fix: recursively extract input/output schema for AgentTool

Fixes: https://github.com/google/adk-python/issues/4154

Co-authored-by: Xuan Yang <xygoogle@google.com>
PiperOrigin-RevId: 859440231

src/google/adk/tools/agent_tool.py

@@ -15,9 +15,11 @@
 from __future__ import annotations
 
 from typing import Any
+from typing import Optional
 from typing import TYPE_CHECKING
 
 from google.genai import types
+from pydantic import BaseModel
 from pydantic import model_validator
 from typing_extensions import override
 
@@ -37,6 +39,56 @@ if TYPE_CHECKING:
   from ..agents.base_agent import BaseAgent
 
 
+def _get_input_schema(agent: BaseAgent) -> Optional[type[BaseModel]]:
+  """Extracts the input_schema from an agent.
+
+  For LlmAgent, returns its input_schema directly.
+  For agents with sub_agents, recursively searches the first sub-agent for an
+  input_schema.
+
+  Args:
+    agent: The agent to extract input_schema from.
+
+  Returns:
+    The input_schema if found, None otherwise.
+  """
+  from ..agents.llm_agent import LlmAgent
+
+  if isinstance(agent, LlmAgent):
+    return agent.input_schema
+
+  # For composite agents, check the first sub-agent
+  if agent.sub_agents:
+    return _get_input_schema(agent.sub_agents[0])
+
+  return None
+
+
+def _get_output_schema(agent: BaseAgent) -> Optional[type[BaseModel]]:
+  """Extracts the output_schema from an agent.
+
+  For LlmAgent, returns its output_schema directly.
+  For agents with sub_agents, recursively searches the last sub-agent for an
+  output_schema.
+
+  Args:
+    agent: The agent to extract output_schema from.
+
+  Returns:
+    The output_schema if found, None otherwise.
+  """
+  from ..agents.llm_agent import LlmAgent
+
+  if isinstance(agent, LlmAgent):
+    return agent.output_schema
+
+  # For composite agents, check the last sub-agent
+  if agent.sub_agents:
+    return _get_output_schema(agent.sub_agents[-1])
+
+  return None
+
+
 class AgentTool(BaseTool):
   """A tool that wraps an agent.
 
@@ -74,12 +126,14 @@ class AgentTool(BaseTool):
 
   @override
   def _get_declaration(self) -> types.FunctionDeclaration:
-    from ..agents.llm_agent import LlmAgent
     from ..utils.variant_utils import GoogleLLMVariant
 
-    if isinstance(self.agent, LlmAgent) and self.agent.input_schema:
+    input_schema = _get_input_schema(self.agent)
+    output_schema = _get_output_schema(self.agent)
+
+    if input_schema:
       result = _automatic_function_calling_util.build_function_declaration(
-          func=self.agent.input_schema, variant=self._api_variant
+          func=input_schema, variant=self._api_variant
       )
       # Override the description with the agent's description
       result.description = self.agent.description
@@ -114,7 +168,7 @@ class AgentTool(BaseTool):
     # Set response schema for non-GEMINI_API variants
     if self._api_variant != GoogleLLMVariant.GEMINI_API:
       # Determine response type based on agent's output schema
-      if isinstance(self.agent, LlmAgent) and self.agent.output_schema:
+      if output_schema:
         # Agent has structured output schema - response is an object
         if is_feature_enabled(FeatureName.JSON_SCHEMA_FOR_FUNC_DECL):
           result.response_json_schema = {'type': 'object'}
@@ -137,15 +191,15 @@ class AgentTool(BaseTool):
       args: dict[str, Any],
       tool_context: ToolContext,
   ) -> Any:
-    from ..agents.llm_agent import LlmAgent
     from ..runners import Runner
     from ..sessions.in_memory_session_service import InMemorySessionService
 
     if self.skip_summarization:
       tool_context.actions.skip_summarization = True
 
