feat(agent-tool): Add event streaming propagation from AgentTool sub-agents to Runner

contributing/samples/agent_tool_event_streaming/README.md

@@ -0,0 +1,62 @@
+# AgentTool Event Streaming Demo
+
+This sample demonstrates the AgentTool event streaming feature (Issue #3984).
+
+
+**Before the fix:**
+- When a coordinator agent delegates to a sub-agent via AgentTool, the sub-agent's execution acts as a "black box"
+- No events are yielded during sub-agent execution
+- Frontend appears unresponsive for the duration of sub-agent execution
+- Only the final result is returned after sub-agent completes
+
+**After the fix:**
+- Sub-agent events are streamed in real-time to the parent Runner
+- Frontend receives immediate feedback about sub-agent progress
+- Users can see intermediate steps, tool calls, and responses as they happen
+- Much better UX for hierarchical multi-agent systems
+
+## Running the Demo
+
+```bash
+cd contributing/samples/agent_tool_event_streaming
+adk web .
+
+```
+
+Then in the web UI, select agent_tool_event_streaming from the dropdown
+1. Ask: "Research the history of artificial intelligence"
+2. Watch the events stream in real-time - you'll see:
+   - Coordinator agent's function call
+   - Research agent's step-by-step progress
+   - Research agent's intermediate responses
+   - Final summary
+
+
+## Expected Behavior
+
+With event streaming enabled, you should see:
+
+1. **Coordinator events:**
+   - Function call to `research_agent`
+
+2. **Research agent events (streamed in real-time):**
+   - "Step 1: Acknowledging task..."
+   - "Step 2: Researching topic..."
+   - "Step 3: Analyzing findings..."
+   - "Final summary: ..."
+
+3. **Coordinator final response:**
+   - Summary of the research
+
+All events should appear progressively, not all at once at the end.
+
+## Before/After Comparison
+
+To see the difference:
+
+1. **Before fix:** Run on a branch without the event streaming feature
+   - You'll see: Coordinator call → (long pause) → Final result
+
+2. **After fix:** Run on this branch
+   - You'll see: Coordinator call → Research steps streaming → Final result
+

contributing/samples/agent_tool_event_streaming/__init__.py

@@ -0,0 +1,17 @@
+# 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 .agent import root_agent
+
+__all__ = ['root_agent']

contributing/samples/agent_tool_event_streaming/agent.py

@@ -0,0 +1,66 @@
+# 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.
+
+"""Sample demonstrating AgentTool event streaming.
+
+This sample shows how events from sub-agents wrapped in AgentTool are
+streamed to the parent Runner in real-time, providing visibility into
+sub-agent execution progress.
+
+Before the fix: Sub-agent events are buffered until completion, making
+the frontend appear unresponsive during long-running sub-agent tasks.
+
+After the fix: Sub-agent events are streamed immediately, providing
+real-time feedback to the frontend.
+"""
+
+from google.adk import Agent
+from google.adk.tools import AgentTool
+
+# Sub-agent that performs a multi-step task
+research_agent = Agent(
+    name='research_agent',
+    model='gemini-2.5-flash-lite',
+    description='A research agent that performs multi-step research tasks',
+    instruction="""
+    You are a research assistant. When given a research task, break it down
+    into steps and report your progress as you work:
+
+    1. First, acknowledge the task and outline your approach
+    2. Then, perform the research (simulate by thinking through the steps)
+    3. Finally, provide a comprehensive summary
+
+    Always be verbose about your progress so the user can see what you're doing.
+    """,
+)
+
+# Coordinator agent that delegates to the research agent
+coordinator_agent = Agent(
+    name='coordinator_agent',
+    model='gemini-2.5-flash-lite',
+    description='A coordinator that delegates research tasks',
+    instruction="""
+    You are a coordinator agent. When users ask research questions, delegate
+    them to the research_agent tool. Always use the research_agent tool for
+    any research-related queries.
+    """,
+    tools=[
+        AgentTool(
+            agent=research_agent,
+            skip_summarization=True,
+        )
+    ],
+)
+
+root_agent = coordinator_agent

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

@@ -678,24 +678,27 @@ async def _postprocess_handle_function_calls_async(
       function_call_event: Event,
       llm_request: LlmRequest,
   ) -> AsyncGenerator[Event, None]:
-    if function_response_event := await functions.handle_function_calls_async(
+    # Handle function calls with AgentTool event streaming (handles both AgentTool and regular calls)
+    function_response_event = None
+    async for (
+        event
+    ) in functions.handle_function_calls_async_with_agent_tool_streaming(
         invocation_context, function_call_event, llm_request.tools_dict
     ):
-      auth_event = functions.generate_auth_event(
-          invocation_context, function_response_event
-      )
-      if auth_event:
-        yield auth_event
-
-      tool_confirmation_event = functions.generate_request_confirmation_event(
-          invocation_context, function_call_event, function_response_event
-      )
-      if tool_confirmation_event:
-        yield tool_confirmation_event
-
-      # Always yield the function response event first
-      yield function_response_event
+      # Track the function response event for post-processing
+      if (
+          event.content
+          and event.content.parts
+          and any(
+              part.function_response
+              for part in event.content.parts
+              if part.function_response
+          )
+      ):
+        function_response_event = event
+      yield event
 
