feat: Add enum constraint to agent_name for transfer_to_agent

src/google/adk/flows/llm_flows/agent_transfer.py

@@ -24,9 +24,8 @@
 from ...agents.invocation_context import InvocationContext
 from ...events.event import Event
 from ...models.llm_request import LlmRequest
-from ...tools.function_tool import FunctionTool
 from ...tools.tool_context import ToolContext
-from ...tools.transfer_to_agent_tool import transfer_to_agent
+from ...tools.transfer_to_agent_tool import TransferToAgentTool
 from ._base_llm_processor import BaseLlmRequestProcessor
 
 if typing.TYPE_CHECKING:
@@ -50,15 +49,18 @@ async def run_async(
     if not transfer_targets:
       return
 
+    transfer_to_agent = TransferToAgentTool(
+        agent_names=[agent.name for agent in transfer_targets]
+    )
+
     llm_request.append_instructions([
         _build_target_agents_instructions(
-            invocation_context.agent, transfer_targets
+            transfer_to_agent.name, invocation_context.agent, transfer_targets
         )
     ])
 
-    transfer_to_agent_tool = FunctionTool(func=transfer_to_agent)
     tool_context = ToolContext(invocation_context)
-    await transfer_to_agent_tool.process_llm_request(
+    await transfer_to_agent.process_llm_request(
         tool_context=tool_context, llm_request=llm_request
     )
 
@@ -80,7 +82,7 @@ def _build_target_agents_info(target_agent: BaseAgent) -> str:
 
 
 def _build_target_agents_instructions(
-    agent: LlmAgent, target_agents: list[BaseAgent]
+    tool_name: str, agent: LlmAgent, target_agents: list[BaseAgent]
 ) -> str:
   si = f"""
 You have a list of other agents to transfer to:
@@ -93,7 +95,7 @@ def _build_target_agents_instructions(
 can answer it.
 
 If another agent is better for answering the question according to its
-description, call `{_TRANSFER_TO_AGENT_FUNCTION_NAME}` function to transfer the
+description, call `{tool_name}` function to transfer the
 question to that agent. When transferring, do not generate any text other than
 the function call.
 """
@@ -107,9 +109,6 @@ def _build_target_agents_instructions(
   return si
 
 
-_TRANSFER_TO_AGENT_FUNCTION_NAME = transfer_to_agent.__name__
-
-
 def _get_transfer_targets(agent: LlmAgent) -> list[BaseAgent]:
   from ...agents.llm_agent import LlmAgent
 

src/google/adk/tools/__init__.py

@@ -29,7 +29,7 @@
 from .long_running_tool import LongRunningFunctionTool
 from .preload_memory_tool import preload_memory_tool as preload_memory
 from .tool_context import ToolContext
-from .transfer_to_agent_tool import transfer_to_agent
+from .transfer_to_agent_tool import TransferToAgentTool
 from .url_context_tool import url_context
 from .vertex_ai_search_tool import VertexAiSearchTool
 
@@ -51,7 +51,7 @@
     'LongRunningFunctionTool',
     'preload_memory',
     'ToolContext',
-    'transfer_to_agent',
+    'TransferToAgentTool',
 ]
 
 

src/google/adk/tools/transfer_to_agent_tool.py

@@ -14,6 +14,12 @@
 
 from __future__ import annotations
 
+from typing import Optional
+
+from google.genai import types
+from typing_extensions import override
+
+from .function_tool import FunctionTool
 from .tool_context import ToolContext
 
 
@@ -27,3 +33,24 @@ def transfer_to_agent(agent_name: str, tool_context: ToolContext) -> None:
     agent_name: the agent name to transfer to.
   """
   tool_context.actions.transfer_to_agent = agent_name
+
+
+class TransferToAgentTool(FunctionTool):
+  """A specialized FunctionTool for agent transfer."""
+
+  def __init__(
+      self,
+      agent_names: list[str],
+  ):
+    super().__init__(func=transfer_to_agent)
+    self._agent_names = agent_names
+
+  @override
+  def _get_declaration(self) -> Optional[types.FunctionDeclaration]:
+    """Add enum constraint to the agent_name."""
+    function_decl = super()._get_declaration()
+    if function_decl and function_decl.parameters:
+      agent_name_schema = function_decl.parameters.properties.get("agent_name")
+      if agent_name_schema:
+        agent_name_schema.enum = self._agent_names
+    return function_decl