fix(chat): preserve messages after summarization (#3280)

* fix(chat): preserve messages after summarization

* make format

* fix(chat): address summarization review comments

backend/tests/test_summarization_middleware.py

@@ -5,7 +5,10 @@ from unittest import mock
 from unittest.mock import MagicMock
 
 import pytest
+from langchain.agents import create_agent
+from langchain_core.language_models import BaseChatModel
 from langchain_core.messages import AIMessage, HumanMessage, RemoveMessage, ToolMessage
+from langchain_core.outputs import ChatGeneration, ChatResult
 
 from deerflow.agents.memory.summarization_hook import memory_flush_hook
 from deerflow.agents.middlewares.dynamic_context_middleware import _DYNAMIC_CONTEXT_REMINDER_KEY, DynamicContextMiddleware
@@ -22,6 +25,23 @@ def _messages() -> list:
     ]
 
 
+class _StaticChatModel(BaseChatModel):
+    text: str = "ok"
+
+    @property
+    def _llm_type(self) -> str:
+        return "static-test-chat-model"
+
+    def bind_tools(self, tools, **kwargs):
+        return self
+
+    def _generate(self, messages, stop=None, run_manager=None, **kwargs):
+        return ChatResult(generations=[ChatGeneration(message=AIMessage(content=self.text))])
+
+    async def _agenerate(self, messages, stop=None, run_manager=None, **kwargs):
+        return self._generate(messages, stop=stop, run_manager=run_manager, **kwargs)
+
+
 def _dynamic_context_reminder(msg_id: str = "reminder-1") -> HumanMessage:
     return HumanMessage(
         content="<system-reminder>\n<current_date>2026-05-08, Friday</current_date>\n</system-reminder>",
@@ -114,6 +134,32 @@ def test_before_summarization_hook_receives_messages_before_compression() -> Non
     assert result["messages"][1].content.startswith("Here is a summary")
 
 
+def test_summarization_middleware_emits_frontend_update_key_in_agent_stream() -> None:
+    middleware = DeerFlowSummarizationMiddleware(
+        model=_StaticChatModel(text="compressed summary"),
+        trigger=("messages", 4),
+        keep=("messages", 2),
+        token_counter=len,
+    )
+    agent = create_agent(
+        model=_StaticChatModel(text="done"),
+        tools=[],
+        middleware=[middleware],
+    )
+
+    chunks = list(agent.stream({"messages": _messages()}, stream_mode="updates"))
+    update = next(
+        (chunk["DeerFlowSummarizationMiddleware.before_model"] for chunk in chunks if "DeerFlowSummarizationMiddleware.before_model" in chunk),
+        None,
+    )
+
+    assert update is not None
+    emitted = update["messages"]
+    assert isinstance(emitted[0], RemoveMessage)
+    assert emitted[1].name == "summary"
+    assert emitted[1].content == ("Here is a summary of the conversation to date:\n\ncompressed summary")
+
+
 def test_dynamic_context_reminder_is_preserved_across_summarization() -> None:
     captured: list[SummarizationEvent] = []
     middleware = _middleware(before_summarization=[captured.append])