feat(threads): Slack-style per-thread notifications (P4-1)

Default = Participating: thread replies notify only when you've posted in the
thread or are @mentioned; per-thread override to All / Mentions-only / Mute via
a bell menu in the thread panel header. Modes sync across devices in
io.lotus.thread_notifications account data (pruned on write: left rooms, >180d,
cap 200/room). Muted threads: no notifications/sounds, chip badge suppressed
(+BellMute glyph), and their counts are subtracted from the room's sidebar
badge (client-side; clamped ≥0).

Also fixes the thread notification path itself: thread replies are now owned by
exactly ONE handler (room-level ThreadEvent.NewReply via a new useRoomsListener
hook, with per-thread dedupe, panel-aware focus suppression, and per-thread OS
tag coalescing) — the existing RoomEvent.Timeline handlers in the notifier and
the unread binder are explicitly thread-guarded, eliminating the previously
un-gated/double path. Room badges now also refresh live on
RoomEvent.UnreadNotifications (surgical per-room PUT; fixes thread-badge lag).

Pure decision core shouldNotifyThreadReply (13-case matrix) + prune + unread
subtraction: +32 tests (648 total). E2EE caveat documented: mentions-only may
under-notify pre-decryption (same class as the existing path).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/components/ThreadNotificationModeSwitcher.tsx

@@ -0,0 +1,127 @@
+import { Box, config, Icon, Icons, IconSrc, Menu, MenuItem, PopOut, RectCords, Text } from 'folds';
+import React, { MouseEventHandler, ReactNode, useMemo, useState } from 'react';
+import FocusTrap from 'focus-trap-react';
+import { stopPropagation } from '../utils/keyboard';
+import { ThreadNotificationMode } from '../utils/threadNotifications';
+import { useSetThreadNotificationMode } from '../hooks/useThreadNotifications';
+import { AsyncStatus } from '../hooks/useAsyncCallback';
+
+export const getThreadNotificationModeIcon = (mode?: ThreadNotificationMode): IconSrc => {
+  if (mode === ThreadNotificationMode.Mute) return Icons.BellMute;
+  if (mode === ThreadNotificationMode.MentionsOnly) return Icons.BellPing;
+  if (mode === ThreadNotificationMode.All) return Icons.BellRing;
+
+  return Icons.Bell;
+};
+
+const useThreadNotificationModes = (): ThreadNotificationMode[] =>
+  useMemo(
+    () => [
+      ThreadNotificationMode.Default,
+      ThreadNotificationMode.All,
+      ThreadNotificationMode.MentionsOnly,
+      ThreadNotificationMode.Mute,
+    ],
+    [],
+  );
+
+const useThreadNotificationModeStr = (): Record<ThreadNotificationMode, string> =>
+  useMemo(
+    () => ({
+      [ThreadNotificationMode.Default]: 'Default (participating)',
+      [ThreadNotificationMode.All]: 'All replies',
+      [ThreadNotificationMode.MentionsOnly]: 'Mentions only',
+      [ThreadNotificationMode.Mute]: 'Mute',
+    }),
+    [],
+  );
+
+type ThreadNotificationModeSwitcherProps = {
+  roomId: string;
+  threadId: string;
+  value?: ThreadNotificationMode;
+  children: (
+    handleOpen: MouseEventHandler<HTMLButtonElement>,
+    opened: boolean,
+    changing: boolean,
+  ) => ReactNode;
+};
+export function ThreadNotificationModeSwitcher({
+  roomId,
+  threadId,
+  value = ThreadNotificationMode.Default,
+  children,
+}: ThreadNotificationModeSwitcherProps) {
+  const modes = useThreadNotificationModes();
+  const modeToStr = useThreadNotificationModeStr();
+
+  const { modeState, setMode } = useSetThreadNotificationMode(roomId, threadId);
+  const changing = modeState.status === AsyncStatus.Loading;
+
+  const [menuCords, setMenuCords] = useState<RectCords>();
+
+  const handleOpenMenu: MouseEventHandler<HTMLButtonElement> = (evt) => {
+    setMenuCords(evt.currentTarget.getBoundingClientRect());
+  };
+
+  const handleClose = () => {
+    setMenuCords(undefined);
+  };
+
+  const handleSelect = (mode: ThreadNotificationMode) => {
+    if (changing) return;
+    setMode(mode);
+    handleClose();
+  };
+
+  return (
+    <PopOut
+      anchor={menuCords}
+      offset={5}
+      position="Bottom"
+      align="End"
+      content={
+        <FocusTrap
+          focusTrapOptions={{
+            initialFocus: false,
+            onDeactivate: handleClose,
+            clickOutsideDeactivates: true,
+            isKeyForward: (evt: KeyboardEvent) =>
+              evt.key === 'ArrowDown' || evt.key === 'ArrowRight',
+            isKeyBackward: (evt: KeyboardEvent) => evt.key === 'ArrowUp' || evt.key === 'ArrowLeft',
+            escapeDeactivates: stopPropagation,
+          }}
+        >
+          <Menu>
+            <Box direction="Column" gap="100" style={{ padding: config.space.S100 }}>
+              {modes.map((mode) => (
+                <MenuItem
+                  key={mode}
+                  size="300"
+                  variant="Surface"
+                  aria-pressed={mode === value}
+                  radii="300"
+                  disabled={changing}
+                  onClick={() => handleSelect(mode)}
+                  before={
+                    <Icon
+                      size="100"
+                      src={getThreadNotificationModeIcon(mode)}
+                      filled={mode === value}
+                    />
+                  }
+                >
+                  <Text size="T300">
+                    {mode === value ? <b>{modeToStr[mode]}</b> : modeToStr[mode]}
+                  </Text>
+                </MenuItem>
+              ))}
+            </Box>
+          </Menu>
+        </FocusTrap>
+      }
+    >
+      {children(handleOpenMenu, !!menuCords, changing)}
+    </PopOut>
+  );
+}

src/app/features/room/thread/ThreadPanel.tsx

@@ -23,12 +23,21 @@ import { useKeyDown } from '../../../hooks/useKeyDown';
 import { useSetting } from '../../../state/hooks/settings';
 import { settingsAtom } from '../../../state/settings';
 import { RoomInput } from '../RoomInput';
+import {
+  getThreadNotificationModeIcon,
+  ThreadNotificationModeSwitcher,
+} from '../../../components/ThreadNotificationModeSwitcher';
+import { useThreadNotificationMode } from '../../../hooks/useThreadNotifications';
+import { ThreadNotificationMode } from '../../../utils/threadNotifications';
 
 type ThreadPanelHeaderProps = {
   room: Room;
+  threadId: string;
   requestClose: () => void;
 };
-function ThreadPanelHeader({ room, requestClose }: ThreadPanelHeaderProps) {
+function ThreadPanelHeader({ room, threadId, requestClose }: ThreadPanelHeaderProps) {
+  const mode = useThreadNotificationMode(room.roomId, threadId);
+
   return (
     <Header className={css.ThreadPanelHeader} variant="Background" size="600">
       <Box grow="Yes" alignItems="Center" gap="200">
@@ -40,7 +49,36 @@ function ThreadPanelHeader({ room, requestClose }: ThreadPanelHeaderProps) {
             {room.name}
           </Text>
         </Box>
-        <Box shrink="No" alignItems="Center">
+        <Box shrink="No" alignItems="Center" gap="100">
+          <ThreadNotificationModeSwitcher roomId={room.roomId} threadId={threadId} value={mode}>
+            {(handleOpen, opened) => (
+              <TooltipProvider
+                position="Bottom"
+                align="End"
+                offset={4}
+                tooltip={
+                  <Tooltip>
+                    <Text>Notifications</Text>
+                  </Tooltip>
+                }
+              >
+                {(triggerRef) => (
+                  <IconButton
+                    ref={triggerRef}
+                    variant="Background"
+                    aria-label="Thread notifications"
+                    aria-pressed={opened}
+                    onClick={handleOpen}
+                  >
+                    <Icon
+                      src={getThreadNotificationModeIcon(mode)}
+                      filled={mode !== ThreadNotificationMode.Default}
+                    />
+                  </IconButton>
+                )}
+              </TooltipProvider>
+            )}
+          </ThreadNotificationModeSwitcher>
           <TooltipProvider
             position="Bottom"
             align="End"
@@ -137,7 +175,7 @@ export function ThreadPanel({ room, threadId, requestClose }: ThreadPanelProps)
       shrink="No"
       direction="Column"
     >
-      <ThreadPanelHeader room={room} requestClose={requestClose} />
+      <ThreadPanelHeader room={room} threadId={threadId} requestClose={requestClose} />
       {!thread ? (
         <Box grow="Yes" alignItems="Center" justifyContent="Center" gap="200">
           <Spinner size="400" variant="Secondary" />

