fix(frontend): deduplicate restored thread messages (#2958)

* fix(frontend): fix duplicate messages when reopening agent sessions (#2957)

* make format

* fix(frontend): retry pending thread history loads

frontend/src/core/threads/hooks.ts

@@ -45,15 +45,60 @@ type SendMessageOptions = {
   additionalKwargs?: Record<string, unknown>;
 };
 
-function mergeMessages(
+function isNonEmptyString(value: string | undefined): value is string {
+  return typeof value === "string" && value.length > 0;
+}
+
+function messageIdentity(message: Message): string | undefined {
+  if (
+    "tool_call_id" in message &&
+    typeof message.tool_call_id === "string" &&
+    message.tool_call_id.length > 0
+  ) {
+    return `tool:${message.tool_call_id}`;
+  }
+  if (typeof message.id === "string" && message.id.length > 0) {
+    return `message:${message.id}`;
+  }
+  return undefined;
+}
+
+function dedupeMessagesByIdentity(messages: Message[]): Message[] {
+  const lastIndexByIdentity = new Map<string, number>();
+
+  messages.forEach((message, index) => {
+    const identity = messageIdentity(message);
+    if (identity) {
+      lastIndexByIdentity.set(identity, index);
+    }
+  });
+
+  return messages.filter((message, index) => {
+    const identity = messageIdentity(message);
+    return !identity || lastIndexByIdentity.get(identity) === index;
+  });
+}
+
+function findLatestUnloadedRunIndex(
+  runs: Run[],
+  loadedRunIds: ReadonlySet<string>,
+): number {
+  for (let i = runs.length - 1; i >= 0; i--) {
+    const run = runs[i];
+    if (run && !loadedRunIds.has(run.run_id)) {
+      return i;
+    }
+  }
+  return -1;
+}
+
+export function mergeMessages(
   historyMessages: Message[],
   threadMessages: Message[],
   optimisticMessages: Message[],
 ): Message[] {
   const threadMessageIds = new Set(
-    threadMessages
+    threadMessages.map(messageIdentity).filter(isNonEmptyString),
-      .map((m) => ("tool_call_id" in m ? m.tool_call_id : m.id))
-      .filter(Boolean),
   );
 
   // The overlap is a contiguous suffix of historyMessages (newest history == oldest thread).
@@ -65,28 +110,19 @@ function mergeMessages(
     if (!msg) {
       continue;
     }
-    if (
+    const identity = messageIdentity(msg);
-      (msg?.id && threadMessageIds.has(msg.id)) ||
+    if (identity && threadMessageIds.has(identity)) {
-      ("tool_call_id" in msg && threadMessageIds.has(msg.tool_call_id))
-    ) {
       cutoff = i;
     } else {
       break;
     }
   }
 
-  return [
+  return dedupeMessagesByIdentity([
     ...historyMessages.slice(0, cutoff),
     ...threadMessages,
     ...optimisticMessages,
-  ];
+  ]);
-}
-
-function messageIdentity(message: Message): string | undefined {
-  if ("tool_call_id" in message) {
-    return message.tool_call_id;
-  }
-  return message.id;
 }
 
 function getMessagesAfterBaseline(
@@ -627,22 +663,51 @@ export function useThreadHistory(threadId: string) {
   const runsRef = useRef(runs.data ?? []);
   const indexRef = useRef(-1);
   const loadingRef = useRef(false);
+  const pendingLoadRef = useRef(false);
+  const loadingRunIdRef = useRef<string | null>(null);
+  const loadedRunIdsRef = useRef<Set<string>>(new Set());
   const [loading, setLoading] = useState(false);
   const [messages, setMessages] = useState<Message[]>([]);
 