-    if isinstance(self.agent, LlmAgent) and self.agent.input_schema:
-      input_value = self.agent.input_schema.model_validate(args)
+    input_schema = _get_input_schema(self.agent)
+    if input_schema:
+      input_value = input_schema.model_validate(args)
       content = types.Content(
           role='user',
           parts=[
@@ -212,10 +266,11 @@ class AgentTool(BaseTool):
     merged_text = '\n'.join(
         p.text for p in last_content.parts if p.text and not p.thought
     )
-    if isinstance(self.agent, LlmAgent) and self.agent.output_schema:
-      tool_result = self.agent.output_schema.model_validate_json(
-          merged_text
-      ).model_dump(exclude_none=True)
+    output_schema = _get_output_schema(self.agent)
+    if output_schema:
+      tool_result = output_schema.model_validate_json(merged_text).model_dump(
+          exclude_none=True
+      )
     else:
       tool_result = merged_text
     return tool_result

tests/unittests/tools/test_agent_tool.py

@@ -942,3 +942,225 @@ async def test_run_async_handles_none_parts_in_response():
   )
 
   assert tool_result == ''
+
+
+class TestAgentToolWithCompositeAgents:
+  """Tests for AgentTool wrapping composite agents (SequentialAgent, etc.)."""
+
+  def test_sequential_agent_with_first_sub_agent_input_schema(self):
+    """Test that AgentTool exposes input_schema from first sub-agent of SequentialAgent."""
+
+    class CustomInput(BaseModel):
+      query: str
+      language: str
+
+    first_agent = Agent(
+        name='first_agent',
+        model=testing_utils.MockModel.create(responses=['response1']),
+        input_schema=CustomInput,
+    )
+
+    second_agent = Agent(
+        name='second_agent',
+        model=testing_utils.MockModel.create(responses=['response2']),
+    )
+
+    sequence = SequentialAgent(
+        name='sequence',
+        description='Process the query through multiple steps',
+        sub_agents=[first_agent, second_agent],
+    )
+
+    agent_tool = AgentTool(agent=sequence)
+    declaration = agent_tool._get_declaration()
+
+    # Should expose CustomInput schema, not fallback to 'request'
+    assert declaration.name == 'sequence'
+    assert declaration.description == 'Process the query through multiple steps'
+    assert declaration.parameters.properties['query'].type == 'STRING'
+    assert declaration.parameters.properties['language'].type == 'STRING'
+    assert 'request' not in declaration.parameters.properties
+
+  def test_sequential_agent_without_input_schema_falls_back_to_request(self):
+    """Test that AgentTool falls back to 'request' when no sub-agent has input_schema."""
+
+    first_agent = Agent(
+        name='first_agent',
+        model=testing_utils.MockModel.create(responses=['response1']),
+    )
+
+    second_agent = Agent(
+        name='second_agent',
+        model=testing_utils.MockModel.create(responses=['response2']),
+    )
+
+    sequence = SequentialAgent(
+        name='sequence',
+        description='Process the query through multiple steps',
+        sub_agents=[first_agent, second_agent],
+    )
+
+    agent_tool = AgentTool(agent=sequence)
+    declaration = agent_tool._get_declaration()
+
+    # Should fall back to 'request' parameter
+    assert declaration.name == 'sequence'
+    assert declaration.parameters.properties['request'].type == 'STRING'
+    assert 'query' not in declaration.parameters.properties
+
+  @mark.parametrize(
+      'env_variables',
+      [
+          'VERTEX',
+      ],
+      indirect=True,
+  )
+  def test_sequential_agent_with_last_sub_agent_output_schema(
+      self, env_variables
+  ):
+    """Test that AgentTool uses output_schema from last sub-agent of SequentialAgent."""
+
+    class CustomOutput(BaseModel):
+      result: str
+
+    first_agent = Agent(
+        name='first_agent',
+        model=testing_utils.MockModel.create(responses=['response1']),
+    )
+
+    second_agent = Agent(
+        name='second_agent',
+        model=testing_utils.MockModel.create(responses=['response2']),
+        output_schema=CustomOutput,
+    )
+
+    sequence = SequentialAgent(