src/app/features/room/thread/ThreadSummary.tsx

@@ -5,6 +5,7 @@ import { useThreadSummary } from '../../../hooks/useThreadSummary';
 import { useSetting } from '../../../state/hooks/settings';
 import { settingsAtom } from '../../../state/settings';
 import { timeDayMonthYear, timeHourMinute, today } from '../../../utils/time';
+import { ThreadNotificationMode } from '../../../utils/threadNotifications';
 
 type ThreadSummaryProps = {
   rootEvent: MatrixEvent;
@@ -12,7 +13,7 @@ type ThreadSummaryProps = {
   onOpen: (threadId: string) => void;
 };
 export function ThreadSummary({ rootEvent, room, onOpen }: ThreadSummaryProps) {
-  const { summary, unread } = useThreadSummary(rootEvent, room);
+  const { summary, unread, mode } = useThreadSummary(rootEvent, room);
   const [hour24Clock] = useSetting(settingsAtom, 'hour24Clock');
 
   if (!summary || summary.count === 0) return null;
@@ -43,6 +44,7 @@ export function ThreadSummary({ rootEvent, room, onOpen }: ThreadSummaryProps) {
           {count === 1 ? '1 reply' : `${count} replies`}
           {latestStr ? ` · ${latestStr}` : ''}
         </Text>
+        {mode === ThreadNotificationMode.Mute && <Icon size="50" src={Icons.BellMute} />}
       </Chip>
     </Box>
   );

src/app/hooks/useRoomsListener.ts

@@ -0,0 +1,64 @@
+import { useEffect, useRef } from 'react';
+import {
+  ClientEvent,
+  MatrixClient,
+  Room,
+  RoomEmittedEvents,
+  RoomEventHandlerMap,
+} from 'matrix-js-sdk';
+
+/**
+ * Attach `handler` for `event` on every joined/known room, including rooms
+ * created after mount (via `ClientEvent.Room`). All listeners are detached on
+ * unmount or when `mx`/`event` change.
+ *
+ * The handler is stored in a ref (mirroring `useTauriEvent`) so callers don't
+ * need to memoize it — changing the handler identity never re-attaches the
+ * per-room listeners.
+ *
+ * The emitting {@link Room} is appended as a FINAL extra argument after the
+ * event's own args: several room-level SDK events (e.g.
+ * `RoomEvent.UnreadNotifications`) don't include the room in their payload,
+ * which callers need for per-room updates. Handlers that don't care can simply
+ * ignore it.
+ */
+export function useRoomsListener<E extends RoomEmittedEvents>(
+  mx: MatrixClient,
+  event: E,
+  handler: (...args: [...Parameters<RoomEventHandlerMap[E]>, Room]) => void,
+): void {
+  const handlerRef = useRef(handler);
+  handlerRef.current = handler;
+
+  useEffect(() => {
+    // Track attached rooms (and their per-room trampolines) so re-emitted
+    // `ClientEvent.Room` (e.g. on membership changes) never double-subscribes,
+    // and cleanup can detach exactly what was attached.
+    const attached = new Map<string, (...args: unknown[]) => void>();
+
+    const attach = (room: Room) => {
+      if (attached.has(room.roomId)) return;
+      // Per-room trampoline: forwards to the current ref value with the
+      // emitting room appended.
+      const roomHandler = (...args: unknown[]) =>
+        (handlerRef.current as (...a: unknown[]) => void)(...args, room);
+      attached.set(room.roomId, roomHandler);
+      // `event`/`roomHandler` are correlated through E but TS can't prove it
+      // for the open generic, so we assert at the boundary.
+      room.on(event, roomHandler as any);
+    };
+
+    mx.getRooms().forEach(attach);
+
+    const handleRoom = (room: Room) => attach(room);
+    mx.on(ClientEvent.Room, handleRoom);
+
+    return () => {
+      mx.removeListener(ClientEvent.Room, handleRoom);
+      attached.forEach((roomHandler, roomId) => {
+        mx.getRoom(roomId)?.removeListener(event, roomHandler as any);
+      });
+      attached.clear();
+    };
+  }, [mx, event]);
+}

src/app/hooks/useThreadNotifications.ts

@@ -0,0 +1,97 @@
+import { useCallback } from 'react';
+import { useAtomValue } from 'jotai';
+import { MatrixClient } from 'matrix-js-sdk';
+import { AccountDataEvent } from '../../types/matrix/accountData';
+import { threadNotificationsAtom } from '../state/threadNotifications';
+import {
+  getThreadNotificationMode,
+  pruneThreadNotifications,
+  ThreadNotificationEntry,
+  ThreadNotificationMode,
+  ThreadNotificationsContent,
+} from '../utils/threadNotifications';
+import { useMatrixClient } from './useMatrixClient';
+import { AsyncState, useAsyncCallback } from './useAsyncCallback';
+
+/** Read the current notification mode for a thread from the bound atom. */
+export function useThreadNotificationMode(
+  roomId: string,
+  threadRootId: string,
+): ThreadNotificationMode {
+  const content = useAtomValue(threadNotificationsAtom);
+  return getThreadNotificationMode(content, roomId, threadRootId);
+}
+
+const readContent = (mx: MatrixClient): ThreadNotificationsContent =>
+  ((mx as any).getAccountData(AccountDataEvent.LotusThreadNotifications)?.getContent() as
+    | ThreadNotificationsContent
+    | undefined) ?? {};
+
+const getJoinedRoomIds = (mx: MatrixClient): Set<string> => {
+  const joined = new Set<string>();
+  mx.getRooms().forEach((room) => {
+    if (room.getMyMembership() === 'join') {
+      joined.add(room.roomId);
+    }
+  });
+  return joined;
+};
+
+const writeThreadNotificationMode = async (
+  mx: MatrixClient,
+  roomId: string,
+  threadRootId: string,
+  mode: ThreadNotificationMode,
+): Promise<void> => {
+  const current = readContent(mx);
+  const now = Date.now();
+
+  // Work on a mutable clone; prune produces a fresh object so the mutations
+  // below never touch the atom's/account-data's current content.
+  const next: ThreadNotificationsContent = {
+    ...current,
+    rooms: Object.fromEntries(
+      Object.entries(current.rooms ?? {}).map(([rid, entries]) => [rid, { ...entries }]),
+    ),
+  };
+
+  const rooms = next.rooms as Record<string, Record<string, ThreadNotificationEntry>>;
+
+  if (mode === ThreadNotificationMode.Default) {
+    if (rooms[roomId]) {
+      delete rooms[roomId][threadRootId];
+      if (Object.keys(rooms[roomId]).length === 0) {
+        delete rooms[roomId];
+      }
+    }
+  } else {
+    if (!rooms[roomId]) {
+      rooms[roomId] = {};
+    }
+    rooms[roomId][threadRootId] = { mode, ts: now };
+  }
+
+  // ALWAYS prune before persisting to keep account data bounded.
+  const finalContent = pruneThreadNotifications(next, getJoinedRoomIds(mx), now);
+
+  await (mx as any).setAccountData(AccountDataEvent.LotusThreadNotifications, finalContent);
+};
+
+export function useSetThreadNotificationMode(
+  roomId: string,
+  threadRootId: string,
+): {
+  modeState: AsyncState<void, Error>;
+  setMode: (mode: ThreadNotificationMode) => Promise<void>;
+} {
+  const mx = useMatrixClient();
+
+  const [modeState, setMode] = useAsyncCallback<void, Error, [ThreadNotificationMode]>(
+    useCallback(
+      (mode: ThreadNotificationMode) => writeThreadNotificationMode(mx, roomId, threadRootId, mode),
+      [mx, roomId, threadRootId],
+    ),
+  );
+
+  return { modeState, setMode };
+}

