diff --git a/apps/mobile/app/(app)/[workspace]/(tabs)/_layout.tsx b/apps/mobile/app/(app)/[workspace]/(tabs)/_layout.tsx
index 4e539827c9..676d1346ea 100644
--- a/apps/mobile/app/(app)/[workspace]/(tabs)/_layout.tsx
+++ b/apps/mobile/app/(app)/[workspace]/(tabs)/_layout.tsx
@@ -30,7 +30,7 @@ import { useColorScheme } from "@/lib/use-color-scheme";
 import { THEME } from "@/lib/theme";
 import {
   useInboxUnreadCount,
-  useChatUnreadSessionCount,
+  useChatUnreadMessageCount,
 } from "@/lib/unread-counts";
 import { MoreTabDropdownAnchor } from "@/components/nav/more-tab-dropdown";
 
@@ -49,15 +49,14 @@ export default function TabsLayout() {
 
   const wsId = useWorkspaceStore((s) => s.currentWorkspaceId);
   const inboxUnread = useInboxUnreadCount(wsId);
-  const chatUnread = useChatUnreadSessionCount(wsId);
+  const chatUnread = useChatUnreadMessageCount(wsId);
 
-  // Truncation aligned with web: inbox 99+, chat 9+ (matches sidebar +
-  // ChatFab respectively). `undefined` makes React Navigation hide the
-  // badge, so zero-count is a free no-op.
+  // Truncation aligned with web's sidebar badges: 99+ for both. `undefined`
+  // makes React Navigation hide the badge, so zero-count is a free no-op.
   const inboxBadge =
     inboxUnread > 0 ? (inboxUnread > 99 ? "99+" : String(inboxUnread)) : undefined;
   const chatBadge =
-    chatUnread > 0 ? (chatUnread > 9 ? "9+" : String(chatUnread)) : undefined;
+    chatUnread > 0 ? (chatUnread > 99 ? "99+" : String(chatUnread)) : undefined;
 
   // Imperative handle into the More tab's dropdown — listeners.tabPress
   // calls .open(); the @rn-primitives Trigger measures itself inside
diff --git a/apps/mobile/data/mutations/chat.ts b/apps/mobile/data/mutations/chat.ts
index be08cfb366..7dbacc336b 100644
--- a/apps/mobile/data/mutations/chat.ts
+++ b/apps/mobile/data/mutations/chat.ts
@@ -70,9 +70,15 @@ export function useMarkChatSessionRead() {
       const key = chatKeys.sessions(wsId);
       await qc.cancelQueries({ queryKey: key });
       const prev = qc.getQueryData<ChatSession[]>(key);
+      // Zero unread_count together with has_unread — the tab badge sums
+      // unread_count (see lib/unread-counts.ts), so clearing only the flag
+      // would leave a stale badge until the settle refetch. Mirrors web's
+      // useMarkChatSessionRead in packages/core/chat/mutations.ts.
       qc.setQueryData<ChatSession[]>(key, (old) =>
         old?.map((s) =>
-          s.id === sessionId ? { ...s, has_unread: false } : s,
+          s.id === sessionId
+            ? { ...s, has_unread: false, unread_count: 0 }
+            : s,
         ),
       );
       return { prev, key };
diff --git a/apps/mobile/data/schemas.ts b/apps/mobile/data/schemas.ts
index 596ad82346..842900aef1 100644
--- a/apps/mobile/data/schemas.ts
+++ b/apps/mobile/data/schemas.ts
@@ -246,6 +246,10 @@ export const ChatSessionSchema: z.ZodType<ChatSession> = z.object({
   // unknown server values fall back to "active" so the row still renders.
   status: z.enum(["active", "archived"]).catch("active"),
   has_unread: z.boolean().default(false),
+  // Unread assistant messages after the read cursor. Optional (not defaulted)
+  // so the badge math can tell "older server didn't send it" from a real 0 —
+  // the tab badge sums `unread_count ?? 0`, same rule as web's sidebar.
+  unread_count: z.number().optional(),
   created_at: z.string().default(""),
   updated_at: z.string().default(""),
 }).loose();
diff --git a/apps/mobile/lib/unread-counts.ts b/apps/mobile/lib/unread-counts.ts
index 60ad74c1e3..c739f07577 100644
--- a/apps/mobile/lib/unread-counts.ts
+++ b/apps/mobile/lib/unread-counts.ts
@@ -3,7 +3,9 @@
  *
  * Mirrors the counting logic from:
  *   - packages/core/inbox/queries.ts::useInboxUnreadCount (inbox)
- *   - packages/views/chat/components/chat-fab.tsx (chat: sessions with unread)
+ *   - packages/core/chat/unread.ts::countUnreadChatMessages (chat — the
+ *     shared pure function IS the definition; web's sidebar calls the same
+ *     one, so the platforms cannot drift apart)
  *
  * Both queries (`inboxListOptions`, `chatSessionsOptions`) are already kept
  * fresh by listing-level realtime hooks mounted in
@@ -15,6 +17,7 @@
  * the N rendered here MUST equal the N web shows for the same user/workspace.
  */
 import { useQuery } from "@tanstack/react-query";
+import { countUnreadChatMessages } from "@multica/core/chat/unread";
 import { inboxListOptions } from "@/data/queries/inbox";
 import { chatSessionsOptions } from "@/data/queries/chat";
 import { deduplicateInboxItems } from "@/lib/inbox-display";
@@ -34,16 +37,23 @@ export function useInboxUnreadCount(wsId: string | null | undefined): number {
 }
 
