diff --git a/src/google/adk/agents/base_agent.py b/src/google/adk/agents/base_agent.py
index a644cb8b90..fccde1cb6f 100644
--- a/src/google/adk/agents/base_agent.py
+++ b/src/google/adk/agents/base_agent.py
@@ -563,6 +563,46 @@ def validate_name(cls, value: str):
       )
     return value
 
+  @field_validator('sub_agents', mode='after')
+  @classmethod
+  def validate_sub_agents_unique_names(
+      cls, value: list[BaseAgent]
+  ) -> list[BaseAgent]:
+    """Validates that all sub-agents have unique names.
+
+    Args:
+      value: The list of sub-agents to validate.
+
+    Returns:
+      The validated list of sub-agents.
+
+    Raises:
+      ValueError: If duplicate sub-agent names are found.
+    """
+    if not value:
+      return value
+
+    seen_names: set[str] = set()
+    duplicates: set[str] = set()
+
+    for sub_agent in value:
+      name = sub_agent.name
+      if name in seen_names:
+        duplicates.add(name)
+      else:
+        seen_names.add(name)
+
+    if duplicates:
+      duplicate_names_str = ', '.join(
+          f'`{name}`' for name in sorted(duplicates)
+      )
+      raise ValueError(
+          f'Found duplicate sub-agent names: {duplicate_names_str}. '
+          'All sub-agents must have unique names.'
+      )
+
+    return value
+
   def __set_parent_agent_for_sub_agents(self) -> BaseAgent:
     for sub_agent in self.sub_agents:
       if sub_agent.parent_agent is not None:
diff --git a/src/google/adk/models/lite_llm.py b/src/google/adk/models/lite_llm.py
index c9712974ea..9e3698b190 100644
--- a/src/google/adk/models/lite_llm.py
+++ b/src/google/adk/models/lite_llm.py
@@ -39,8 +39,8 @@
 from litellm import acompletion
 from litellm import ChatCompletionAssistantMessage
 from litellm import ChatCompletionAssistantToolCall
-from litellm import ChatCompletionDeveloperMessage
 from litellm import ChatCompletionMessageToolCall
+from litellm import ChatCompletionSystemMessage
 from litellm import ChatCompletionToolMessage
 from litellm import ChatCompletionUserMessage
 from litellm import completion
@@ -983,8 +983,8 @@ def _get_completion_inputs(
   if llm_request.config.system_instruction:
     messages.insert(
         0,
-        ChatCompletionDeveloperMessage(
-            role="developer",
+        ChatCompletionSystemMessage(
+            role="system",
             content=llm_request.config.system_instruction,
         ),
     )
diff --git a/src/google/adk/tools/agent_tool.py b/src/google/adk/tools/agent_tool.py
index 702f6e43aa..46d8616619 100644
--- a/src/google/adk/tools/agent_tool.py
+++ b/src/google/adk/tools/agent_tool.py
@@ -45,11 +45,22 @@ class AgentTool(BaseTool):
   Attributes:
     agent: The agent to wrap.
     skip_summarization: Whether to skip summarization of the agent output.
+    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):
+  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
 
     super().__init__(name=agent.name, description=agent.description)
 
@@ -68,6 +79,8 @@ def _get_declaration(self) -> types.FunctionDeclaration:
       result = _automatic_function_calling_util.build_function_declaration(
           func=self.agent.input_schema, variant=self._api_variant
       )
+      # Override the description with the agent's description
+      result.description = self.agent.description
     else:
       result = types.FunctionDeclaration(
           parameters=types.Schema(
@@ -130,6 +143,11 @@ async def run_async(
         invocation_context.app_name if invocation_context else None
     )
     child_app_name = parent_app_name or self.agent.name
+    plugins = (
+        tool_context._invocation_context.plugin_manager.plugins
+        if self.include_plugins
+        else None
+    )
     runner = Runner(
         app_name=child_app_name,
         agent=self.agent,
@@ -137,7 +155,7 @@ async def run_async(
         session_service=InMemorySessionService(),
         memory_service=InMemoryMemoryService(),
         credential_service=tool_context._invocation_context.credential_service,
-        plugins=list(tool_context._invocation_context.plugin_manager.plugins),
+        plugins=plugins,
     )
 
     state_dict = {
@@ -192,7 +210,9 @@ 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,
     )
 
 
@@ -204,3 +224,6 @@ 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."""
diff --git a/tests/unittests/agents/test_base_agent.py b/tests/unittests/agents/test_base_agent.py
index 663179f670..860cc8d4f0 100644
--- a/tests/unittests/agents/test_base_agent.py
+++ b/tests/unittests/agents/test_base_agent.py
@@ -854,6 +854,110 @@ def test_set_parent_agent_for_sub_agent_twice(
     )
 