src/app/hooks/useThreadSummary.ts

@@ -1,4 +1,5 @@
 import { useEffect, useState } from 'react';
+import { useAtomValue } from 'jotai';
 import {
   MatrixEvent,
   NotificationCountType,
@@ -8,6 +9,8 @@ import {
   ThreadEvent,
 } from 'matrix-js-sdk';
 import { getThreadSummary, ThreadSummaryData } from '../features/room/thread/threadSummary';
+import { threadNotificationsAtom } from '../state/threadNotifications';
+import { getThreadNotificationMode, ThreadNotificationMode } from '../utils/threadNotifications';
 
 /**
  * Reactive thread summary + unread count for a root event's "N replies" chip.
@@ -18,9 +21,14 @@ import { getThreadSummary, ThreadSummaryData } from '../features/room/thread/thr
 export const useThreadSummary = (
   rootEvent: MatrixEvent,
   room: Room,
-): { summary: ThreadSummaryData | undefined; unread: number } => {
+): { summary: ThreadSummaryData | undefined; unread: number; mode: ThreadNotificationMode } => {
   const threadId = rootEvent.getId();
 
+  const threadNotifications = useAtomValue(threadNotificationsAtom);
+  const mode = threadId
+    ? getThreadNotificationMode(threadNotifications, room.roomId, threadId)
+    : ThreadNotificationMode.Default;
+
   const [summary, setSummary] = useState<ThreadSummaryData | undefined>(() =>
     getThreadSummary(rootEvent),
   );
@@ -53,5 +61,7 @@ export const useThreadSummary = (
     };
   }, [rootEvent, room, threadId]);
 
-  return { summary, unread };
+  const muted = mode === ThreadNotificationMode.Mute;
+
+  return { summary, unread: muted ? 0 : unread, mode };
 };

src/app/pages/client/ClientNonUIFeatures.tsx

@@ -1,7 +1,13 @@
 import { useAtomValue, useSetAtom } from 'jotai';
 import React, { ReactNode, useCallback, useEffect, useRef } from 'react';
 import { useNavigate } from 'react-router-dom';
-import { RoomEvent, RoomEventHandlerMap } from 'matrix-js-sdk';
+import {
+  MatrixEvent,
+  Room,
+  RoomEvent,
+  RoomEventHandlerMap,
+  ThreadEvent,
+} from 'matrix-js-sdk';
 import { focusAssistActiveAtom } from '../../state/focusAssist';
 import { roomToUnreadAtom, unreadEqual, unreadInfoToUnread } from '../../state/room/roomToUnread';
 import LogoSVG from '../../../../public/res/lotus.png';
@@ -35,6 +41,14 @@ import { toastQueueAtom } from '../../state/toast';
 import { useReminders } from '../../hooks/useReminders';
 import { useTauriUpdater } from '../../hooks/useTauriUpdater';
 import { TauriDesktopFeatures } from '../../components/TauriDesktopFeatures';
+import { useRoomsListener } from '../../hooks/useRoomsListener';
+import { threadNotificationsAtom } from '../../state/threadNotifications';
+import { roomIdToActiveThreadIdAtomFamily } from '../../state/room/thread';
+import {
+  getThreadNotificationMode,
+  shouldNotifyThreadReply,
+  THREAD_NOTIFICATIONS_FALLBACK_BEHAVIOR,
+} from '../../utils/threadNotifications';
 
 function isInQuietHours(start: string, end: string): boolean {
   const now = new Date();
@@ -212,6 +226,8 @@ function PresenceUpdater() {
 function MessageNotifications() {
   const audioRef = useRef<HTMLAudioElement>(null);
   const unreadCacheRef = useRef<Map<string, UnreadInfo>>(new Map());
+  // Per-thread dedupe: threadId -> last notified eventId.
+  const lastNotifiedThreadRef = useRef<Map<string, string>>(new Map());
   const mx = useMatrixClient();
   const useAuthentication = useMediaAuthentication();
   const [showNotifications] = useSetting(settingsAtom, 'showNotifications');
@@ -242,6 +258,8 @@ function MessageNotifications() {
   const navigate = useNavigate();
   const notificationSelected = useInboxNotificationsSelected();
   const selectedRoomId = useSelectedRoom();
+  const threadPrefs = useAtomValue(threadNotificationsAtom);
+  const activeThreadId = useAtomValue(roomIdToActiveThreadIdAtomFamily(selectedRoomId ?? ''));
 
   const notify = useCallback(
     ({
@@ -252,6 +270,7 @@ function MessageNotifications() {
       eventId,
       body,
       encrypted,
+      threadId,
     }: {
       roomName: string;
       roomAvatar?: string;
@@ -260,6 +279,7 @@ function MessageNotifications() {
       eventId: string;
       body?: string;
       encrypted?: boolean;
+      threadId?: string;
     }) => {
       const roomPath = mDirects.has(roomId)
         ? getDirectRoomPath(roomId, eventId)
@@ -294,7 +314,9 @@ function MessageNotifications() {
           silent: true,
           // Coalesce repeated notifications for the same room (replaces the old
           // manual notifRef.close() dedup, which a SW notification can't hold).
-          tag: roomId,
+          // For thread replies widen the tag to room:thread so each thread
+          // coalesces independently instead of clobbering the room's bucket.
+          tag: threadId ? `${roomId}:${threadId}` : roomId,
           data: { path: roomPath },
         },
         () => {
@@ -326,6 +348,69 @@ function MessageNotifications() {
     audioElement?.play();
   }, []);
 
+  // Shared delivery tail for both the main timeline and per-thread paths:
+  // room-level unread dedup → avatar resolution → OS/toast notify → sound, all
+  // behind the quiet-hours / focus-assist gate. `threadId` (when set) widens the
+  // OS coalescing tag so each thread notifies independently; the click path
+  // stays the room path (RoomTimeline deep-links thread events into the panel).
+  const deliverNotification = useCallback(
+    (room: Room, mEvent: MatrixEvent, threadId?: string) => {
+      const sender = mEvent.getSender();
+      const eventId = mEvent.getId();
+      if (!sender || !eventId) return;
+
+      const unreadInfo = getUnreadInfo(room);
+      const cachedUnreadInfo = unreadCacheRef.current.get(room.roomId);
+      unreadCacheRef.current.set(room.roomId, unreadInfo);
+
+      if (unreadInfo.total === 0) return;
+      if (
+        cachedUnreadInfo &&
+        unreadEqual(unreadInfoToUnread(cachedUnreadInfo), unreadInfoToUnread(unreadInfo))
+      ) {
+        return;
+      }
+
+      const quietActive =
+        focusAssistActive || (quietHoursEnabled && isInQuietHours(quietHoursStart, quietHoursEnd));
+      if (quietActive) return;
+
+      if (showNotifications && notificationPermission('granted')) {
+        const avatarMxc =
+          room.getAvatarFallbackMember()?.getMxcAvatarUrl() ?? room.getMxcAvatarUrl();
+        notify({
+          roomName: room.name ?? 'Unknown',
+          roomAvatar: avatarMxc
+            ? (mxcUrlToHttp(mx, avatarMxc, useAuthentication, 96, 96, 'crop') ?? undefined)
+            : undefined,
+          username: getMemberDisplayName(room, sender) ?? getMxIdLocalPart(sender) ?? sender,
+          roomId: room.roomId,
+          eventId,
+          body: (mEvent.getContent().body as string | undefined) ?? '',
+          encrypted: room.hasEncryptionStateEvent(),
+          threadId,
+        });
+      }
+
+      if (notificationSound && messageSoundId !== 'none') {
+        playSound();
+      }
+    },
+    [
+      mx,
+      notify,
+      playSound,
+      showNotifications,
+      notificationSound,
+      useAuthentication,
+      quietHoursEnabled,
+      quietHoursStart,
+      quietHoursEnd,
+      focusAssistActive,
+      messageSoundId,
+    ],
+  );
+
   useEffect(() => {
     const handleTimelineEvent: RoomEventHandlerMap[RoomEvent.Timeline] = (
       mEvent,
@@ -349,61 +434,64 @@ function MessageNotifications() {
       const sender = mEvent.getSender();
       const eventId = mEvent.getId();
       if (!sender || !eventId || mEvent.getSender() === mx.getUserId()) return;
-      const unreadInfo = getUnreadInfo(room);
-      const cachedUnreadInfo = unreadCacheRef.current.get(room.roomId);
-      unreadCacheRef.current.set(room.roomId, unreadInfo);
+      // Single-owner rule: thread replies are delivered by the ThreadEvent.NewReply
+      // handler below (per-thread gating), so ignore them here — a reply notifies once.
+      if (mEvent.threadRootId && mEvent.getId() !== mEvent.threadRootId) return;
 