-  loadingRef.current = loading;
   const loadMessages = useCallback(async () => {
+    if (loadingRef.current) {
+      const pendingRunIndex = findLatestUnloadedRunIndex(
+        runsRef.current,
+        loadedRunIdsRef.current,
+      );
+      const pendingRun = runsRef.current[pendingRunIndex];
+      if (pendingRun && pendingRun.run_id !== loadingRunIdRef.current) {
+        pendingLoadRef.current = true;
+      }
+      return;
+    }
     if (runsRef.current.length === 0) {
       return;
     }
-    const run = runsRef.current[indexRef.current];
+
-    if (!run || loadingRef.current) {
+    loadingRef.current = true;
+    setLoading(true);
+
+    try {
+      do {
+        pendingLoadRef.current = false;
+
+        const nextRunIndex = findLatestUnloadedRunIndex(
+          runsRef.current,
+          loadedRunIdsRef.current,
+        );
+        indexRef.current = nextRunIndex;
+
+        const run = runsRef.current[nextRunIndex];
+        if (!run) {
+          indexRef.current = -1;
           return;
         }
-    try {
+
-      setLoading(true);
+        const requestThreadId = threadIdRef.current;
+        loadingRunIdRef.current = run.run_id;
         const result: { data: RunMessage[]; hasMore: boolean } = await fetch(
-        `${getBackendBaseURL()}/api/threads/${encodeURIComponent(threadIdRef.current)}/runs/${encodeURIComponent(run.run_id)}/messages`,
+          `${getBackendBaseURL()}/api/threads/${encodeURIComponent(requestThreadId)}/runs/${encodeURIComponent(run.run_id)}/messages`,
           {
             method: "GET",
             headers: {
@@ -656,19 +721,47 @@ export function useThreadHistory(threadId: string) {
         const _messages = result.data
           .filter((m) => !m.metadata.caller?.startsWith("middleware:"))
           .map((m) => m.content);
-      setMessages((prev) => [..._messages, ...prev]);
+        if (threadIdRef.current !== requestThreadId) {
-      indexRef.current -= 1;
+          return;
+        }
+        setMessages((prev) =>
+          dedupeMessagesByIdentity([..._messages, ...prev]),
+        );
+        loadedRunIdsRef.current.add(run.run_id);
+        indexRef.current = findLatestUnloadedRunIndex(
+          runsRef.current,
+          loadedRunIdsRef.current,
+        );
+      } while (pendingLoadRef.current);
     } catch (err) {
       console.error(err);
     } finally {
+      loadingRef.current = false;
+      loadingRunIdRef.current = null;
       setLoading(false);
     }
   }, []);
   useEffect(() => {
+    const threadChanged = threadIdRef.current !== threadId;
     threadIdRef.current = threadId;
+
+    if (threadChanged) {
+      runsRef.current = [];
+      indexRef.current = -1;
+      pendingLoadRef.current = false;
+      loadingRunIdRef.current = null;
+      loadedRunIdsRef.current = new Set();
+      loadingRef.current = false;
+      setLoading(false);
+      setMessages([]);
+    }
+
     if (runs.data && runs.data.length > 0) {
       runsRef.current = runs.data ?? [];
-      indexRef.current = runs.data.length - 1;
+      indexRef.current = findLatestUnloadedRunIndex(
+        runs.data,
+        loadedRunIdsRef.current,
+      );
     }
     loadMessages().catch(() => {
       toast.error("Failed to load thread history.");
@@ -677,7 +770,7 @@ export function useThreadHistory(threadId: string) {
 
   const appendMessages = useCallback((_messages: Message[]) => {
     setMessages((prev) => {
-      return [...prev, ..._messages];
+      return dedupeMessagesByIdentity([...prev, ..._messages]);
     });
   }, []);
   const hasMore = indexRef.current >= 0 || !runs.data;

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

@@ -0,0 +1,64 @@
+import type { Message } from "@langchain/langgraph-sdk";
+import { expect, test } from "vitest";
+
+import { mergeMessages } from "@/core/threads/hooks";
+
+test("mergeMessages removes duplicate messages already present in history", () => {
+  const human = {
+    id: "human-1",
+    type: "human",
+    content: "Design an agent",
+  } as Message;
+  const ai = {
+    id: "ai-1",
+    type: "ai",
+    content: "Let's design it.",
+  } as Message;
+
+  expect(mergeMessages([human, ai, human, ai], [], [])).toEqual([human, ai]);
+});
+
+test("mergeMessages lets live thread messages replace overlapping history", () => {
+  const oldHuman = {
+    id: "human-1",
+    type: "human",
+    content: "old",
+  } as Message;
+  const liveHuman = {
+    id: "human-1",
+    type: "human",
+    content: "live",
+  } as Message;
+  const oldAi = {
+    id: "ai-1",
+    type: "ai",
+    content: "old",
+  } as Message;
+  const liveAi = {
+    id: "ai-1",
+    type: "ai",
+    content: "live",
+  } as Message;
+
+  expect(mergeMessages([oldHuman, oldAi], [liveHuman, liveAi], [])).toEqual([
+    liveHuman,
+    liveAi,
+  ]);
+});
+
+test("mergeMessages deduplicates tool messages by tool_call_id", () => {
+  const oldTool = {
+    id: "tool-message-old",
+    type: "tool",
+    tool_call_id: "call-1",
+    content: "old",
+  } as Message;
+  const liveTool = {
+    id: "tool-message-live",
+    type: "tool",
+    tool_call_id: "call-1",
+    content: "live",
+  } as Message;
+
+  expect(mergeMessages([oldTool], [liveTool], [])).toEqual([liveTool]);
+});