature_decorator import working_in_progress
+from .eval_case import Invocation
+from .eval_case import ToolUseWithResponse
+from .eval_metrics import EvalMetric
+from .evaluator import EvalStatus
+from .evaluator import EvaluationResult
+from .evaluator import PerInvocationResult
+from .llm_as_judge import LlmAsJudge
+from .llm_as_judge_utils import get_eval_status
+from .llm_as_judge_utils import get_text_from_content
+
+logger = logging.getLogger("google_adk." + __name__)
+
+
+HALLUCINATIONS_V1_PROMPT = """
+You are a helpful and harmless AI assistant. You will be provided with a textual context and a model-generated response.
+Your task is to analyze the response sentence by sentence and classify each sentence according to its relationship with the provided context.
+
+**Instructions:**
+
+1. **Decompose the response into individual sentences.**
+2. **For each sentence, assign one of the following labels:**
+    * **`supported`**: The sentence is entailed by the given context. Provide a supporting excerpt from the context. The supporting except must *fully* entail the sentence.
+    * **`unsupported`**: The sentence is not entailed by the given context. No excerpt is needed for this label.
+    * **`contradictory`**: The sentence is falsified by the given context. Provide a contradicting excerpt from the context.
+    * **`disputed`**: The given context contains both supporting and contradicting information. Provide both supporting and contradicting excerpt from the context.
+    * **`no_rad`**: The sentence does not require factual attribution (e.g., opinions, planning steps, greetings, questions, disclaimers, mathematical calculation).
+3. **For each label, provide a short rationale explaining your decision.** The rationale should be separate from the excerpt.
+4. **Be very strict with your `supported`, `contradictory` and `disputed` decisions.** Unless you can find straightforward, indisputable evidence excepts *in the context* that a sentence is `supported`, `contradictory` or `disputed`, consider it `unsupported`.  You should not employ world knowledge unless it is truly trivial.
+5. "tool_outputs" blocks contain code execution results of the "tool_code" blocks immediately above them. If any sentence is based on "tool_outputs" results, first analyze if the corresponding "tool_code" is supported and if the results are error-free. Only if the "tool_code" block is supported, you can treat code execution results as correct.
+6. If you need to cite multiple supporting excerpts, simply concatenate them. Excerpt could be summary from the context if it is too long.
+
+**Input Format:**
+
+The input will consist of two parts, clearly separated:
+
+* **Context:**  The textual context used to generate the response.
+* **Response:** The model-generated response to be analyzed.
+
+**Output Format:**
+
+For each sentence in the response, output a block of text with the following fields:
+
+* sentence: The sentence being analyzed.
+* label: One of `supported`, `unsupported`, `contradictory`, `disputed` or `no_rad`.
+* rationale: A brief explanation for the assessment
+* supporting_excerpt: A relevant excerpt from the context that supports the sentence. Only required for `supported` and `disputed` labels.
+* contradicting_excerpt: A relevant excerpt from the context that contradicts with the sentence. Only required for `contradictory` and `disputed` labels.
+
+**Example:**
+
+**Input:**
+
+**Context Begin**
+Apples are red fruits. Bananas are yellow fruits. Pears are purple fruits. Pears are blue fruits.
+**Context End**
+
+**Response Begin**
+Apples are red. Bananas are green. Pears are purple. Bananas are cheaper than apples. Enjoy your fruit!
+**Response End**
+
+**Output:**
+sentence: Apples are red.
+label: supported
+rationale: The context explicitly states that apples are red.
+supporting_excerpt: Apples are red fruits.
+contradicting_excerpt: null
+
+sentence: Bananas are green.
+label: contradictory
+rationale: The context states that bananas are yellow, not green.
+supporting_excerpt: null
+contradicting_excerpt: Bananas are yellow fruits.
+
+sentence: Pears are purple.
+label: disputed
+rationale: The context states that pears are purple but it also states that pears are blue.
+supporting_excerpt: Pears are purple fruits
+contradicting_excerpt: Pears are blue fruits
+
+sentence: Bananas are cheaper than apples.
+label: unsupported
+rationale: The context does not mention the price of bananas or apples.
+supporting_excerpt: null
+contradicting_excerpt: null
+
+sentence: Enjoy your fruit!
+label: no_rad
+rationale: This is a general expression and does not require factual attribution.