-      if (unreadInfo.total === 0) return;
-      if (
-        cachedUnreadInfo &&
-        unreadEqual(unreadInfoToUnread(cachedUnreadInfo), unreadInfoToUnread(unreadInfo))
-      ) {
-        return;
-      }
-
-      const quietActive =
-        focusAssistActive || (quietHoursEnabled && isInQuietHours(quietHoursStart, quietHoursEnd));
-      if (!quietActive) {
-        if (showNotifications && notificationPermission('granted')) {
-          const avatarMxc =
-            room.getAvatarFallbackMember()?.getMxcAvatarUrl() ?? room.getMxcAvatarUrl();
-          notify({
-            roomName: room.name ?? 'Unknown',
-            roomAvatar: avatarMxc
-              ? (mxcUrlToHttp(mx, avatarMxc, useAuthentication, 96, 96, 'crop') ?? undefined)
-              : undefined,
-            username: getMemberDisplayName(room, sender) ?? getMxIdLocalPart(sender) ?? sender,
-            roomId: room.roomId,
-            eventId,
-            body: (mEvent.getContent().body as string | undefined) ?? '',
-            encrypted: room.hasEncryptionStateEvent(),
-          });
-        }
-
-        if (notificationSound && messageSoundId !== 'none') {
-          playSound();
-        }
-      }
+      deliverNotification(room, mEvent);
     };
     mx.on(RoomEvent.Timeline, handleTimelineEvent);
     return () => {
       mx.removeListener(RoomEvent.Timeline, handleTimelineEvent);
     };
-  }, [
-    mx,
-    notificationSound,
-    notificationSelected,
-    showNotifications,
-    playSound,
-    notify,
-    selectedRoomId,
-    useAuthentication,
-    quietHoursEnabled,
-    quietHoursStart,
-    quietHoursEnd,
-    focusAssistActive,
-    messageSoundId,
-  ]);
+  }, [mx, notificationSelected, selectedRoomId, deliverNotification]);
+
+  const handleNewReply = useCallback<RoomEventHandlerMap[ThreadEvent.NewReply]>(
+    (thread, mEvent) => {
+      if (mx.getSyncState() !== 'SYNCING') return;
+      const room = mx.getRoom(thread.roomId);
+      if (!room || room.isSpaceRoom()) return;
+      if (!isNotificationEvent(mEvent) || mEvent.isSending()) return;
+      const sender = mEvent.getSender();
+      if (!sender || sender === mx.getUserId()) return;
+      // Suppress when the user is actively looking at this thread (or the inbox).
+      if (
+        document.hasFocus() &&
+        (notificationSelected ||
+          (selectedRoomId === thread.roomId && activeThreadId === thread.id))
+      ) {
+        return;
+      }
+
+      // Per-thread dedupe: a NewReply can re-fire for the same event as the
+      // thread (re)populates; notify at most once per (thread, event).
+      const eventId = mEvent.getId();
+      if (eventId) {
+        if (lastNotifiedThreadRef.current.get(thread.id) === eventId) return;
+        lastNotifiedThreadRef.current.set(thread.id, eventId);
+      }
+
+      const content = threadPrefs;
+      const mode = getThreadNotificationMode(content, room.roomId, thread.id);
+      const actions = mx.getPushActionsForEvent(mEvent);
+      const decision = shouldNotifyThreadReply({
+        mode,
+        defaultBehavior: content.default ?? THREAD_NOTIFICATIONS_FALLBACK_BEHAVIOR,
+        participated: thread.hasCurrentUserParticipated,
+        highlight: !!actions?.tweaks?.highlight,
+        notify: !!actions?.notify,
+        roomMuted: getNotificationType(mx, room.roomId) === NotificationType.Mute,
+      });
+      if (decision === 'none') return;
+
+      // E2EE caveat: NewReply can fire before decryption, so MentionsOnly may
+      // under-notify in encrypted rooms (same class as the main timeline path).
+      // Plaintext body suppression for encrypted rooms is handled inside notify().
+      deliverNotification(room, mEvent, thread.id);
+    },
+    [mx, notificationSelected, selectedRoomId, activeThreadId, threadPrefs, deliverNotification],
+  );
+  useRoomsListener(mx, ThreadEvent.NewReply, handleNewReply);
 
   return (
     <audio ref={audioRef} style={{ display: 'none' }}>

src/app/state/hooks/useBindAtoms.ts

@@ -5,12 +5,14 @@ import { mDirectAtom, useBindMDirectAtom } from '../mDirectList';
 import { roomToUnreadAtom, useBindRoomToUnreadAtom } from '../room/roomToUnread';
 import { roomToParentsAtom, useBindRoomToParentsAtom } from '../room/roomToParents';
 import { roomIdToTypingMembersAtom, useBindRoomIdToTypingMembersAtom } from '../typingMembers';
+import { threadNotificationsAtom, useBindThreadNotificationsAtom } from '../threadNotifications';
 
 export const useBindAtoms = (mx: MatrixClient) => {
   useBindMDirectAtom(mx, mDirectAtom);
   useBindAllInvitesAtom(mx, allInvitesAtom);
   useBindAllRoomsAtom(mx, allRoomsAtom);
   useBindRoomToParentsAtom(mx, roomToParentsAtom);
+  useBindThreadNotificationsAtom(mx, threadNotificationsAtom);
   useBindRoomToUnreadAtom(mx, roomToUnreadAtom);
 
   useBindRoomIdToTypingMembersAtom(mx, roomIdToTypingMembersAtom);

src/app/state/room/roomToUnread.ts

@@ -1,15 +1,16 @@
 import { produce } from 'immer';
-import { atom, useSetAtom } from 'jotai';
+import { atom, useAtomValue, useSetAtom } from 'jotai';
 import {
   IRoomTimelineData,
   MatrixClient,
   MatrixEvent,
+  NotificationCount,
   Room,
   RoomEvent,
   SyncState,
 } from 'matrix-js-sdk';
 import { ReceiptContent, ReceiptType } from 'matrix-js-sdk/lib/@types/read_receipts';
-import { useCallback, useEffect } from 'react';
+import { useCallback, useEffect, useRef } from 'react';
 import {
   Membership,
   NotificationType,
@@ -29,6 +30,9 @@ import { roomToParentsAtom } from './roomToParents';
 import { useStateEventCallback } from '../../hooks/useStateEventCallback';
 import { useSyncState } from '../../hooks/useSyncState';
 import { useRoomsNotificationPreferencesContext } from '../../hooks/useRoomsNotificationPreferences';
+import { useRoomsListener } from '../../hooks/useRoomsListener';
+import { threadNotificationsAtom } from '../threadNotifications';
+import { getMutedThreads } from '../../utils/threadNotifications';
 
