fix: hide summarization LLM output from frontend during context compression

Fixes #2804

  The useStream hook tracks messages-tuple mode which captures ALL LLM
  events, including the internal summarization LLM call. This caused the
  English summary text to briefly appear as an AI message in the UI before
  the REMOVE_ALL_MESSAGES state update replaced it.

  Fix by overriding _create_summary/_acreate_summary to pass callbacks=[]
  when invoking the summary model, preventing LangGraph from forwarding
  the internal LLM events to the frontend stream. Also add
  hide_from_ui=True to the summary HumanMessage's additional_kwargs as a
  belt-and-suspenders safety net alongside the existing name=summary
  check.

backend/tests/test_summarization_middleware.py

@@ -634,3 +634,62 @@ def test_memory_flush_hook_preserves_agent_scoped_memory(monkeypatch: pytest.Mon
 
     queue.add_nowait.assert_called_once()
     assert queue.add_nowait.call_args.kwargs["agent_name"] == "research-agent"
+
+
+# ---------------------------------------------------------------------------
+# Issue #2804: summary text must not leak to the frontend via streaming
+# ---------------------------------------------------------------------------
+
+
+def test_build_new_messages_sets_hide_from_ui() -> None:
+    """The summary HumanMessage must carry hide_from_ui so the frontend filters it."""
+    middleware = _middleware()
+    messages = middleware._build_new_messages("test summary")
+
+    assert len(messages) == 1
+    msg = messages[0]
+    assert msg.name == "summary"
+    assert msg.additional_kwargs.get("hide_from_ui") is True
+    assert "test summary" in msg.content
+
+
+def test_create_summary_suppresses_callbacks() -> None:
+    """_create_summary must pass callbacks=[] to prevent the internal LLM call
+    from producing visible streaming events in the frontend (issue #2804)."""
+    middleware = _middleware()
+
+    middleware._create_summary(_messages())
+
+    middleware.model.invoke.assert_called_once()
+    call_config = middleware.model.invoke.call_args.kwargs.get("config") or middleware.model.invoke.call_args[1].get("config")
+    assert call_config is not None
+    assert call_config.get("callbacks") == []
+    assert call_config.get("metadata", {}).get("lc_source") == "summarization"
+
+
+@pytest.mark.anyio
+async def test_acreate_summary_suppresses_callbacks() -> None:
+    """_acreate_summary must pass callbacks=[] to prevent the internal LLM call
+    from producing visible streaming events in the frontend (issue #2804)."""
+    middleware = _middleware()
+    middleware.model.ainvoke = mock.AsyncMock(return_value=SimpleNamespace(text="async summary"))
+
+    await middleware._acreate_summary(_messages())
+
+    middleware.model.ainvoke.assert_called_once()
+    call_config = middleware.model.ainvoke.call_args.kwargs.get("config") or middleware.model.ainvoke.call_args[1].get("config")
+    assert call_config is not None
+    assert call_config.get("callbacks") == []
+    assert call_config.get("metadata", {}).get("lc_source") == "summarization"
+
+
+def test_before_model_summary_message_has_hide_from_ui() -> None:
+    """End-to-end: the emitted state update contains a summary message with hide_from_ui."""
+    middleware = _middleware()
+
+    result = middleware.before_model({"messages": _messages()}, _runtime())
+
+    emitted = result["messages"]
+    summary_msg = emitted[1]
+    assert summary_msg.name == "summary"
+    assert summary_msg.additional_kwargs.get("hide_from_ui") is True