feat(agent-tool): Make include_plugins keyword-only with default True

- Change include_plugins default from False to True (matches existing behavior)
- Make only include_plugins keyword-only (preserves backward compatibility)
- Enhance docstring to explain plugin propagation behavior
- Add include_plugins field to AgentToolConfig
- Update from_config to pass include_plugins parameter

Tests:
- Add test_include_plugins_default_true (verifies default propagation)
- Add test_include_plugins_explicit_true (verifies explicit True)
- Add test_include_plugins_false (verifies opt-out behavior)
- All 18 tests pass (6 new test variants for GOOGLE_AI and VERTEX)

Author: Jacksunwei

src/google/adk/tools/agent_tool.py

@@ -45,10 +45,19 @@ class AgentTool(BaseTool):
   Attributes:
     agent: The agent to wrap.
     skip_summarization: Whether to skip summarization of the agent output.
-    include_plugins: Whether to include plugins from parent runner context
+    include_plugins: Whether to propagate plugins from the parent runner context
+      to the agent's runner. When True (default), the agent will inherit all
+      plugins from its parent. Set to False to run the agent with an isolated
+      plugin environment.
   """
 
-  def __init__(self, *, agent: BaseAgent, skip_summarization: bool = False, include_plugins: bool = False):
+  def __init__(
+      self,
+      agent: BaseAgent,
+      skip_summarization: bool = False,
+      *,
+      include_plugins: bool = True,
+  ):
     self.agent = agent
     self.skip_summarization: bool = skip_summarization
     self.include_plugins = include_plugins
@@ -127,15 +136,19 @@ async def run_async(
           role='user',
           parts=[types.Part.from_text(text=args['request'])],
       )
-    plugins = tool_context._invocation_context.plugin_manager.plugins if self.include_plugins else None
+    plugins = (
+        tool_context._invocation_context.plugin_manager.plugins
+        if self.include_plugins
+        else None
+    )
     runner = Runner(
         app_name=self.agent.name,
         agent=self.agent,
         artifact_service=ForwardingArtifactService(tool_context),
         session_service=InMemorySessionService(),
         memory_service=InMemoryMemoryService(),
         credential_service=tool_context._invocation_context.credential_service,
-        plugins=plugins
+        plugins=plugins,
     )
     session = await runner.session_service.create_session(
         app_name=self.agent.name,
@@ -180,7 +193,9 @@ def from_config(
         agent_tool_config.agent, config_abs_path
     )
     return cls(
-        agent=agent, skip_summarization=agent_tool_config.skip_summarization
+        agent=agent,
+        skip_summarization=agent_tool_config.skip_summarization,
+        include_plugins=agent_tool_config.include_plugins,
     )
 
 
@@ -192,3 +207,6 @@ class AgentToolConfig(BaseToolConfig):
 
   skip_summarization: bool = False
   """Whether to skip summarization of the agent output."""
+
+  include_plugins: bool = True
+  """Whether to include plugins from parent runner context."""

tests/unittests/tools/test_agent_tool.py

@@ -15,6 +15,7 @@
 from google.adk.agents.callback_context import CallbackContext
 from google.adk.agents.llm_agent import Agent
 from google.adk.agents.sequential_agent import SequentialAgent
+from google.adk.plugins.base_plugin import BasePlugin
 from google.adk.tools.agent_tool import AgentTool
 from google.adk.utils.variant_utils import GoogleLLMVariant
 from google.genai import types
@@ -355,3 +356,109 @@ class CustomInput(BaseModel):
   # Should have string response schema for VERTEX_AI when no output_schema
   assert declaration.response is not None
   assert declaration.response.type == types.Type.STRING
+
+
+def test_include_plugins_default_true():
+  """Test that plugins are propagated by default (include_plugins=True)."""
+
+  # Create a test plugin that tracks callbacks
+  class TrackingPlugin(BasePlugin):
+    def __init__(self, name: str):
+      super().__init__(name)
+      self.before_agent_calls = 0
+
+    async def before_agent_callback(self, **kwargs):
+      self.before_agent_calls += 1
+
+  tracking_plugin = TrackingPlugin(name='tracking')
+
+  mock_model = testing_utils.MockModel.create(
+      responses=[function_call_no_schema, 'response1', 'response2']
+  )
+
+  tool_agent = Agent(
+      name='tool_agent',
+      model=mock_model,
+  )
+
+  root_agent = Agent(
+      name='root_agent',
+      model=mock_model,
+      tools=[AgentTool(agent=tool_agent)],  # Default include_plugins=True
+  )
+
+  runner = testing_utils.InMemoryRunner(root_agent, plugins=[tracking_plugin])
+  runner.run('test1')
+
+  # Plugin should be called for both root_agent and tool_agent
+  assert tracking_plugin.before_agent_calls == 2
+
+
+def test_include_plugins_explicit_true():
+  """Test that plugins are propagated when include_plugins=True."""
+
+  class TrackingPlugin(BasePlugin):
+    def __init__(self, name: str):
+      super().__init__(name)
+      self.before_agent_calls = 0
+
+    async def before_agent_callback(self, **kwargs):
+      self.before_agent_calls += 1
+
+  tracking_plugin = TrackingPlugin(name='tracking')
+
+  mock_model = testing_utils.MockModel.create(
+      responses=[function_call_no_schema, 'response1', 'response2']
+  )
+
+  tool_agent = Agent(
+      name='tool_agent',
+      model=mock_model,
+  )
+
+  root_agent = Agent(
+      name='root_agent',
+      model=mock_model,
+      tools=[AgentTool(agent=tool_agent, include_plugins=True)],
+  )
+
+  runner = testing_utils.InMemoryRunner(root_agent, plugins=[tracking_plugin])
+  runner.run('test1')
+
+  # Plugin should be called for both root_agent and tool_agent
+  assert tracking_plugin.before_agent_calls == 2
+
+
+def test_include_plugins_false():
+  """Test that plugins are NOT propagated when include_plugins=False."""
+
+  class TrackingPlugin(BasePlugin):
+    def __init__(self, name: str):
+      super().__init__(name)
+      self.before_agent_calls = 0
+
+    async def before_agent_callback(self, **kwargs):
+      self.before_agent_calls += 1
+
+  tracking_plugin = TrackingPlugin(name='tracking')
+
+  mock_model = testing_utils.MockModel.create(
+      responses=[function_call_no_schema, 'response1', 'response2']
+  )
+
+  tool_agent = Agent(
+      name='tool_agent',
+      model=mock_model,
+  )
+
+  root_agent = Agent(
+      name='root_agent',
+      model=mock_model,
+      tools=[AgentTool(agent=tool_agent, include_plugins=False)],
+  )
+
+  runner = testing_utils.InMemoryRunner(root_agent, plugins=[tracking_plugin])
+  runner.run('test1')
+
+  # Plugin should only be called for root_agent, not tool_agent
+  assert tracking_plugin.before_agent_calls == 1