 
+def test_validate_sub_agents_unique_names_single_duplicate(
+    request: pytest.FixtureRequest,
+):
+  """Test that duplicate sub-agent names raise ValueError."""
+  duplicate_name = f'{request.function.__name__}_duplicate_agent'
+  sub_agent_1 = _TestingAgent(name=duplicate_name)
+  sub_agent_2 = _TestingAgent(name=duplicate_name)
+
+  with pytest.raises(ValueError, match='Found duplicate sub-agent names'):
+    _ = _TestingAgent(
+        name=f'{request.function.__name__}_parent',
+        sub_agents=[sub_agent_1, sub_agent_2],
+    )
+
+
+def test_validate_sub_agents_unique_names_multiple_duplicates(
+    request: pytest.FixtureRequest,
+):
+  """Test that multiple duplicate sub-agent names are all reported."""
+  duplicate_name_1 = f'{request.function.__name__}_duplicate_1'
+  duplicate_name_2 = f'{request.function.__name__}_duplicate_2'
+
+  sub_agents = [
+      _TestingAgent(name=duplicate_name_1),
+      _TestingAgent(name=f'{request.function.__name__}_unique'),
+      _TestingAgent(name=duplicate_name_1),  # First duplicate
+      _TestingAgent(name=duplicate_name_2),
+      _TestingAgent(name=duplicate_name_2),  # Second duplicate
+  ]
+
+  with pytest.raises(ValueError) as exc_info:
+    _ = _TestingAgent(
+        name=f'{request.function.__name__}_parent',
+        sub_agents=sub_agents,
+    )
+
+  error_message = str(exc_info.value)
+  # Verify each duplicate name appears exactly once in the error message
+  assert error_message.count(duplicate_name_1) == 1
+  assert error_message.count(duplicate_name_2) == 1
+  # Verify both duplicate names are present
+  assert duplicate_name_1 in error_message
+  assert duplicate_name_2 in error_message
+
+
+def test_validate_sub_agents_unique_names_triple_duplicate(
+    request: pytest.FixtureRequest,
+):
+  """Test that a name appearing three times is reported only once."""
+  duplicate_name = f'{request.function.__name__}_triple_duplicate'
+
+  sub_agents = [
+      _TestingAgent(name=duplicate_name),
+      _TestingAgent(name=f'{request.function.__name__}_unique'),
+      _TestingAgent(name=duplicate_name),  # Second occurrence
+      _TestingAgent(name=duplicate_name),  # Third occurrence
+  ]
+
+  with pytest.raises(ValueError) as exc_info:
+    _ = _TestingAgent(
+        name=f'{request.function.__name__}_parent',
+        sub_agents=sub_agents,
+    )
+
+  error_message = str(exc_info.value)
+  # Verify the duplicate name appears exactly once in the error message
+  # (not three times even though it appears three times in the list)
+  assert error_message.count(duplicate_name) == 1
+  assert duplicate_name in error_message
+
+
+def test_validate_sub_agents_unique_names_no_duplicates(
+    request: pytest.FixtureRequest,
+):
+  """Test that unique sub-agent names pass validation."""
+  sub_agents = [
+      _TestingAgent(name=f'{request.function.__name__}_sub_agent_1'),
+      _TestingAgent(name=f'{request.function.__name__}_sub_agent_2'),
+      _TestingAgent(name=f'{request.function.__name__}_sub_agent_3'),
+  ]
+
+  parent = _TestingAgent(
+      name=f'{request.function.__name__}_parent',
+      sub_agents=sub_agents,
+  )
+
+  assert len(parent.sub_agents) == 3
+  assert parent.sub_agents[0].name == f'{request.function.__name__}_sub_agent_1'
+  assert parent.sub_agents[1].name == f'{request.function.__name__}_sub_agent_2'
+  assert parent.sub_agents[2].name == f'{request.function.__name__}_sub_agent_3'
+
+
+def test_validate_sub_agents_unique_names_empty_list(
+    request: pytest.FixtureRequest,
+):
+  """Test that empty sub-agents list passes validation."""
+  parent = _TestingAgent(
+      name=f'{request.function.__name__}_parent',
+      sub_agents=[],
+  )
+
+  assert len(parent.sub_agents) == 0
+
+
 if __name__ == '__main__':
   pytest.main([__file__])
 
diff --git a/tests/unittests/models/test_litellm.py b/tests/unittests/models/test_litellm.py
index c486806d37..f65fc77a61 100644
--- a/tests/unittests/models/test_litellm.py
+++ b/tests/unittests/models/test_litellm.py
@@ -1195,7 +1195,7 @@ async def test_generate_content_async_with_system_instruction(
 
   _, kwargs = mock_acompletion.call_args
   assert kwargs["model"] == "test_model"
-  assert kwargs["messages"][0]["role"] == "developer"
+  assert kwargs["messages"][0]["role"] == "system"
   assert kwargs["messages"][0]["content"] == "Test system instruction"
   assert kwargs["messages"][1]["role"] == "user"
   assert kwargs["messages"][1]["content"] == "Test prompt"
diff --git a/tests/unittests/tools/test_agent_tool.py b/tests/unittests/tools/test_agent_tool.py
index 85e8b9caa1..a9723b4347 100644
--- a/tests/unittests/tools/test_agent_tool.py
+++ b/tests/unittests/tools/test_agent_tool.py
@@ -570,3 +570,135 @@ 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
+
+
+def test_agent_tool_description_with_input_schema():
+  """Test that agent description is propagated when using input_schema."""
+
+  class CustomInput(BaseModel):
+    """This is the Pydantic model docstring."""
+
+    custom_input: str
+
+  agent_description = 'This is the agent description that should be used'
+  tool_agent = Agent(
+      name='tool_agent',
+      model=testing_utils.MockModel.create(responses=['test response']),
+      description=agent_description,
+      input_schema=CustomInput,
+  )
+
+  agent_tool = AgentTool(agent=tool_agent)
+  declaration = agent_tool._get_declaration()
+
+  # The description should come from the agent, not the Pydantic model
+  assert declaration.description == agent_description