 /**
- * Number of chat sessions that have at least one unread assistant reply.
- * Matches web ChatFab's `sessions.filter(s => s.has_unread).length` — this
- * is a session count, not a message count.
+ * Total unread assistant *messages* across chat sessions (IM-style), the
+ * same number web/desktop's sidebar Chat badge shows. Was a session count
+ * before MUL-4286; that matched the (since removed) web ChatFab badge and
+ * disagreed with the sidebar.
+ *
+ * No excludeSessionId here: the chat tab renders the active conversation
+ * itself, and the focused-screen auto mark-read (chat.tsx) plus the
+ * mutation's optimistic unread_count reset clear that session's share of
+ * the badge immediately — a badge decrementing on the tab you are already
+ * inside is normal IM behavior, not a phantom.
  */
-export function useChatUnreadSessionCount(
+export function useChatUnreadMessageCount(
   wsId: string | null | undefined,
 ): number {
   const { data } = useQuery({
     ...chatSessionsOptions(wsId ?? null),
-    select: (sessions) => sessions.filter((s) => s.has_unread).length,
+    select: (sessions) => countUnreadChatMessages(sessions),
   });
   return data ?? 0;
 }
diff --git a/packages/core/chat/unread.test.ts b/packages/core/chat/unread.test.ts
new file mode 100644
index 0000000000..977e878f48
--- /dev/null
+++ b/packages/core/chat/unread.test.ts
@@ -0,0 +1,46 @@
+import { describe, expect, it } from "vitest";
+import { countUnreadChatMessages } from "./unread";
+import type { ChatSession } from "../types/chat";
+
+function session(id: string, unread_count?: number): ChatSession {
+  return {
+    id,
+    workspace_id: "ws-1",
+    agent_id: "agent-1",
+    creator_id: "user-1",
+    title: id,
+    status: "active",
+    has_unread: (unread_count ?? 0) > 0,
+    unread_count,
+    created_at: "2026-07-10T00:00:00Z",
+    updated_at: "2026-07-10T00:00:00Z",
+  };
+}
+
+describe("countUnreadChatMessages", () => {
+  it("sums unread messages across sessions (not session count)", () => {
+    expect(
+      countUnreadChatMessages([session("a", 2), session("b", 3), session("c", 0)]),
+    ).toBe(5);
+  });
+
+  it("treats a missing unread_count (older server) as 0", () => {
+    expect(countUnreadChatMessages([session("a"), session("b", 4)])).toBe(4);
+  });
+
+  it("returns 0 for an empty or unloaded list", () => {
+    expect(countUnreadChatMessages([])).toBe(0);
+    expect(countUnreadChatMessages(undefined)).toBe(0);
+  });
+
+  it("excludes the session being viewed from the sum", () => {
+    const sessions = [session("a", 2), session("b", 3)];
+    expect(countUnreadChatMessages(sessions, "a")).toBe(3);
+    expect(countUnreadChatMessages(sessions, "b")).toBe(2);
+  });
+
+  it("ignores an exclude id that is not in the list", () => {
+    expect(countUnreadChatMessages([session("a", 2)], "ghost")).toBe(2);
+    expect(countUnreadChatMessages([session("a", 2)], null)).toBe(2);
+  });
+});
diff --git a/packages/core/chat/unread.ts b/packages/core/chat/unread.ts
new file mode 100644
index 0000000000..dd647ccf4b
--- /dev/null
+++ b/packages/core/chat/unread.ts
@@ -0,0 +1,31 @@
+import type { ChatSession } from "../types/chat";
+
+/**
+ * The chat unread badge number: total unread assistant *messages* across
+ * sessions (IM-style), NOT the number of sessions with unread.
+ *
+ * Single source of truth for every surface that renders an aggregate chat
+ * unread count (web/desktop sidebar, mobile tab bar) — the platforms must
+ * show the same N for the same account state (see "Behavioral parity" in
+ * apps/mobile/CLAUDE.md). Pure function so mobile can import it directly.
+ *
+ * `excludeSessionId` drops one session from the sum: the session the user is
+ * currently reading. The chat thread list renders that session's own badge
+ * as 0 (its unread is about to be cleared by the viewer's auto mark-read),
+ * so an aggregate that still counted it would show a number the list can't
+ * account for. Callers pass it only while a chat surface is actually showing
+ * that session; a background pointer (e.g. a remembered selection while the
+ * chat page is closed) must NOT be excluded, or new replies would never
+ * badge.
+ */
+export function countUnreadChatMessages(
+  sessions: readonly ChatSession[] | undefined,
+  excludeSessionId?: string | null,
+): number {
+  if (!sessions) return 0;
+  return sessions.reduce(
+    (sum, s) =>
+      s.id === excludeSessionId ? sum : sum + (s.unread_count ?? 0),
+    0,
+  );
+}
diff --git a/packages/core/package.json b/packages/core/package.json
index 714e7aca68..4ce09d0045 100644
--- a/packages/core/package.json
+++ b/packages/core/package.json
@@ -50,6 +50,7 @@
     "./chat": "./chat/index.ts",
     "./chat/queries": "./chat/queries.ts",
     "./chat/mutations": "./chat/mutations.ts",
+    "./chat/unread": "./chat/unread.ts",
     "./runtimes": "./runtimes/index.ts",
     "./runtimes/queries": "./runtimes/queries.ts",
     "./runtimes/mutations": "./runtimes/mutations.ts",
diff --git a/packages/views/layout/app-sidebar.test.tsx b/packages/views/layout/app-sidebar.test.tsx
index 094188e653..4d383140fc 100644
--- a/packages/views/layout/app-sidebar.test.tsx
+++ b/packages/views/layout/app-sidebar.test.tsx
@@ -3,8 +3,9 @@ import { beforeEach, describe, expect, it, vi } from "vitest";
 import { ApiError } from "@multica/core/api";
 import { AppSidebar } from "./app-sidebar";
 