+supporting_excerpt: null
+contradicting_excerpt: null
+
+**Now, please analyze the following context and response:**
+
+**Input:**
+
+**Context Begin**
+{context}
+**Context End**
+
+**Response Begin**
+{response}
+**Response End**
+
+**Output:**
+""".strip()
+
+
+def _format_function_call(function_call: genai_types.FunctionCall) -> str:
+  """Formats a function call as a string."""
+  formatted_function_call = f"Function call\nName: {function_call.name}\n"
+  formatted_function_call += f"Args: {json.dumps(function_call.args)}"
+  return formatted_function_call
+
+
+def _format_function_response(
+    function_response: genai_types.FunctionResponse,
+) -> str:
+  """Formats a function response as a string."""
+  formatted_function_response = (
+      f"Function response\nName: {function_response.name}\n"
+  )
+  formatted_function_response += (
+      f"Response: {json.dumps(function_response.response)}"
+  )
+  return formatted_function_response
+
+
+def _format_tool_use_with_response(
+    tool_use_with_response: ToolUseWithResponse,
+) -> str:
+  """Formats a tool use with response as a string."""
+  formatted_tool_use = {
+      "name": tool_use_with_response.function_call.name,
+      "args": tool_use_with_response.function_call.args,
+  }
+  if tool_use_with_response.function_response:
+    formatted_tool_use["response"] = (
+        tool_use_with_response.function_response.response
+    )
+  return json.dumps(formatted_tool_use)
+
+
+def _extract_labels_from_critique(response: str) -> list[str]:
+  """Extracts the label from the LLM critique."""
+  label_matches = re.findall(
+      r"label: (supported|unsupported|contradictory|disputed|no_rad)", response
+  )
+  return label_matches
+
+
+@working_in_progress
+class HallucinationsV1Evaluator(LlmAsJudge):
+  """LLM-based evaluator to judge factuality and whether the response contains hallucinations."""
+
+  def __init__(
+      self,
+      eval_metric: EvalMetric,
+  ):
+    super().__init__(eval_metric)
+    self._auto_rater_prompt_template = HALLUCINATIONS_V1_PROMPT
+
+  @override
+  def format_auto_rater_prompt(
+      self, actual_invocation: Invocation, expected_invocation: Invocation
+  ) -> str:
+    response = get_text_from_content(actual_invocation.final_response)
+    user_content = get_text_from_content(expected_invocation.user_content)
+    context = [f"User prompt: {user_content}"]
+    if not actual_invocation.intermediate_data:
+      return self._auto_rater_prompt_template.format(
+          context=context[0], response=response
+      )
+    # Only support text and function calls/tool uses for now.
+    for (
+        tool_use
+    ) in actual_invocation.intermediate_data.tool_uses_with_responses:
+      context.append(f"Tool use:\n{_format_tool_use_with_response(tool_use)}")
+    for (
+        author,
+        parts,
+    ) in actual_invocation.intermediate_data.intermediate_responses:
+      parts_formatted = []
+      for part in parts:
+        if part.text:
+          parts_formatted.append(part.text)
+        elif part.function_call:
+          parts_formatted.append(_format_function_call(part.function_call))
+        elif part.function_response:
+          parts_formatted.append(
+              _format_function_response(part.function_response)
+          )
+      parts_formatted = "\n".join(parts_formatted)
+      context.append(f"Sub-agent {author} response:\n{parts_formatted}")
+    return self._auto_rater_prompt_template.format(
+        context="\n\n".join(context), response=response
+    )
+
+  @override
+  def convert_auto_rater_response_to_score(
+      self, llm_response: LlmResponse
+  ) -> Optional[float]:
+    text = get_text_from_content(llm_response.content)
+    labels = _extract_labels_from_critique(text)
+    if not labels:
+      return None
+    final_score = 0.0
+    # If label is 'disputed' or 'no_rad', the score is not affected.
+    for label in labels:
+      if label == "supported":
+        final_score += 1.0
+      elif label in ["unsupported", "contradictory"]:
+        final_score -= 1.0
+    return final_score / len(labels)
+
+  @override
+  def aggregate_per_invocation_samples(
+      self, per_invocation_samples: list[PerInvocationResult]
+  ) -> PerInvocationResult:
+    """Computes the fraction of invocation samples that are valid."""
