Python: fix(python): wire ChatHistoryReducer into ChatCompletionAgent invocation (#12303) (#14607)

### Motivation and Context

Fixes #12303

1. **Why is this change required?**
Currently, configuring a `ChatHistorySummarizationReducer` or
`ChatHistoryTruncationReducer` on a `ChatCompletionAgent` (e.g. via
`arguments=KernelArguments(chat_history=reducer)` or thread injection)
fails to reduce the chat history before dispatching calls to the LLM.
The full, unbounded conversation history is passed in the request
payload, causing context window overflows and unnecessary token
consumption.

2. **What problem does it solve?**
- Enables `ChatCompletionAgent` to respect active `ChatHistoryReducer`
implementations (such as summarization and truncation reducers).
- Allows users and multi-agent orchestrations to manage context size
efficiently without token explosion.

3. **What scenario does it contribute to?**
Multi-turn agent interactions and multi-agent group chats where
long-running conversations require summarizing or truncating older
messages to stay within LLM context boundaries.

---

### Description

- **Auto-reduction on thread updates:** In
`ChatHistoryAgentThread._on_new_message`, check if `self._chat_history`
is a `ChatHistoryReducer` with `auto_reduce=True`. If so, invoke `await
self._chat_history.add_message_async(new_message)` so the reduction
boundary triggers as designed.
- **Reducer propagation from KernelArguments:** In
`ChatCompletionAgent.get_response()`, `invoke()`, and `invoke_stream()`,
check if `arguments` contains an active `chat_history` reducer and pass
it into the constructor when initializing `ChatHistoryAgentThread`.
- **Pre-invocation reduction pass:** Before constructing the final
`chat_history` payload for `_inner_invoke()` / `_inner_invoke_stream()`,
check if the active thread contains a `ChatHistoryReducer` and call
`await thread.reduce()`.
- **Unit test suite:** Added unit tests verifying `ChatCompletionAgent`
respects both thread-injected and argument-injected reducers during
`get_response()`.

---

### Contribution Checklist

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

---------

Signed-off-by: sparkysolutions10-pixelmy <sparkysolutions10@gmail.com>

Author: sparkysolutions10-pixel

python/semantic_kernel/agents/chat_completion/chat_completion_agent.py

@@ -89,7 +89,10 @@ async def _on_new_message(self, new_message: str | ChatMessageContent) -> None:
             or "thread_id" not in new_message.metadata
             or new_message.metadata["thread_id"] != self._id
         ):
-            self._chat_history.add_message(new_message)
+            if isinstance(self._chat_history, ChatHistoryReducer) and self._chat_history.auto_reduce:
+                await self._chat_history.add_message_async(new_message)
+            else:
+                self._chat_history.add_message(new_message)
 
     async def get_messages(self) -> AsyncIterable[ChatMessageContent]:
         """Retrieve the current chat history.
@@ -295,14 +298,23 @@ async def get_response(
         Returns:
             An AgentResponseItem of type ChatMessageContent.
         """
+        merged_args = self._merge_arguments(arguments)
+        initial_history = (
+            merged_args.get("chat_history")
+            if merged_args and isinstance(merged_args.get("chat_history"), ChatHistory)
+            else None
+        )
         thread = await self._ensure_thread_exists_with_messages(
             messages=messages,
             thread=thread,
-            construct_thread=lambda: ChatHistoryAgentThread(),
+            construct_thread=lambda: ChatHistoryAgentThread(chat_history=initial_history),
             expected_type=ChatHistoryAgentThread,
         )
         assert thread.id is not None  # nosec
 
+        if isinstance(thread, ChatHistoryAgentThread) and isinstance(thread._chat_history, ChatHistoryReducer):
+            await thread.reduce()
+
         chat_history = ChatHistory()
         async for message in thread.get_messages():
             chat_history.add_message(message)
@@ -349,14 +361,23 @@ async def invoke(
         Returns:
             An async iterable of AgentResponseItem of type ChatMessageContent.
         """
+        merged_args = self._merge_arguments(arguments)
+        initial_history = (
+            merged_args.get("chat_history")
+            if merged_args and isinstance(merged_args.get("chat_history"), ChatHistory)
+            else None
+        )
         thread = await self._ensure_thread_exists_with_messages(
             messages=messages,
             thread=thread,
-            construct_thread=lambda: ChatHistoryAgentThread(),
+            construct_thread=lambda: ChatHistoryAgentThread(chat_history=initial_history),
             expected_type=ChatHistoryAgentThread,
         )
         assert thread.id is not None  # nosec
 