 export type RoomToUnreadAction =
   | {
@@ -169,11 +173,17 @@ export const roomToUnreadAtom = atom<RoomToUnread, [RoomToUnreadAction], undefin
 export const useBindRoomToUnreadAtom = (mx: MatrixClient, unreadAtom: typeof roomToUnreadAtom) => {
   const setUnreadAtom = useSetAtom(unreadAtom);
   const roomsNotificationPreferences = useRoomsNotificationPreferencesContext();
+  const threadNotifications = useAtomValue(threadNotificationsAtom);
+  // Latest thread-notification prefs for the SDK-event handlers below, read via a
+  // ref so changing prefs never re-attaches the (many) per-room listeners. The
+  // dedicated reset effect keyed on `threadNotifications` handles mute/unmute.
+  const threadNotificationsRef = useRef(threadNotifications);
+  threadNotificationsRef.current = threadNotifications;
 
   useEffect(() => {
     setUnreadAtom({
       type: 'RESET',
-      unreadInfos: getUnreadInfos(mx),
+      unreadInfos: getUnreadInfos(mx, threadNotificationsRef.current),
     });
   }, [mx, setUnreadAtom]);
 
@@ -187,7 +197,7 @@ export const useBindRoomToUnreadAtom = (mx: MatrixClient, unreadAtom: typeof roo
         ) {
           setUnreadAtom({
             type: 'RESET',
-            unreadInfos: getUnreadInfos(mx),
+            unreadInfos: getUnreadInfos(mx, threadNotificationsRef.current),
           });
         }
       },
@@ -204,6 +214,10 @@ export const useBindRoomToUnreadAtom = (mx: MatrixClient, unreadAtom: typeof roo
       data: IRoomTimelineData,
     ) => {
       if (!room || !data.liveEvent || room.isSpaceRoom() || !isNotificationEvent(mEvent)) return;
+      // Single-owner rule: thread replies drive the room badge via
+      // RoomEvent.UnreadNotifications below — ignore them here so the count is
+      // never double-driven / mis-attributed to the main timeline.
+      if (mEvent.threadRootId && mEvent.getId() !== mEvent.threadRootId) return;
       if (getNotificationType(mx, room.roomId) === NotificationType.Mute) {
         setUnreadAtom({
           type: 'DELETE',
@@ -213,7 +227,13 @@ export const useBindRoomToUnreadAtom = (mx: MatrixClient, unreadAtom: typeof roo
       }
 
       if (mEvent.getSender() === mx.getUserId()) return;
-      setUnreadAtom({ type: 'PUT', unreadInfo: getUnreadInfo(room) });
+      setUnreadAtom({
+        type: 'PUT',
+        unreadInfo: getUnreadInfo(
+          room,
+          getMutedThreads(threadNotificationsRef.current, room.roomId),
+        ),
+      });
     };
     mx.on(RoomEvent.Timeline, handleTimelineEvent);
     return () => {
@@ -246,10 +266,51 @@ export const useBindRoomToUnreadAtom = (mx: MatrixClient, unreadAtom: typeof roo
   useEffect(() => {
     setUnreadAtom({
       type: 'RESET',
-      unreadInfos: getUnreadInfos(mx),
+      unreadInfos: getUnreadInfos(mx, threadNotificationsRef.current),
     });
   }, [mx, setUnreadAtom, roomsNotificationPreferences]);
 
+  // Mute/unmute of a thread rewrites `threadNotificationsAtom`; recompute badges
+  // immediately so muted-thread subtraction takes effect without waiting for the
+  // next timeline / unread event (mirrors the notification-preferences reset).
+  useEffect(() => {
+    setUnreadAtom({
+      type: 'RESET',
+      unreadInfos: getUnreadInfos(mx, threadNotifications),
+    });
+  }, [mx, setUnreadAtom, threadNotifications]);
+
+  // RoomEvent.UnreadNotifications is emitted room-level only (never re-emitted
+  // client-side), so the main Timeline pathway misses thread-count changes and
+  // room badges lag. useRoomsListener appends the emitting Room as the final
+  // arg, making this a surgical per-room PUT (not a full RESET per emit) with
+  // muted-thread subtraction re-applied. Room-mute keeps its DELETE semantics.
+  useRoomsListener(
+    mx,
+    RoomEvent.UnreadNotifications,
+    useCallback(
+      (
+        _unreadNotifications: NotificationCount | undefined,
+        _threadId: string | undefined,
+        room: Room,
+      ) => {
+        if (room.isSpaceRoom()) return;
+        if (getNotificationType(mx, room.roomId) === NotificationType.Mute) {
+          setUnreadAtom({ type: 'DELETE', roomId: room.roomId });
+          return;
+        }
+        setUnreadAtom({
+          type: 'PUT',
+          unreadInfo: getUnreadInfo(
+            room,
+            getMutedThreads(threadNotificationsRef.current, room.roomId),
+          ),
+        });
+      },
+      [mx, setUnreadAtom],
+    ),
+  );
+
   useEffect(() => {
     const handleMembershipChange = (room: Room, membership: string) => {
       if (membership !== Membership.Join) {

src/app/state/threadNotifications.ts

@@ -0,0 +1,36 @@
+import { atom, useSetAtom } from 'jotai';
+import { ClientEvent, MatrixClient, MatrixEvent } from 'matrix-js-sdk';
+import { useEffect } from 'react';
+import { AccountDataEvent } from '../../types/matrix/accountData';
+import { ThreadNotificationsContent } from '../utils/threadNotifications';
+
+// Holds the parsed `io.lotus.thread_notifications` account data. Seeded and
+// kept in sync by `useBindThreadNotificationsAtom`.
+export const threadNotificationsAtom = atom<ThreadNotificationsContent>({});
+
+const readContent = (mx: MatrixClient): ThreadNotificationsContent =>
+  ((mx as any).getAccountData(AccountDataEvent.LotusThreadNotifications)?.getContent() as
+    | ThreadNotificationsContent
+    | undefined) ?? {};
+
+export const useBindThreadNotificationsAtom = (
+  mx: MatrixClient,
+  threadNotifications: typeof threadNotificationsAtom,
+) => {
+  const setContent = useSetAtom(threadNotifications);
+
+  useEffect(() => {
+    setContent(readContent(mx));
+
+    const handleAccountData = (event: MatrixEvent) => {
+      if (event.getType() === AccountDataEvent.LotusThreadNotifications) {
+        setContent(event.getContent<ThreadNotificationsContent>() ?? {});
+      }
+    };
+
+    mx.on(ClientEvent.AccountData, handleAccountData);
+    return () => {
+      mx.removeListener(ClientEvent.AccountData, handleAccountData);
+    };
+  }, [mx, setContent]);
+};

src/app/utils/room.test.ts

@@ -388,6 +388,63 @@ test('getUnreadInfo uses highlight when it exceeds total', () => {
   assert.deepEqual(getUnreadInfo(room2), { roomId: '!r:y', highlight: 1, total: 7 });
 });
 
