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

* fix(chat): preserve messages after summarization

* make format

* fix(chat): address summarization review comments

frontend/tests/unit/core/threads/message-merge.test.ts

@@ -2,6 +2,7 @@ import type { Message } from "@langchain/langgraph-sdk";
 import { expect, test } from "vitest";
 
 import {
+  getSummarizationMiddlewareMessages,
   getVisibleOptimisticMessages,
   mergeMessages,
 } from "@/core/threads/hooks";
@@ -66,6 +67,104 @@ test("mergeMessages deduplicates tool messages by tool_call_id", () => {
   expect(mergeMessages([oldTool], [liveTool], [])).toEqual([liveTool]);
 });
 
+test("mergeMessages keeps a visible history message when a hidden live message reuses its id", () => {
+  const historyHuman = {
+    id: "human-1",
+    type: "human",
+    content: "visible user prompt",
+  } as Message;
+  const hiddenReminder = {
+    id: "human-1",
+    type: "human",
+    content: "<system-reminder>hidden</system-reminder>",
+    additional_kwargs: { hide_from_ui: true },
+  } as Message;
+  const liveAi = {
+    id: "ai-1",
+    type: "ai",
+    content: "live answer",
+  } as Message;
+
+  expect(mergeMessages([historyHuman], [hiddenReminder, liveAi], [])).toEqual([
+    historyHuman,
+    liveAi,
+  ]);
+});
+
+test("mergeMessages lets a visible live message replace overlapping hidden history", () => {
+  const hiddenHistoryHuman = {
+    id: "human-1",
+    type: "human",
+    content: "<system-reminder>hidden</system-reminder>",
+    additional_kwargs: { hide_from_ui: true },
+  } as Message;
+  const liveHuman = {
+    id: "human-1",
+    type: "human",
+    content: "visible user prompt",
+  } as Message;
+
+  expect(mergeMessages([hiddenHistoryHuman], [liveHuman], [])).toEqual([
+    liveHuman,
+  ]);
+});
+
+test("getSummarizationMiddlewareMessages matches DeerFlow summarization update keys", () => {
+  const removeAll = {
+    id: "__remove_all__",
+    type: "remove",
+    content: "",
+  } as Message;
+  const summary = {
+    id: "summary-1",
+    type: "human",
+    name: "summary",
+    content: "summary",
+  } as Message;
+
+  expect(
+    getSummarizationMiddlewareMessages({
+      "DeerFlowSummarizationMiddleware.before_model": {
+        messages: [removeAll, summary],
+      },
+    }),
+  ).toEqual([removeAll, summary]);
+});
+
+test("getSummarizationMiddlewareMessages matches base LangChain summarization update keys", () => {
+  const summary = {
+    id: "summary-1",
+    type: "human",
+    name: "summary",
+    content: "summary",
+  } as Message;
+
+  expect(
+    getSummarizationMiddlewareMessages({
+      "SummarizationMiddleware.before_model": {
+        messages: [summary],
+      },
+    }),
+  ).toEqual([summary]);
+});
+
+test("getSummarizationMiddlewareMessages ignores unrelated suffix-sharing update keys", () => {
+  const summary = {
+    id: "summary-1",
+    type: "human",
+    name: "summary",
+    content: "summary",
+  } as Message;
+
+  expect(
+    getSummarizationMiddlewareMessages({
+      "OtherSummarizationMiddleware.before_model": {
+        messages: [summary],
+      },
+    }),
+  ).toBeUndefined();
+});
+
 test("getVisibleOptimisticMessages hides optimistic user input after server human arrives", () => {
   const optimisticHuman = {
     id: "opt-human-1",