+    if function_response_event:
       # Check if this is a set_model_response function response
       if json_response := _output_schema_processor.get_structured_model_response(
           function_response_event

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

@@ -186,6 +186,137 @@ def generate_request_confirmation_event(
   )
 
 
+async def handle_function_calls_async_with_agent_tool_streaming(
+    invocation_context: InvocationContext,
+    function_call_event: Event,
+    tools_dict: dict[str, BaseTool],
+) -> AsyncGenerator[Event, None]:
+  """Handles function calls with event streaming for AgentTool.
+
+  Yields events from AgentTool sub-agents as they are generated, then
+  yields the final function response event.
+  """
+  from ...agents.llm_agent import LlmAgent
+  from ...tools.agent_tool import AgentTool
+
+  function_calls = function_call_event.get_function_calls()
+  if not function_calls:
+    return
+
+  agent_tool_calls = []
+  regular_calls = []
+
+  # Separate AgentTool calls from regular calls
+  for function_call in function_calls:
+    tool = tools_dict.get(function_call.name)
+    if isinstance(tool, AgentTool):
+      agent_tool_calls.append((function_call, tool))
+    else:
+      regular_calls.append(function_call)
+
+  # If no AgentTool calls, use normal flow
+  if not agent_tool_calls:
+    function_response_event = await handle_function_calls_async(
+        invocation_context, function_call_event, tools_dict
+    )
+    if function_response_event:
+      auth_event = generate_auth_event(
+          invocation_context, function_response_event
+      )
+      if auth_event:
+        yield auth_event
+      tool_confirmation_event = generate_request_confirmation_event(
+          invocation_context, function_call_event, function_response_event
+      )
+      if tool_confirmation_event:
+        yield tool_confirmation_event
+      yield function_response_event
+    return
+
+  # Stream events from AgentTool sub-agents
+  agent_tool_results = {}
+  for function_call, agent_tool in agent_tool_calls:
+    tool_context = _create_tool_context(invocation_context, function_call, None)
+    last_content = None
+
+    async for event in agent_tool.run_async_with_events(
+        args=function_call.args or {}, tool_context=tool_context
+    ):
+      yield event
+      if event.content:
+        last_content = event.content
+
+    # Build final result from last content using AgentTool helper method
+    tool_result = agent_tool._build_tool_result_from_content(last_content)
+    # Wrap non-dict results for function response format
+    if not isinstance(tool_result, dict):
+      tool_result = {'result': tool_result}
+    agent_tool_results[function_call.id] = tool_result
+
+  # Handle regular calls if any
+  regular_response_event = None
+  if regular_calls:
+    regular_call_event = Event(
+        invocation_id=function_call_event.invocation_id,
+        author=function_call_event.author,
+        content=types.Content(
+            role='user',
+            parts=[
+                part
+                for part in (function_call_event.content.parts or [])
+                if part.function_call
+                and part.function_call.name
+                not in [fc.name for fc, _ in agent_tool_calls]
+            ],
+        ),
+        branch=function_call_event.branch,
+    )
+    regular_response_event = await handle_function_calls_async(
+        invocation_context, regular_call_event, tools_dict
+    )
+
+  # Build AgentTool response events
+  agent_tool_response_events = []
+  for function_call, agent_tool in agent_tool_calls:
+    if function_call.id in agent_tool_results:
+      tool_context = _create_tool_context(
+          invocation_context, function_call, None
+      )
+      response_event = __build_response_event(
+          agent_tool,
+          agent_tool_results[function_call.id],
+          tool_context,
+          invocation_context,
+      )
+      agent_tool_response_events.append(response_event)
+
+  # Merge all response events
+  all_events = []
+  if regular_response_event:
+    all_events.append(regular_response_event)
+  all_events.extend(agent_tool_response_events)
+
+  if all_events:
+    if len(all_events) == 1:
+      final_response_event = all_events[0]
+    else:
+      final_response_event = merge_parallel_function_response_events(all_events)
+
+    # Yield auth and confirmation events
+    auth_event = generate_auth_event(invocation_context, final_response_event)
+    if auth_event:
+      yield auth_event
+
+    tool_confirmation_event = generate_request_confirmation_event(
+        invocation_context, function_call_event, final_response_event
+    )
+    if tool_confirmation_event:
+      yield tool_confirmation_event
+
+    # Yield the final function response event
+    yield final_response_event
+
+
 async def handle_function_calls_async(
     invocation_context: InvocationContext,
     function_call_event: Event,