+        if isinstance(thread, ChatHistoryAgentThread) and isinstance(thread._chat_history, ChatHistoryReducer):
+            await thread.reduce()
+
         chat_history = ChatHistory()
         async for message in thread.get_messages():
             chat_history.add_message(message)
@@ -398,14 +419,23 @@ async def invoke_stream(
         Returns:
             An async generator of AgentResponseItem of type StreamingChatMessageContent.
         """
+        merged_args = self._merge_arguments(arguments)
+        initial_history = (
+            merged_args.get("chat_history")
+            if merged_args and isinstance(merged_args.get("chat_history"), ChatHistory)
+            else None
+        )
         thread = await self._ensure_thread_exists_with_messages(
             messages=messages,
             thread=thread,
-            construct_thread=lambda: ChatHistoryAgentThread(),
+            construct_thread=lambda: ChatHistoryAgentThread(chat_history=initial_history),
             expected_type=ChatHistoryAgentThread,
         )
         assert thread.id is not None  # nosec
 
+        if isinstance(thread, ChatHistoryAgentThread) and isinstance(thread._chat_history, ChatHistoryReducer):
+            await thread.reduce()
+
         chat_history = ChatHistory()
         async for message in thread.get_messages():
             chat_history.add_message(message)

python/tests/unit/agents/chat_completion/test_chat_completion_agent.py

@@ -469,3 +469,95 @@ async def test_prepare_agent_chat_history_without_formatted_instructions():
         mock_format_instructions.assert_awaited_once_with(dummy_kernel, dummy_args)
         assert len(result_history.messages) == 1
         assert result_history.messages[0] == user_message
+
+
+async def test_chat_completion_agent_with_chat_history_reducer():
+    mock_service = AsyncMock(spec=ChatCompletionClientBase)
+    mock_service.service_id = "test_service"
+    mock_service.get_prompt_execution_settings_class.return_value = PromptExecutionSettings
+    mock_service.get_chat_message_contents.return_value = [
+        ChatMessageContent(role=AuthorRole.ASSISTANT, content="Agent answer")
+    ]
+
+    reducer = ChatHistoryTruncationReducer(
+        target_count=2,
+        threshold_count=0,
+        auto_reduce=True,
+    )
+
+    thread = ChatHistoryAgentThread(chat_history=reducer)
+    for i in range(5):
+        await thread.on_new_message(ChatMessageContent(role=AuthorRole.USER, content=f"Message {i}"))
+
+    agent = ChatCompletionAgent(
+        name="ReducerAgent",
+        service=mock_service,
+    )
+
+    response = await agent.get_response(thread=thread)
+    assert response.message.content == "Agent answer"
+
+    # Verify that the chat_history passed to get_chat_message_contents was reduced
+    passed_history = mock_service.get_chat_message_contents.call_args.kwargs["chat_history"]
+    user_messages = [m.content for m in passed_history.messages if m.role == AuthorRole.USER]
+    assert user_messages == ["Message 3", "Message 4"]
+
+
+async def test_chat_completion_agent_auto_reduce_via_arguments():
+    mock_service = AsyncMock(spec=ChatCompletionClientBase)
+    mock_service.service_id = "test_service"
+    mock_service.get_prompt_execution_settings_class.return_value = PromptExecutionSettings
+    mock_service.get_chat_message_contents.return_value = [
+        ChatMessageContent(role=AuthorRole.ASSISTANT, content="Agent answer")
+    ]
+
+    reducer = ChatHistoryTruncationReducer(
+        target_count=2,
+        threshold_count=0,
+        auto_reduce=False,
+    )
+
+    for i in range(6):
+        reducer.add_message(ChatMessageContent(role=AuthorRole.USER, content=f"Msg {i}"))
+
+    agent = ChatCompletionAgent(
+        name="ReducerAgentArguments",
+        service=mock_service,
+    )
+
+    response = await agent.get_response(arguments=KernelArguments(chat_history=reducer))
+    assert response.message.content == "Agent answer"
+
+    passed_history = mock_service.get_chat_message_contents.call_args.kwargs["chat_history"]
+    user_messages = [m.content for m in passed_history.messages if m.role == AuthorRole.USER]
+    assert user_messages == ["Msg 4", "Msg 5"]
+
+
+async def test_chat_completion_agent_with_constructor_arguments_reducer():
+    mock_service = AsyncMock(spec=ChatCompletionClientBase)
+    mock_service.service_id = "test_service"
+    mock_service.get_prompt_execution_settings_class.return_value = PromptExecutionSettings
+    mock_service.get_chat_message_contents.return_value = [
+        ChatMessageContent(role=AuthorRole.ASSISTANT, content="Agent answer")
+    ]
+
+    reducer = ChatHistoryTruncationReducer(
+        target_count=2,
+        threshold_count=0,
+        auto_reduce=False,
+    )
+    for i in range(4):
+        reducer.add_message(ChatMessageContent(role=AuthorRole.USER, content=f"ConstMsg {i}"))
+
+    agent = ChatCompletionAgent(
+        name="ReducerAgentConstructorArgs",
+        service=mock_service,
+        arguments=KernelArguments(chat_history=reducer),
+    )
+
+    response = await agent.get_response()
+    assert response.message.content == "Agent answer"
+
+    passed_history = mock_service.get_chat_message_contents.call_args.kwargs["chat_history"]
+    user_messages = [m.content for m in passed_history.messages if m.role == AuthorRole.USER]
+    assert user_messages == ["ConstMsg 2", "ConstMsg 3"]