+const mockRoomWithThreadCounts = (
+  total: number,
+  highlight: number,
+  threadCounts: Record<string, { total: number; highlight: number }>,
+): Room =>
+  ({
+    roomId: '!r:x',
+    getUnreadNotificationCount: (type: NotificationCountType) =>
+      type === NotificationCountType.Total ? total : highlight,
+    getThreadUnreadNotificationCount: (threadId: string, type: NotificationCountType) =>
+      type === NotificationCountType.Total
+        ? (threadCounts[threadId]?.total ?? 0)
+        : (threadCounts[threadId]?.highlight ?? 0),
+  }) as unknown as Room;
+
+test('getUnreadInfo subtracts muted thread counts from room totals', () => {
+  const room = mockRoomWithThreadCounts(5, 2, { $t1: { total: 3, highlight: 1 } });
+  assert.deepEqual(getUnreadInfo(room, new Set(['$t1'])), {
+    roomId: '!r:x',
+    highlight: 1,
+    total: 2,
+  });
+});
+
+test('getUnreadInfo subtracts multiple muted threads', () => {
+  const room = mockRoomWithThreadCounts(9, 3, {
+    $t1: { total: 3, highlight: 1 },
+    $t2: { total: 2, highlight: 1 },
+  });
+  assert.deepEqual(getUnreadInfo(room, new Set(['$t1', '$t2'])), {
+    roomId: '!r:x',
+    highlight: 1,
+    total: 4,
+  });
+});
+
+test('getUnreadInfo clamps subtracted counts at zero', () => {
+  const room = mockRoomWithThreadCounts(2, 1, { $t1: { total: 5, highlight: 4 } });
+  assert.deepEqual(getUnreadInfo(room, new Set(['$t1'])), {
+    roomId: '!r:x',
+    highlight: 0,
+    total: 0,
+  });
+});
+
+test('getUnreadInfo leaves counts untouched without muted threads', () => {
+  const room = mockRoomWithThreadCounts(4, 1, { $t1: { total: 3, highlight: 1 } });
+  // undefined muted set (backward compat)
+  assert.deepEqual(getUnreadInfo(room), { roomId: '!r:x', highlight: 1, total: 4 });
+  // empty muted set is a no-op too
+  assert.deepEqual(getUnreadInfo(room, new Set<string>()), {
+    roomId: '!r:x',
+    highlight: 1,
+    total: 4,
+  });
+});
+
 // --- getRoomIconSrc -------------------------------------------------------
 
 test('getRoomIconSrc selects icon by room type and join rule', () => {

src/app/utils/room.ts

@@ -29,6 +29,7 @@ import {
   StateEvent,
   UnreadInfo,
 } from '../../types/matrix/room';
+import { getMutedThreads, ThreadNotificationsContent } from './threadNotifications';
 
 export const getStateEvent = (
   room: Room,
@@ -233,9 +234,23 @@ export const roomHaveUnread = (mx: MatrixClient, room: Room) => {
   return true;
 };
 
-export const getUnreadInfo = (room: Room): UnreadInfo => {
-  const total = room.getUnreadNotificationCount(NotificationCountType.Total);
-  const highlight = room.getUnreadNotificationCount(NotificationCountType.Highlight);
+export const getUnreadInfo = (room: Room, mutedThreads?: Set<string>): UnreadInfo => {
+  let total = room.getUnreadNotificationCount(NotificationCountType.Total);
+  let highlight = room.getUnreadNotificationCount(NotificationCountType.Highlight);
+
+  // Server room totals INCLUDE per-thread notification counts, so subtract any
+  // explicitly muted thread's counts back out (clamped at zero) to keep muted
+  // threads from contributing to the room badge (P4-1).
+  if (mutedThreads && mutedThreads.size > 0) {
+    mutedThreads.forEach((threadId) => {
+      total -= room.getThreadUnreadNotificationCount(threadId, NotificationCountType.Total) ?? 0;
+      highlight -=
+        room.getThreadUnreadNotificationCount(threadId, NotificationCountType.Highlight) ?? 0;
+    });
+    if (total < 0) total = 0;
+    if (highlight < 0) highlight = 0;
+  }
+
   return {
     roomId: room.roomId,
     highlight,
@@ -243,14 +258,18 @@ export const getUnreadInfo = (room: Room): UnreadInfo => {
   };
 };
 
-export const getUnreadInfos = (mx: MatrixClient): UnreadInfo[] => {
+export const getUnreadInfos = (
+  mx: MatrixClient,
+  content?: ThreadNotificationsContent,
+): UnreadInfo[] => {
   const unreadInfos = mx.getRooms().reduce<UnreadInfo[]>((unread, room) => {
     if (room.isSpaceRoom()) return unread;
     if (room.getMyMembership() !== 'join') return unread;
     if (getNotificationType(mx, room.roomId) === NotificationType.Mute) return unread;
 
     if (roomHaveNotification(room) || roomHaveUnread(mx, room)) {
-      unread.push(getUnreadInfo(room));
+      const mutedThreads = content ? getMutedThreads(content, room.roomId) : undefined;
+      unread.push(getUnreadInfo(room, mutedThreads));
     }
 
     return unread;

