feat: Implement sub-room functionality

- Added support for sub-rooms in the SpaceHierarchy component, allowing rooms to have nested child rooms.
- Introduced UnjoinedSubRoomItem component to display unjoined sub-rooms with a join button.
- Created SubRooms settings page for managing sub-rooms, including adding and removing sub-rooms.
- Updated RoomNavItem to include options for creating sub-rooms.
- Enhanced room fetching logic to filter out sub-rooms from the main room list.
- Added hooks for managing sub-rooms state and permissions.
- Updated relevant state management to handle sub-room creation and association with parent rooms.

src/app/features/create-room/CreateRoomModal.tsx

@@ -1,4 +1,4 @@
-import React from 'react';
+import React, { useCallback } from 'react';
 import {
   Box,
   config,
@@ -14,6 +14,7 @@ import {
   Text,
 } from 'folds';
 import FocusTrap from 'focus-trap-react';
+import { useAtomValue } from 'jotai';
 import { useAllJoinedRoomsSet, useGetRoom } from '../../hooks/useGetRoom';
 import { SpaceProvider } from '../../hooks/useSpace';
 import { CreateRoomForm } from './CreateRoom';
@@ -23,17 +24,96 @@ import {
 } from '../../state/hooks/createRoomModal';
 import { CreateRoomModalState } from '../../state/createRoomModal';
 import { stopPropagation } from '../../utils/keyboard';
+import { useMatrixClient } from '../../hooks/useMatrixClient';
+import { StateEvent } from '../../../types/matrix/room';
+import { PaarrotSubRoomsContent } from '../../hooks/useRoomSubRooms';
+import { roomToParentsAtom } from '../../state/room/roomToParents';
+import { isSpace } from '../../utils/room';
 
 type CreateRoomModalProps = {
   state: CreateRoomModalState;
 };
 function CreateRoomModal({ state }: CreateRoomModalProps) {
-  const { spaceId } = state;
+  const { spaceId, parentSubRoomId } = state;
+  const mx = useMatrixClient();
   const closeDialog = useCloseCreateRoomModal();
+  const roomToParents = useAtomValue(roomToParentsAtom);
 
   const allJoinedRooms = useAllJoinedRoomsSet();
   const getRoom = useGetRoom(allJoinedRooms);
   const space = spaceId ? getRoom(spaceId) : undefined;
+  const parentRoom = parentSubRoomId ? getRoom(parentSubRoomId) : undefined;
+
+  const handleCreate = useCallback(
+    async (newRoomId: string) => {
+      // If creating a sub-room, add it to the parent's sub-rooms and set parent relationship
+      if (parentSubRoomId && parentRoom) {
+        try {
+          // Extract server from parent room ID for via servers
+          const serverName = parentSubRoomId.split(':')[1] || '';
+          const viaServers = serverName ? [serverName] : [];
+
+          // Set m.space.parent on the new room pointing to parent
+          await mx.sendStateEvent(
+            newRoomId,
+            StateEvent.SpaceParent,
+            {
+              canonical: true,
+              via: viaServers,
+            },
+            parentSubRoomId
+          );
+
+          // Add new room to parent's sub-rooms list
+          const stateEvent = parentRoom.currentState.getStateEvents(StateEvent.PaarrotSubRooms, '');
+          const currentContent = stateEvent?.getContent() as PaarrotSubRoomsContent | undefined;
+          const currentChildren = currentContent?.children ?? [];
+          const newContent: PaarrotSubRoomsContent = {
+            children: [...currentChildren, newRoomId],
+          };
+          // eslint-disable-next-line @typescript-eslint/no-explicit-any
+          await mx.sendStateEvent(parentSubRoomId, StateEvent.PaarrotSubRooms as any, newContent, '');
+
+          // Also add sub-room to parent's space(s) for joining/permissions
+          // The UI will hide it from the top-level display and only show it nested
+          const parentSpaces = roomToParents.get(parentSubRoomId);
+          if (parentSpaces) {
+            const spaceIds = Array.from(parentSpaces);
+            await Promise.all(
+              spaceIds.map(async (parentSpaceId) => {
+                const parentSpaceRoom = mx.getRoom(parentSpaceId);
+                // Only add to actual spaces (not other sub-room parents)
+                if (parentSpaceRoom && isSpace(parentSpaceRoom)) {
+                  try {
+                    const spaceServerName = parentSpaceId.split(':')[1] || '';
+                    const spaceViaServers = spaceServerName ? [spaceServerName] : [];
+                    // Add as m.space.child to the space
+                    await mx.sendStateEvent(
+                      parentSpaceId,
+                      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+                      StateEvent.SpaceChild as any,
+                      { via: spaceViaServers },
+                      newRoomId
+                    );
+                  } catch (spaceError) {
+                    // eslint-disable-next-line no-console
+                    console.warn(`Failed to add sub-room to space ${parentSpaceId}:`, spaceError);
+                  }
+                }
+              })
+            );
+          }
+        } catch (error) {
+          // eslint-disable-next-line no-console
+          console.error('Failed to set up sub-room relationship:', error);
+        }
+      }
+      closeDialog();
+    },
+    [mx, parentSubRoomId, parentRoom, roomToParents, closeDialog]
+  );
+
+  const modalTitle = parentSubRoomId ? 'New Sub-Room' : 'New Room';
 