-const { chatSessions, detail, deletePin, navigation, pins, summary, workspaces } = vi.hoisted(() => ({
-  chatSessions: { current: [] as { unread_count?: number }[] },
+const { chatSessions, chatStore, detail, deletePin, navigation, pins, summary, workspaces } = vi.hoisted(() => ({
+  chatSessions: { current: [] as { id?: string; unread_count?: number }[] },
+  chatStore: { current: { activeSessionId: null as string | null, isOpen: false } },
   detail: { current: { isPending: false, isError: false, data: null as unknown, error: null as unknown } },
   deletePin: vi.fn(),
   navigation: { current: { pathname: "/acme/issues" } },
@@ -98,6 +99,14 @@ vi.mock("@multica/ui/components/common/actor-avatar", () => ({ ActorAvatar: () =
 vi.mock("@multica/core/auth", () => ({
   useAuthStore: (selector: (state: { user: { id: string } }) => unknown) => selector({ user: { id: "user-1" } }),
 }));
+// Callable-store shape (selectorFn + getState) per the repo testing rules.
+vi.mock("@multica/core/chat", () => ({
+  useChatStore: Object.assign(
+    (selector: (state: { activeSessionId: string | null; isOpen: boolean }) => unknown) =>
+      selector(chatStore.current),
+    { getState: () => chatStore.current },
+  ),
+}));
 vi.mock("@multica/core/paths", () => ({
   paths: { workspace: (slug: string) => ({ issues: () => `/${slug}/issues` }) },
   useCurrentWorkspace: () => ({ id: "ws-1", name: "Acme", slug: "acme" }),
@@ -286,6 +295,7 @@ describe("personal nav — Chat", () => {
   beforeEach(() => {
     chatSessions.current = [];
     navigation.current = { pathname: "/acme/issues" };
+    chatStore.current = { activeSessionId: null, isOpen: false };
   });
 
   // The mocked SidebarMenuButton exposes the AppLink target as `data-href`
@@ -299,14 +309,43 @@ describe("personal nav — Chat", () => {
   });
 
   it("badges the Chat nav with the summed unread_count of chat sessions", () => {
-    chatSessions.current = [{ unread_count: 3 }, { unread_count: 2 }, { unread_count: 0 }];
+    chatSessions.current = [{ id: "a", unread_count: 3 }, { id: "b", unread_count: 2 }, { id: "c", unread_count: 0 }];
     const { container } = render(<AppSidebar />);
     expect(chatNav(container)?.textContent).toContain("5");
   });
 