src/app/utils/threadNotifications.test.ts

@@ -0,0 +1,260 @@
+import { describe, it } from 'node:test';
+import assert from 'node:assert/strict';
+import {
+  getMutedThreads,
+  getThreadNotificationMode,
+  pruneThreadNotifications,
+  shouldNotifyThreadReply,
+  ThreadDefaultBehavior,
+  ThreadNotificationMode,
+  ThreadNotificationsContent,
+  ThreadNotifyDecision,
+} from './threadNotifications';
+
+const DAY = 24 * 60 * 60 * 1000;
+
+const decide = (
+  overrides: Partial<Parameters<typeof shouldNotifyThreadReply>[0]>,
+): ThreadNotifyDecision =>
+  shouldNotifyThreadReply({
+    mode: ThreadNotificationMode.Default,
+    defaultBehavior: 'participating',
+    participated: false,
+    highlight: false,
+    notify: false,
+    roomMuted: false,
+    ...overrides,
+  });
+
+describe('shouldNotifyThreadReply', () => {
+  it('roomMuted trumps everything, even mode All + highlight', () => {
+    assert.equal(
+      decide({ roomMuted: true, mode: ThreadNotificationMode.All, highlight: true }),
+      'none',
+    );
+  });
+
+  it('roomMuted trumps Default + participating + participated', () => {
+    assert.equal(decide({ roomMuted: true, participated: true }), 'none');
+  });
+
+  it('mode Mute is none regardless of highlight/participation', () => {
+    assert.equal(
+      decide({ mode: ThreadNotificationMode.Mute, highlight: true, participated: true }),
+      'none',
+    );
+  });
+
+  it('mode All + highlight => loud', () => {
+    assert.equal(decide({ mode: ThreadNotificationMode.All, highlight: true }), 'loud');
+  });
+
+  it('mode All + no highlight => notify', () => {
+    assert.equal(decide({ mode: ThreadNotificationMode.All, highlight: false }), 'notify');
+  });
+
+  it('mode MentionsOnly + highlight => loud', () => {
+    assert.equal(decide({ mode: ThreadNotificationMode.MentionsOnly, highlight: true }), 'loud');
+  });
+
+  it('mode MentionsOnly + no highlight => none (even if participated)', () => {
+    assert.equal(
+      decide({ mode: ThreadNotificationMode.MentionsOnly, highlight: false, participated: true }),
+      'none',
+    );
+  });
+
+  it('Default + behavior all + highlight => loud', () => {
+    assert.equal(decide({ defaultBehavior: 'all', highlight: true }), 'loud');
+  });
+
+  it('Default + behavior all + no highlight => notify', () => {
+    assert.equal(decide({ defaultBehavior: 'all', highlight: false }), 'notify');
+  });
+
+  it('Default + participating + highlight => loud (even if not participated)', () => {
+    assert.equal(
+      decide({ defaultBehavior: 'participating', highlight: true, participated: false }),
+      'loud',
+    );
+  });
+
+  it('Default + participating + no highlight + participated => notify', () => {
+    assert.equal(
+      decide({ defaultBehavior: 'participating', highlight: false, participated: true }),
+      'notify',
+    );
+  });
+
+  it('Default + participating + no highlight + not participated => none', () => {
+    assert.equal(
+      decide({ defaultBehavior: 'participating', highlight: false, participated: false }),
+      'none',
+    );
+  });
+
+  it('ignores the `notify` input entirely in v1', () => {
+    // notify=true must not upgrade a "none" decision.
+    assert.equal(
+      decide({ defaultBehavior: 'participating', participated: false, notify: true }),
+      'none',
+    );
+    // notify=false must not downgrade an "all" mode notify.
+    assert.equal(decide({ mode: ThreadNotificationMode.All, notify: false }), 'notify');
+  });
+});
+
+describe('getThreadNotificationMode', () => {
+  it('returns Default for undefined content', () => {
+    assert.equal(getThreadNotificationMode(undefined, '!r', '$t'), ThreadNotificationMode.Default);
+  });
+
+  it('returns Default when room or thread is absent', () => {
+    const content: ThreadNotificationsContent = {
+      rooms: { '!r': { $other: { mode: ThreadNotificationMode.All, ts: 1 } } },
+    };
+    assert.equal(getThreadNotificationMode(content, '!r', '$t'), ThreadNotificationMode.Default);
+    assert.equal(getThreadNotificationMode(content, '!x', '$t'), ThreadNotificationMode.Default);
+  });
+
+  it('returns the stored mode', () => {
+    const content: ThreadNotificationsContent = {
+      rooms: { '!r': { $t: { mode: ThreadNotificationMode.Mute, ts: 1 } } },
+    };
+    assert.equal(getThreadNotificationMode(content, '!r', '$t'), ThreadNotificationMode.Mute);
+  });
+
+  it('is safe against malformed entries', () => {
+    const bad = {
+      rooms: {
+        '!r': {
+          $badMode: { mode: 'nonsense', ts: 1 },
+          $noTs: { mode: ThreadNotificationMode.All },
+          $notObj: 'oops',
+          $nullEntry: null,
+        },
+      },
+    } as unknown as ThreadNotificationsContent;
+    assert.equal(getThreadNotificationMode(bad, '!r', '$badMode'), ThreadNotificationMode.Default);
+    assert.equal(getThreadNotificationMode(bad, '!r', '$noTs'), ThreadNotificationMode.Default);
+    assert.equal(getThreadNotificationMode(bad, '!r', '$notObj'), ThreadNotificationMode.Default);
+    assert.equal(
+      getThreadNotificationMode(bad, '!r', '$nullEntry'),
+      ThreadNotificationMode.Default,
+    );
+  });
+
+  it('is safe when rooms is not an object', () => {
+    const bad = { rooms: 'oops' } as unknown as ThreadNotificationsContent;
+    assert.equal(getThreadNotificationMode(bad, '!r', '$t'), ThreadNotificationMode.Default);
+  });
+});
+
+describe('getMutedThreads', () => {
+  it('returns empty set for undefined/absent room', () => {
+    assert.deepEqual(getMutedThreads(undefined, '!r'), new Set());
+    assert.deepEqual(getMutedThreads({ rooms: {} }, '!r'), new Set());
+  });
+
+  it('collects only Mute entries', () => {
+    const content: ThreadNotificationsContent = {
+      rooms: {
+        '!r': {
+          $a: { mode: ThreadNotificationMode.Mute, ts: 1 },
+          $b: { mode: ThreadNotificationMode.All, ts: 1 },
+          $c: { mode: ThreadNotificationMode.Mute, ts: 1 },
+        },
+      },
+    };
+    assert.deepEqual(getMutedThreads(content, '!r'), new Set(['$a', '$c']));
+  });
+
+  it('ignores malformed entries', () => {
+    const bad = {
+      rooms: { '!r': { $a: { mode: 'mute-ish', ts: 1 }, $b: null } },
+    } as unknown as ThreadNotificationsContent;
+    assert.deepEqual(getMutedThreads(bad, '!r'), new Set());
+  });
+});
+
+describe('pruneThreadNotifications', () => {
+  const now = 1_000_000_000_000;
+
+  it('drops rooms not in joinedRoomIds', () => {
+    const content: ThreadNotificationsContent = {
+      rooms: {
+        '!keep': { $t: { mode: ThreadNotificationMode.All, ts: now } },
+        '!left': { $t: { mode: ThreadNotificationMode.All, ts: now } },
+      },
+    };
+    const out = pruneThreadNotifications(content, new Set(['!keep']), now);
+    assert.deepEqual(Object.keys(out.rooms ?? {}), ['!keep']);
+  });
+
+  it('drops entries older than 180 days', () => {
+    const content: ThreadNotificationsContent = {
+      rooms: {
+        '!r': {
+          $fresh: { mode: ThreadNotificationMode.All, ts: now - 179 * DAY },
+          $old: { mode: ThreadNotificationMode.All, ts: now - 181 * DAY },
+        },
+      },
+    };
+    const out = pruneThreadNotifications(content, new Set(['!r']), now);
+    assert.deepEqual(Object.keys(out.rooms?.['!r'] ?? {}), ['$fresh']);
+  });
+
+  it('caps a room at 200 entries, evicting oldest ts first', () => {
+    const entries: Record<string, { mode: ThreadNotificationMode.All; ts: number }> = {};
+    // 205 entries, ts ascending with the id index.
+    for (let i = 0; i < 205; i += 1) {
+      entries[`$t${i}`] = { mode: ThreadNotificationMode.All, ts: now - (205 - i) * 1000 };
+    }
+    const content: ThreadNotificationsContent = { rooms: { '!r': entries } };
+    const out = pruneThreadNotifications(content, new Set(['!r']), now);
+    const kept = out.rooms?.['!r'] ?? {};
+    assert.equal(Object.keys(kept).length, 200);
+    // Oldest 5 ($t0..$t4) evicted; newest retained.
+    assert.equal(kept.$t0, undefined);
+    assert.equal(kept.$t4, undefined);
+    assert.notEqual(kept.$t5, undefined);
+    assert.notEqual(kept.$t204, undefined);
+  });
+
+  it('drops rooms left with no entries', () => {
+    const content: ThreadNotificationsContent = {
+      rooms: {
+        '!r': { $old: { mode: ThreadNotificationMode.All, ts: now - 200 * DAY } },
+      },
+    };
+    const out = pruneThreadNotifications(content, new Set(['!r']), now);
+    assert.equal(out.rooms, undefined);
+  });
+
+  it('preserves the default behavior field', () => {
+    const behavior: ThreadDefaultBehavior = 'all';
+    const content: ThreadNotificationsContent = { default: behavior, rooms: {} };
+    const out = pruneThreadNotifications(content, new Set(), now);
+    assert.equal(out.default, 'all');
+  });
+
+  it('never mutates the input', () => {
+    const content: ThreadNotificationsContent = {
+      default: 'participating',
+      rooms: {
+        '!r': { $t: { mode: ThreadNotificationMode.All, ts: now } },
+        '!left': { $t: { mode: ThreadNotificationMode.All, ts: now } },
+      },
+    };
+    const snapshot = JSON.parse(JSON.stringify(content));
+    const out = pruneThreadNotifications(content, new Set(['!r']), now);
+    assert.deepEqual(content, snapshot);
+    // Output room objects are fresh, not shared references with the input.
+    assert.notEqual(out.rooms?.['!r'], content.rooms?.['!r']);
+  });
+
+  it('handles malformed rooms container safely', () => {
+    const bad = { rooms: 'oops' } as unknown as ThreadNotificationsContent;
+    assert.deepEqual(pruneThreadNotifications(bad, new Set(['!r']), now), {});
+  });
+});