+    final_score = 0.0
+    num_evaluated = 0
+    for sample in per_invocation_samples:
+      if sample.score is None or sample.eval_status == EvalStatus.NOT_EVALUATED:
+        continue
+      num_evaluated += 1
+      final_score += sample.score
+    final_score /= num_evaluated
+    return PerInvocationResult(
+        actual_invocation=per_invocation_samples[0].actual_invocation,
+        expected_invocation=per_invocation_samples[0].expected_invocation,
+        score=final_score,
+        eval_status=get_eval_status(final_score, self._eval_metric.threshold),
+    )
+
+  @override
+  def aggregate_invocation_results(
+      self, per_invocation_results: list[PerInvocationResult]
+  ) -> EvaluationResult:
+    """Computes the fraction of invocation results that are valid."""
+    final_score = 0.0
+    num_evaluated = 0
+    for result in per_invocation_results:
+      if result.score is None or result.eval_status == EvalStatus.NOT_EVALUATED:
+        continue
+      num_evaluated += 1
+      final_score += result.score
+    final_score /= num_evaluated
+    return EvaluationResult(
+        overall_score=final_score,
+        overall_eval_status=get_eval_status(
+            final_score, self._eval_metric.threshold
+        ),
+        per_invocation_results=per_invocation_results,
+    )
diff --git a/tests/unittests/evaluation/test_hallucinations_v1.py b/tests/unittests/evaluation/test_hallucinations_v1.py
new file mode 100644
index 000000000..b63c2e58d
--- /dev/null
+++ b/tests/unittests/evaluation/test_hallucinations_v1.py
@@ -0,0 +1,233 @@
+# Copyright 2025 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.eval_case import Invocation
+from google.adk.evaluation.eval_case import ToolUseWithResponse
+from google.adk.evaluation.eval_metrics import EvalMetric
+from google.adk.evaluation.eval_metrics import JudgeModelOptions
+from google.adk.evaluation.evaluator import EvalStatus
+from google.adk.evaluation.evaluator import PerInvocationResult
+from google.adk.evaluation.hallucinations_v1 import _format_function_call
+from google.adk.evaluation.hallucinations_v1 import _format_function_response
+from google.adk.evaluation.hallucinations_v1 import _format_tool_use_with_response
+from google.adk.evaluation.hallucinations_v1 import HallucinationsV1Evaluator
+from google.adk.models.llm_response import LlmResponse
+from google.genai import types as genai_types
+import pytest
+
+
+def test_format_function_call():
+  function_call = genai_types.FunctionCall(
+      id="test_function_id",
+      name="test_function",
+      args={"arg1": "arg1_value", "arg2": "arg2_value"},
+  )
+  formatted_function_call = _format_function_call(function_call)
+  assert formatted_function_call == """Function call
+Name: test_function
+Args: {"arg1": "arg1_value", "arg2": "arg2_value"}"""
+
+
+def test_format_function_response():
+  function_response = genai_types.FunctionResponse(
+      id="test_function_id",
+      name="test_function",
+      response={"result": "return_value"},
+  )
+  formatted_function_response = _format_function_response(function_response)
+  assert formatted_function_response == """Function response
+Name: test_function
+Response: {"result": "return_value"}"""
+
+
+def test_format_tool_use_with_response():
+  tool_use_with_response = ToolUseWithResponse(
+      function_call=genai_types.FunctionCall(
+          id="test_function_id",
+          name="test_function",
+          args={"arg1": "arg1_value", "arg2": "arg2_value"},
+      ),
+      function_response=genai_types.FunctionResponse(
+          id="test_function_id",
+          name="test_function",
+          response={"result": "return_value"},
+      ),
+  )
+  formatted_tool_use_with_response = _format_tool_use_with_response(
+      tool_use_with_response
+  )
+  assert formatted_tool_use_with_response == (
+      """{"name": "test_function", "args": {"arg1": "arg1_value", "arg2": "arg2_value"}, "response": {"result": "return_value"}}"""
+  )
+
+
+def _create_test_evaluator_gemini(
+    threshold: float,
+) -> HallucinationsV1Evaluator:
+  evaluator = HallucinationsV1Evaluator(
+      EvalMetric(
+          metric_name="hallucinations_v1",
+          threshold=threshold,
+          judge_model_options=JudgeModelOptions(
+              judge_model="gemini-2.5-flash",
+              num_samples=5,
+          ),
+      ),
+  )
+  return evaluator
+
+
+def test_convert_auto_rater_response_to_score():
+  llm_response = LlmResponse(
+      content=genai_types.Content(
+          parts=[
+              genai_types.Part(
+                  text=(
+                      "label: supported\n"
+                      "rationale: The sentence is supported.\n\n"
+                      "label: unsupported\n"
+                      "rationale: The sentence is unsupported.\n\n"
+                      "label: contradictory\n"
+                      "rationale: The sentence is contradictory.\n\n"
+                      "label: disputed\n"
+                      "rationale: The sentence is disputed.\n\n"
+                      "label: no_rad\n"
+                      "rationale: The sentence is no_rad.\n\n"
+                  )
+              )
+          ],
+          role="model",
+      )
+  )
+  evaluator = _create_test_evaluator_gemini(threshold=0.8)
+  score = evaluator.convert_auto_rater_response_to_score(llm_response)
+  # The score is the average of the labels, with 1.0 for supported and -1.0 for
+  # unsupported and contradictory.
