diff --git a/packages/views/chat/components/use-chat-controller.test.tsx b/packages/views/chat/components/use-chat-controller.test.tsx
index 6c2861552e..2f7d3d771f 100644
--- a/packages/views/chat/components/use-chat-controller.test.tsx
+++ b/packages/views/chat/components/use-chat-controller.test.tsx
@@ -1,4 +1,4 @@
-import { describe, it, expect, beforeEach, vi } from "vitest";
+import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
 import { act, renderHook } from "@testing-library/react";
 import type { Agent, ChatSession } from "@multica/core/types";
 
@@ -17,6 +17,7 @@ const h = vi.hoisted(() => {
   return {
     store,
     archivedMutate: vi.fn(),
+    markReadMutate: vi.fn(),
     // useQuery reads these so each test can vary the loaded data.
     sessions: [] as ChatSession[],
     agents: [] as Agent[],
@@ -45,7 +46,7 @@ vi.mock("@multica/core/hooks/use-file-upload", () => ({
 }));
 vi.mock("@multica/core/chat/mutations", () => ({
   useCreateChatSession: () => ({ mutateAsync: vi.fn() }),
-  useMarkChatSessionRead: () => ({ mutate: vi.fn() }),
+  useMarkChatSessionRead: () => ({ mutate: h.markReadMutate }),
   useSetChatSessionArchived: () => ({ mutate: h.archivedMutate }),
 }));
 vi.mock("@multica/core/chat", () => ({
@@ -193,3 +194,55 @@ describe("useChatController.archiveSession", () => {
     expect(h.archivedMutate).toHaveBeenCalledWith({ sessionId: "sA", archived: true });
   });
 });
+
+// MUL-4360 mount race: `activeSessionId` is persisted, so on a bare `/chat`
+// navigation the page restores the last session as active for one frame before
+// its URL→store effect clears it back to null. The auto-mark-read must NOT fire
+// for that transiently-active session — otherwise the badge vanishes though the
+// user never opened it (the exact "no active session yet the red dot cleared"
+// report). The read is deferred a tick and cancelled when activeSessionId moves.
+describe("useChatController auto mark-read", () => {
+  const unread = makeSession({
+    id: "sA",
+    agent_id: "agent-a",
+    has_unread: true,
+    unread_count: 2,
+  });
+
+  beforeEach(() => {
+    vi.useFakeTimers();
+    h.store.activeSessionId = null;
+    h.store.selectedAgentId = null;
+    h.markReadMutate.mockClear();
+    h.sessions = [unread];
+    h.agents = [agentA];
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  it("marks read a session that stays active past the tick", () => {
+    h.store.activeSessionId = "sA";
+    renderHook(() => useChatController());
+
+    // Deferred, not synchronous — nothing fires on the mount frame.
+    expect(h.markReadMutate).not.toHaveBeenCalled();
+
+    act(() => vi.advanceTimersByTime(1));
+    expect(h.markReadMutate).toHaveBeenCalledWith("sA");
+  });
+
+  it("does NOT mark read a session that was only momentarily active on mount", () => {
+    // Mount restores sA as active (persisted, unread)...
+    h.store.activeSessionId = "sA";
+    const { rerender } = renderHook(() => useChatController());
+
+    // ...then the page's URL→store effect clears it before the tick elapses.
+    h.store.activeSessionId = null;
+    rerender();
+
+    act(() => vi.advanceTimersByTime(1));
+    expect(h.markReadMutate).not.toHaveBeenCalled();
+  });
+});
diff --git a/packages/views/chat/components/use-chat-controller.ts b/packages/views/chat/components/use-chat-controller.ts
index 74aedea202..a181c99651 100644
--- a/packages/views/chat/components/use-chat-controller.ts
+++ b/packages/views/chat/components/use-chat-controller.ts
@@ -308,13 +308,31 @@ export function useChatController(opts?: { isActive?: boolean }) {
   // Auto mark-as-read whenever the user is actively looking at a session with
   // unread state. `isActive` lets the caller say "my surface is on screen":
   // the floating overlay passes `isOpen`, the tab passes `true`.
+  //
+  // The read is deferred by a tick and cancelled on cleanup, so a session that
+  // is only *momentarily* active never gets marked read. This is the fix for
+  // MUL-4360's mount race: `activeSessionId` is persisted, so on a bare `/chat`
+  // navigation the page restores the last session for one frame before its
+  // URL→store effect (which runs AFTER this hook's effects, since the hook is
+  // called first) clears it back to null. Without the defer, that restored-but-
+  // never-opened session was marked read in that gap — its badge vanished
+  // though the user never opened it (right pane still shows "select a chat").
+  // Deferring lets the subsequent activeSessionId change cancel the pending
+  // read via cleanup; the store re-check is a belt-and-suspenders guard. Only a
+  // session that stays active past the tick — a real select, deep link, or
+  // refresh — is read.
   const currentHasUnread =
     sessions.find((s) => s.id === activeSessionId)?.has_unread ?? false;
   useEffect(() => {
     if (!isActive || !activeSessionId) return;
     if (!currentHasUnread) return;
-    uiLogger.info("auto markRead", { sessionId: activeSessionId });
-    markRead.mutate(activeSessionId);
+    const sessionId = activeSessionId;
+    const timer = setTimeout(() => {
+      if (useChatStore.getState().activeSessionId !== sessionId) return;
+      uiLogger.info("auto markRead", { sessionId });
+      markRead.mutate(sessionId);
+    }, 0);
+    return () => clearTimeout(timer);
     // eslint-disable-next-line react-hooks/exhaustive-deps -- markRead ref stable
   }, [isActive, activeSessionId, currentHasUnread]);