feat(llm_flows): support async before/after tool callbacks

Previously, callbacks were treated as purely synchronous,
so passing an async coroutine caused “was never awaited”
errors and Pydantic serialization failures.

Now we detect awaitable return values from
before_tool_callback and after_tool_callback,
and `await` them if necessary.

Fixes: #380

pyproject.toml

@@ -147,3 +147,4 @@ line_length = 200
 [tool.pytest.ini_options]
 testpaths = ["tests"]
 asyncio_default_fixture_loop_scope = "function"
+asyncio_mode = "auto"

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

@@ -152,27 +152,35 @@ async def handle_function_calls_async(
     # in python debugger.
     function_args = function_call.args or {}
     function_response = None
-    # Calls the tool if before_tool_callback does not exist or returns None.
+    # # Calls the tool if before_tool_callback does not exist or returns None.
+    # if agent.before_tool_callback:
+    #   function_response = agent.before_tool_callback(
+    #       tool=tool, args=function_args, tool_context=tool_context
+    #   )
+    # Short-circuit via before_tool_callback (sync *or* async)
     if agent.before_tool_callback:
-      function_response = agent.before_tool_callback(
+      _maybe = agent.before_tool_callback(
           tool=tool, args=function_args, tool_context=tool_context
       )
-
+      if inspect.isawaitable(_maybe):
+          _maybe = await _maybe
+      function_response = _maybe
     if not function_response:
       function_response = await __call_tool_async(
           tool, args=function_args, tool_context=tool_context
       )
-
     # Calls after_tool_callback if it exists.
     if agent.after_tool_callback:
-      new_response = agent.after_tool_callback(
-          tool=tool,
-          args=function_args,
-          tool_context=tool_context,
-          tool_response=function_response,
+      _maybe2 = agent.after_tool_callback(
+        tool=tool,
+        args=function_args,
+        tool_context=tool_context,
+        tool_response=function_response,
       )
-      if new_response:
-        function_response = new_response
+      if inspect.isawaitable(_maybe2):
+        _maybe2 = await _maybe2
+      if _maybe2 is not None:
+        function_response = _maybe2
 
     if tool.is_long_running:
       # Allow long running function to return None to not provide function response.
@@ -223,11 +231,18 @@ async def handle_function_calls_live(
     # in python debugger.
     function_args = function_call.args or {}
     function_response = None
-    # Calls the tool if before_tool_callback does not exist or returns None.
+    # # Calls the tool if before_tool_callback does not exist or returns None.
+    # if agent.before_tool_callback:
+    #   function_response = agent.before_tool_callback(
+    #       tool, function_args, tool_context
+    #   )
     if agent.before_tool_callback:
-      function_response = agent.before_tool_callback(
-          tool, function_args, tool_context
-      )
+      _maybe = agent.before_tool_callback(
+        tool=tool, args=function_args, tool_context=tool_context
+        )
+      if inspect.isawaitable(_maybe):
+        _maybe = await _maybe
+        function_response = _maybe
 
     if not function_response:
       function_response = await _process_function_live_helper(
@@ -235,15 +250,26 @@ async def handle_function_calls_live(
       )
 
     # Calls after_tool_callback if it exists.
+    # if agent.after_tool_callback:
+    #   new_response = agent.after_tool_callback(
+    #       tool,
+    #       function_args,
+    #       tool_context,
+    #       function_response,
+    #   )
+    #   if new_response:
+    #     function_response = new_response
     if agent.after_tool_callback:
-      new_response = agent.after_tool_callback(
-          tool,
-          function_args,
-          tool_context,
-          function_response,
-      )
-      if new_response:
-        function_response = new_response
+      _maybe2 = agent.after_tool_callback(
+        tool=tool,
+        args=function_args,
+        tool_context=tool_context,
+        tool_response=function_response,
+        )
+      if inspect.isawaitable(_maybe2):
+        _maybe2 = await _maybe2
+      if _maybe2 is not None:
+        function_response = _maybe2
 
     if tool.is_long_running:
       # Allow async function to return None to not provide function response.

tests/unittests/flows/llm_flows/test_async_tool_callbacks.py

@@ -0,0 +1,107 @@
+# 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 typing import Any, Dict, Optional
+
+import pytest
+
+from google.adk.agents import Agent
+from google.adk.tools.function_tool import FunctionTool
+from google.adk.tools.tool_context import ToolContext
+from google.adk.flows.llm_flows.functions import handle_function_calls_async
+from google.adk.events.event import Event
+from google.genai import types
+
+from ... import utils
+
+
+class AsyncBeforeToolCallback:
+    def __init__(self, mock_response: Dict[str, Any]):
+        self.mock_response = mock_response
+
+    async def __call__(
+        self,
+        tool: FunctionTool,
+        args: Dict[str, Any],
+        tool_context: ToolContext,
+    ) -> Optional[Dict[str, Any]]:
+        return self.mock_response
+
+
+class AsyncAfterToolCallback:
+    def __init__(self, mock_response: Dict[str, Any]):
+        self.mock_response = mock_response
+
+    async def __call__(
+        self,
+        tool: FunctionTool,
+        args: Dict[str, Any],
+        tool_context: ToolContext,
+        tool_response: Dict[str, Any],
+    ) -> Optional[Dict[str, Any]]:
+        return self.mock_response
+
+
+async def invoke_tool_with_callbacks(
+    before_cb=None, after_cb=None
+) -> Optional[Event]:
+    def simple_fn(**kwargs) -> Dict[str, Any]:
+        return {"initial": "response"}
+
+    tool = FunctionTool(simple_fn)
+    model = utils.MockModel.create(responses=[])
+    agent = Agent(
+        name="agent",
+        model=model,
+        tools=[tool],
+        before_tool_callback=before_cb,
+        after_tool_callback=after_cb,
+    )
+    invocation_context = utils.create_invocation_context(
+        agent=agent, user_content=""
+    )
+    # Build function call event
+    function_call = types.FunctionCall(name=tool.name, args={})
+    content = types.Content(parts=[types.Part(function_call=function_call)])
+    event = Event(
+        invocation_id=invocation_context.invocation_id,
+        author=agent.name,
+        content=content,
+    )
+    tools_dict = {tool.name: tool}
+    return await handle_function_calls_async(
+        invocation_context,
+        event,
+        tools_dict, 
+        )
+
+
+@pytest.mark.asyncio
+async def test_async_before_tool_callback():
+    mock_resp = {"test": "before_tool_callback"}
+    before_cb = AsyncBeforeToolCallback(mock_resp)
+    result_event = await invoke_tool_with_callbacks(before_cb=before_cb)
+    assert result_event is not None
+    part = result_event.content.parts[0]
+    assert part.function_response.response == mock_resp
+
+
+@pytest.mark.asyncio
+async def test_async_after_tool_callback():
+    mock_resp = {"test": "after_tool_callback"}
+    after_cb = AsyncAfterToolCallback(mock_resp)
+    result_event = await invoke_tool_with_callbacks(after_cb=after_cb)
+    assert result_event is not None
+    part = result_event.content.parts[0]
+    assert part.function_response.response == mock_resp