+  assert score == -1 / 5
+
+
+def test_aggregate_per_invocation_samples():
+  per_invocation_samples = [
+      PerInvocationResult(
+          actual_invocation=Invocation(
+              user_content=genai_types.Content(
+                  parts=[genai_types.Part(text="This is a test query.")],
+                  role="user",
+              ),
+              final_response=genai_types.Content(
+                  parts=[genai_types.Part(text="This is a test response.")],
+                  role="model",
+              ),
+          ),
+          expected_invocation=Invocation(
+              user_content=genai_types.Content(
+                  parts=[genai_types.Part(text="This is a test query.")],
+                  role="user",
+              ),
+              final_response=genai_types.Content(
+                  parts=[genai_types.Part(text="This is a test response.")],
+                  role="model",
+              ),
+          ),
+          score=0.5,
+          eval_status=EvalStatus.FAILED,
+      ),
+      PerInvocationResult(
+          actual_invocation=Invocation(
+              user_content=genai_types.Content(
+                  parts=[genai_types.Part(text="This is a test query.")],
+                  role="user",
+              ),
+              final_response=genai_types.Content(
+                  parts=[genai_types.Part(text="This is a test response.")],
+                  role="model",
+              ),
+          ),
+          expected_invocation=Invocation(
+              user_content=genai_types.Content(
+                  parts=[genai_types.Part(text="This is a test query.")],
+                  role="user",
+              ),
+              final_response=genai_types.Content(
+                  parts=[genai_types.Part(text="This is a test response.")],
+                  role="model",
+              ),
+          ),
+          score=1.0,
+          eval_status=EvalStatus.PASSED,
+      ),
+  ]
+  evaluator = _create_test_evaluator_gemini(threshold=0.8)
+  result = evaluator.aggregate_per_invocation_samples(per_invocation_samples)
+  assert result.score == 0.75
+  assert result.eval_status == EvalStatus.FAILED
+
+
+def test_aggregate_invocation_results():
+  per_invocation_samples = [
+      PerInvocationResult(
+          actual_invocation=Invocation(
+              user_content=genai_types.Content(
+                  parts=[genai_types.Part(text="This is a test query.")],
+                  role="user",
+              ),
+              final_response=genai_types.Content(
+                  parts=[genai_types.Part(text="This is a test response.")],
+                  role="model",
+              ),
+          ),
+          expected_invocation=Invocation(
+              user_content=genai_types.Content(
+                  parts=[genai_types.Part(text="This is a test query.")],
+                  role="user",
+              ),
+              final_response=genai_types.Content(
+                  parts=[genai_types.Part(text="This is a test response.")],
+                  role="model",
+              ),
+          ),
+          score=0.5,
+          eval_status=EvalStatus.FAILED,
+      ),
+      PerInvocationResult(
+          actual_invocation=Invocation(
+              user_content=genai_types.Content(
+                  parts=[genai_types.Part(text="This is a test query.")],
+                  role="user",
+              ),
+              final_response=genai_types.Content(
+                  parts=[genai_types.Part(text="This is a test response.")],
+                  role="model",
+              ),
+          ),
+          expected_invocation=Invocation(
+              user_content=genai_types.Content(
+                  parts=[genai_types.Part(text="This is a test query.")],
+                  role="user",
+              ),
+              final_response=genai_types.Content(
+                  parts=[genai_types.Part(text="This is a test response.")],
+                  role="model",
+              ),
+          ),
+          score=1.0,
+          eval_status=EvalStatus.PASSED,
+      ),
+  ]
+  evaluator = _create_test_evaluator_gemini(threshold=0.7)
+  result = evaluator.aggregate_per_invocation_samples(per_invocation_samples)
+  assert result.score == 0.75
+  assert result.eval_status == EvalStatus.PASSED