+        name='sequence',
+        description='Process the query',
+        sub_agents=[first_agent, second_agent],
+    )
+
+    agent_tool = AgentTool(agent=sequence)
+    declaration = agent_tool._get_declaration()
+
+    # Should have object response schema from last sub-agent
+    assert declaration.response is not None
+    assert declaration.response.type == types.Type.OBJECT
+
+  def test_nested_sequential_agent_input_schema(self):
+    """Test that AgentTool recursively finds input_schema in nested composite agents."""
+
+    class CustomInput(BaseModel):
+      deep_query: str
+
+    inner_agent = Agent(
+        name='inner_agent',
+        model=testing_utils.MockModel.create(responses=['response1']),
+        input_schema=CustomInput,
+    )
+
+    inner_sequence = SequentialAgent(
+        name='inner_sequence',
+        sub_agents=[inner_agent],
+    )
+
+    outer_sequence = SequentialAgent(
+        name='outer_sequence',
+        description='Nested sequence',
+        sub_agents=[inner_sequence],
+    )
+
+    agent_tool = AgentTool(agent=outer_sequence)
+    declaration = agent_tool._get_declaration()
+
+    # Should recursively find CustomInput from inner_agent
+    assert declaration.name == 'outer_sequence'
+    assert 'deep_query' in declaration.parameters.properties
+    assert declaration.parameters.properties['deep_query'].type == 'STRING'
+    assert 'request' not in declaration.parameters.properties
+
+  @mark.parametrize(
+      'env_variables',
+      [
+          'GOOGLE_AI',
+          'VERTEX',
+      ],
+      indirect=True,
+  )
+  def test_sequential_agent_custom_schema_end_to_end(self, env_variables):
+    """Test end-to-end flow with SequentialAgent using custom input/output schema."""
+
+    class CustomInput(BaseModel):
+      custom_input: str
+
+    class CustomOutput(BaseModel):
+      custom_output: str
+
+    function_call_seq = Part.from_function_call(
+        name='sequence', args={'custom_input': 'test_input'}
+    )
+
+    mock_model = testing_utils.MockModel.create(
+        responses=[
+            function_call_seq,
+            '{"custom_output": "step1_response"}',
+            '{"custom_output": "final_response"}',
+            'root_response',
+        ]
+    )
+
+    first_agent = Agent(
+        name='first_agent',
+        model=mock_model,
+        input_schema=CustomInput,
+    )
+
+    second_agent = Agent(
+        name='second_agent',
+        model=mock_model,
+        output_schema=CustomOutput,
+        output_key='seq_output',
+    )
+
+    sequence = SequentialAgent(
+        name='sequence',
+        description='A sequential pipeline',
+        sub_agents=[first_agent, second_agent],
+    )
+
+    root_agent = Agent(
+        name='root_agent',
+        model=mock_model,
+        tools=[AgentTool(agent=sequence)],
+    )
+
+    runner = testing_utils.InMemoryRunner(root_agent)
+    runner.run('test1')
+
+    # Verify the tool declaration sent to LLM has the correct schema
+    # The first request is from root_agent, which should have the tool declaration
+    first_request = mock_model.requests[0]
+    tool_declarations = first_request.config.tools
+    assert len(tool_declarations) == 1
+
+    sequence_tool = tool_declarations[0].function_declarations[0]
+    assert sequence_tool.name == 'sequence'
+    # Should have 'custom_input' parameter from first sub-agent's input_schema
+    assert 'custom_input' in sequence_tool.parameters.properties
+    # Should NOT have the fallback 'request' parameter
+    assert 'request' not in sequence_tool.parameters.properties
+
+  def test_empty_sequential_agent_falls_back_to_request(self):
+    """Test that AgentTool with empty SequentialAgent falls back to 'request'."""
+
+    sequence = SequentialAgent(
+        name='empty_sequence',
+        description='An empty sequence',
+        sub_agents=[],
+    )
+
+    agent_tool = AgentTool(agent=sequence)
+    declaration = agent_tool._get_declaration()
+
+    # Should fall back to 'request' parameter
+    assert declaration.parameters.properties['request'].type == 'STRING'