src/app/utils/threadNotifications.ts

@@ -0,0 +1,196 @@
+// Per-thread notification modes (P4-1). Stored in the
+// `io.lotus.thread_notifications` account data event. The functions in this
+// module are PURE — they never touch React or matrix-js-sdk objects so they
+// can be unit-tested in isolation and reused by the pipeline/UI agents.
+
+export enum ThreadNotificationMode {
+  Default = 'default',
+  All = 'all',
+  MentionsOnly = 'mentions',
+  Mute = 'mute',
+}
+
+export type ThreadDefaultBehavior = 'all' | 'participating';
+
+export type ThreadNotifyDecision = 'loud' | 'notify' | 'none';
+
+export type ThreadNotificationEntry = {
+  mode: Exclude<ThreadNotificationMode, ThreadNotificationMode.Default>;
+  ts: number;
+};
+
+export type ThreadNotificationsContent = {
+  default?: ThreadDefaultBehavior;
+  rooms?: Record<string, Record<string, ThreadNotificationEntry>>;
+};
+
+// DEFAULT behavior when the user has not chosen a global default. Fixed to
+// 'participating': notify only if the current user participated in the thread
+// or the reply mentions them.
+export const THREAD_NOTIFICATIONS_FALLBACK_BEHAVIOR: ThreadDefaultBehavior = 'participating';
+
+// Entries older than this are pruned on write to keep account data bounded.
+const PRUNE_MAX_AGE_MS = 180 * 24 * 60 * 60 * 1000;
+
+// Maximum stored entries per room; oldest are evicted first.
+const PRUNE_MAX_ENTRIES_PER_ROOM = 200;
+
+const STORED_MODES: ReadonlySet<string> = new Set([
+  ThreadNotificationMode.All,
+  ThreadNotificationMode.MentionsOnly,
+  ThreadNotificationMode.Mute,
+]);
+
+const isStoredMode = (
+  value: unknown,
+): value is Exclude<ThreadNotificationMode, ThreadNotificationMode.Default> =>
+  typeof value === 'string' && STORED_MODES.has(value);
+
+const readEntry = (
+  content: ThreadNotificationsContent | undefined,
+  roomId: string,
+  threadRootId: string,
+): ThreadNotificationEntry | undefined => {
+  const entry = content?.rooms?.[roomId]?.[threadRootId];
+  if (!entry || typeof entry !== 'object') return undefined;
+  if (!isStoredMode(entry.mode) || typeof entry.ts !== 'number') return undefined;
+  return entry;
+};
+
+/**
+ * Resolve the stored notification mode for a thread. Absent or malformed
+ * content resolves to `ThreadNotificationMode.Default`.
+ */
+export function getThreadNotificationMode(
+  content: ThreadNotificationsContent | undefined,
+  roomId: string,
+  threadRootId: string,
+): ThreadNotificationMode {
+  const entry = readEntry(content, roomId, threadRootId);
+  return entry ? entry.mode : ThreadNotificationMode.Default;
+}
+
+/**
+ * All thread root ids explicitly muted within a room. Malformed content yields
+ * an empty set.
+ */
+export function getMutedThreads(
+  content: ThreadNotificationsContent | undefined,
+  roomId: string,
+): Set<string> {
+  const muted = new Set<string>();
+  const roomEntries = content?.rooms?.[roomId];
+  if (!roomEntries || typeof roomEntries !== 'object') return muted;
+  Object.keys(roomEntries).forEach((threadRootId) => {
+    const entry = roomEntries[threadRootId];
+    if (entry && isStoredMode(entry.mode) && entry.mode === ThreadNotificationMode.Mute) {
+      muted.add(threadRootId);
+    }
+  });
+  return muted;
+}
+
+/**
+ * Decide whether a thread reply should notify.
+ *
+ * NOTE: the `notify` input reflects the base matrix push rule outcome and is
+ * accepted for forward-compatibility, but is intentionally IGNORED in v1: the
+ * per-thread mode fully determines the decision, so honoring `notify` would let
+ * server push rules silently override an explicit "All" thread override. Kept
+ * in the signature so the pipeline can start plumbing it without a later break.
+ */
+export function shouldNotifyThreadReply(input: {
+  mode: ThreadNotificationMode;
+  defaultBehavior: ThreadDefaultBehavior;
+  participated: boolean;
+  highlight: boolean;
+  notify: boolean;
+  roomMuted: boolean;
+}): ThreadNotifyDecision {
+  const { mode, defaultBehavior, participated, highlight, roomMuted } = input;
+
+  if (roomMuted) return 'none';
+  if (mode === ThreadNotificationMode.Mute) return 'none';
+
+  if (mode === ThreadNotificationMode.All) {
+    return highlight ? 'loud' : 'notify';
+  }
+
+  if (mode === ThreadNotificationMode.MentionsOnly) {
+    return highlight ? 'loud' : 'none';
+  }
+
+  // ThreadNotificationMode.Default
+  if (defaultBehavior === 'all') {
+    return highlight ? 'loud' : 'notify';
+  }
+
+  // defaultBehavior === 'participating'
+  if (highlight) return 'loud';
+  return participated ? 'notify' : 'none';
+}
+
+/**
+ * Return a NEW content object with stale/oversized data removed. Never mutates
+ * the input.
+ *
+ * (1) drop rooms not in `joinedRoomIds`
+ * (2) drop entries older than 180 days (`ts < now - PRUNE_MAX_AGE_MS`)
+ * (3) cap each room at 200 entries, evicting the oldest `ts` first
+ * (4) drop rooms left with no entries
+ */
+export function pruneThreadNotifications(
+  content: ThreadNotificationsContent,
+  joinedRoomIds: Set<string>,
+  now: number,
+): ThreadNotificationsContent {
+  const minTs = now - PRUNE_MAX_AGE_MS;
+  const pruned: ThreadNotificationsContent = {};
+  if (content.default !== undefined) {
+    pruned.default = content.default;
+  }
+
+  const rooms = content.rooms;
+  if (!rooms || typeof rooms !== 'object') {
+    return pruned;
+  }
+
+  const prunedRooms: Record<string, Record<string, ThreadNotificationEntry>> = {};
+
+  Object.keys(rooms).forEach((roomId) => {
+    if (!joinedRoomIds.has(roomId)) return;
+    const roomEntries = rooms[roomId];
+    if (!roomEntries || typeof roomEntries !== 'object') return;
+
+    // Keep only well-formed, non-expired entries.
+    const kept: Array<[string, ThreadNotificationEntry]> = [];
+    Object.keys(roomEntries).forEach((threadRootId) => {
+      const entry = roomEntries[threadRootId];
+      if (!entry || !isStoredMode(entry.mode) || typeof entry.ts !== 'number') return;
+      if (entry.ts < minTs) return;
+      kept.push([threadRootId, { mode: entry.mode, ts: entry.ts }]);
+    });
+
+    if (kept.length === 0) return;
+
+    // Cap per room, evicting oldest first (ascending ts sort, keep the tail).
+    let capped = kept;
+    if (kept.length > PRUNE_MAX_ENTRIES_PER_ROOM) {
+      capped = [...kept]
+        .sort((a, b) => a[1].ts - b[1].ts)
+        .slice(kept.length - PRUNE_MAX_ENTRIES_PER_ROOM);
+    }
+
+    const nextRoom: Record<string, ThreadNotificationEntry> = {};
+    capped.forEach(([threadRootId, entry]) => {
+      nextRoom[threadRootId] = entry;
+    });
+    prunedRooms[roomId] = nextRoom;
+  });
+
+  if (Object.keys(prunedRooms).length > 0) {
+    pruned.rooms = prunedRooms;
+  }
+
+  return pruned;
+}

src/types/matrix/accountData.ts

@@ -14,6 +14,10 @@ export enum AccountDataEvent {
   // devices like custom emoji/sticker packs).
   LotusSoundboard = 'io.lotus.soundboard',
 
+  // [P4-1] Per-thread notification mode overrides (All/Mentions/Mute) plus the
+  // global default behavior for threads.
+  LotusThreadNotifications = 'io.lotus.thread_notifications',
+
   SecretStorageDefaultKey = 'm.secret_storage.default_key',
 
   CrossSigningMaster = 'm.cross_signing.master',