   it("shows no Chat unread badge when every session is read", () => {
-    chatSessions.current = [{ unread_count: 0 }, {}];
+    chatSessions.current = [{ id: "a", unread_count: 0 }, { id: "b" }];
     const { container } = render(<AppSidebar />);
     expect(chatNav(container)?.textContent ?? "").not.toMatch(/\d/);
   });
+
+  it("excludes the session being viewed on the chat page from the badge", () => {
+    // The thread list zeroes the open session's row badge; the aggregate
+    // must follow, or a reply landing in the open conversation flashes a
+    // count with no matching row.
+    chatSessions.current = [{ id: "a", unread_count: 2 }, { id: "b", unread_count: 3 }];
+    navigation.current = { pathname: "/acme/chat" };
+    chatStore.current = { activeSessionId: "a", isOpen: false };
+    const { container } = render(<AppSidebar />);
+    expect(chatNav(container)?.textContent).toContain("3");
+  });
+
+  it("excludes the viewed session when the floating chat window is open off-route", () => {
+    chatSessions.current = [{ id: "a", unread_count: 2 }, { id: "b", unread_count: 3 }];
+    navigation.current = { pathname: "/acme/issues" };
+    chatStore.current = { activeSessionId: "a", isOpen: true };
+    const { container } = render(<AppSidebar />);
+    expect(chatNav(container)?.textContent).toContain("3");
+  });
+
+  it("still counts a remembered selection when no chat surface is showing it", () => {
+    // activeSessionId persists after the chat page closes; with both
+    // surfaces closed nothing will auto mark-read, so the badge must count.
+    chatSessions.current = [{ id: "a", unread_count: 2 }, { id: "b", unread_count: 3 }];
+    navigation.current = { pathname: "/acme/issues" };
+    chatStore.current = { activeSessionId: "a", isOpen: false };
+    const { container } = render(<AppSidebar />);
+    expect(chatNav(container)?.textContent).toContain("5");
+  });
 });
diff --git a/packages/views/layout/app-sidebar.tsx b/packages/views/layout/app-sidebar.tsx
index b5da730443..c8912a5aa8 100644
--- a/packages/views/layout/app-sidebar.tsx
+++ b/packages/views/layout/app-sidebar.tsx
@@ -73,6 +73,8 @@ import { resolvePublicFileUrl } from "@multica/core/workspace/avatar-url";
 import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
 import { inboxKeys, deduplicateInboxItems, inboxUnreadSummaryOptions, hasOtherWorkspaceUnread, unreadWorkspaceIds } from "@multica/core/inbox/queries";
 import { chatSessionsOptions } from "@multica/core/chat/queries";
+import { countUnreadChatMessages } from "@multica/core/chat/unread";
+import { useChatStore } from "@multica/core/chat";
 import { api, ApiError } from "@multica/core/api";
 import { useModalStore } from "@multica/core/modals";
 import { useConfigStore } from "@multica/core/config";
@@ -369,15 +371,30 @@ export function AppSidebar({ topSlot, searchSlot, headerClassName, headerStyle }
     () => deduplicateInboxItems(inboxItems).filter((i) => !i.read).length,
     [inboxItems],
   );
-  // Chat tab unread badge: number of chat threads with a fresh reply.
+  // Chat tab unread badge: IM-style total of unread *messages* across chat
+  // threads (countUnreadChatMessages is the shared definition — mobile's tab
+  // badge derives from the same function, keeping the platforms in agreement).
   const { data: chatSessions = [] } = useQuery({
     ...chatSessionsOptions(wsId ?? ""),
     enabled: !!wsId,
   });
-  // IM-style: total unread *messages* across all chat threads (not thread count).
+  // The session the user is reading right now must not count: the thread list
+  // renders its row badge as 0 (auto mark-read is about to clear it), and a
+  // reply landing in the open conversation would otherwise flash a sidebar
+  // count with no matching row. "Reading right now" = a session is active AND
+  // a chat surface is actually showing it (chat page route or the floating
+  // window). A remembered selection while both surfaces are closed still
+  // counts — auto mark-read won't fire there, so the badge must.
+  const activeChatSessionId = useChatStore((s) => s.activeSessionId);
+  const floatingChatOpen = useChatStore((s) => s.isOpen);
+  const chatHref = p.chat();
+  const viewedChatSessionId =
+    floatingChatOpen || isNavActive(pathname, chatHref)
+      ? activeChatSessionId
+      : null;
   const chatUnreadCount = React.useMemo(
-    () => chatSessions.reduce((sum, s) => sum + (s.unread_count ?? 0), 0),
-    [chatSessions],
+    () => countUnreadChatMessages(chatSessions, viewedChatSessionId),
+    [chatSessions, viewedChatSessionId],
   );
   // Cross-workspace unread summary backs the workspace-switcher dot. One
   // shared cache entry across workspaces; gated on an active workspace since