   return (
     <SpaceProvider value={space ?? null}>
@@ -57,7 +137,7 @@ function CreateRoomModal({ state }: CreateRoomModalProps) {
                   }}
                 >
                   <Box grow="Yes">
-                    <Text size="H4">New Room</Text>
+                    <Text size="H4">{modalTitle}</Text>
                   </Box>
                   <Box shrink="No">
                     <IconButton size="300" radii="300" onClick={closeDialog}>
@@ -74,7 +154,7 @@ function CreateRoomModal({ state }: CreateRoomModalProps) {
                     direction="Column"
                     gap="500"
                   >
-                    <CreateRoomForm space={space} onCreate={closeDialog} />
+                    <CreateRoomForm space={space} onCreate={handleCreate} />
                   </Box>
                 </Scroll>
               </Box>

src/app/features/lobby/SpaceHierarchy.tsx

@@ -18,6 +18,7 @@ import { RoomType, StateEvent } from '../../../types/matrix/room';
 import { SequenceCard } from '../../components/sequence-card';
 import { getRoomCreatorsForRoomId } from '../../hooks/useRoomCreators';
 import { getRoomPermissionsAPI } from '../../hooks/useRoomPermissions';
+import { PaarrotSubRoomsContent } from '../../hooks/useRoomSubRooms';
 
 type SpaceHierarchyProps = {
   summary: IHierarchyRoom | undefined;
@@ -99,7 +100,36 @@ export const SpaceHierarchy = forwardRef<HTMLDivElement, SpaceHierarchyProps>(
       onSpacesFound(Array.from(subspaces.values()));
     }, [subspaces, onSpacesFound]);
 
-    let childItems = roomItems?.filter((i) => !subspaces.has(i.roomId));
+    // Build a global set of all sub-room IDs by checking ALL joined rooms
+    // This ensures sub-rooms are hidden even if their parent isn't in this space
+    const globalSubRoomIds = useMemo(() => {
+      const subRoomIds = new Set<string>();
+      mx.getRooms().forEach((room) => {
+        const subRoomsEvent = room.currentState.getStateEvents(StateEvent.PaarrotSubRooms, '');
+        const content = subRoomsEvent?.getContent<PaarrotSubRoomsContent>();
+        content?.children?.forEach((childId: string) => subRoomIds.add(childId));
+      });
+      return subRoomIds;
+    }, [mx, allJoinedRooms]);
+
+    // Build a map of parent room -> sub-room IDs for nested rendering (only for rooms in this space)
+    const subRoomsMap = useMemo(() => {
+      const parentToSubRooms = new Map<string, string[]>();
+      roomItems?.forEach((item) => {
+        const room = mx.getRoom(item.roomId);
+        if (room) {
+          const subRoomsEvent = room.currentState.getStateEvents(StateEvent.PaarrotSubRooms, '');
+          const content = subRoomsEvent?.getContent<PaarrotSubRoomsContent>();
+          const children = content?.children ?? [];
+          if (children.length > 0) {
+            parentToSubRooms.set(item.roomId, children);
+          }
+        }
+      });
+      return parentToSubRooms;
+    }, [mx, roomItems]);
+
+    let childItems = roomItems?.filter((i) => !subspaces.has(i.roomId) && !globalSubRoomIds.has(i.roomId));
     if (!spacePermissions?.stateEvent(StateEvent.SpaceChild, mx.getSafeUserId())) {
       // hide unknown rooms for normal user
       childItems = childItems?.filter((i) => {
@@ -166,40 +196,89 @@ export const SpaceHierarchy = forwardRef<HTMLDivElement, SpaceHierarchyProps>(
                 draggingItem?.roomId === roomItem.roomId &&
                 draggingItem.parentId === roomItem.parentId;
 
+              // Get sub-rooms for this room
+              const subRoomIds = subRoomsMap.get(roomItem.roomId) ?? [];
+
               return (
-                <RoomItemCard
-                  key={roomItem.roomId}
-                  item={roomItem}
-                  loading={fetching}
-                  error={error}
-                  summary={roomSummary}
-                  dm={mDirects.has(roomItem.roomId)}
-                  onOpen={onOpenRoom}
-                  getRoom={getRoom}
-                  canReorder={
-                    !!spacePermissions?.stateEvent(StateEvent.SpaceChild, mx.getSafeUserId()) &&
-                    !disabledReorder
-                  }
-                  options={
-                    <HierarchyItemMenu
-                      item={roomItem}
-                      powerLevels={roomPowerLevels}
-                      joined={allJoinedRooms.has(roomItem.roomId)}
-                      canEditChild={
-                        !!spacePermissions?.stateEvent(StateEvent.SpaceChild, mx.getSafeUserId())
-                      }
-                    />
-                  }
-                  after={
-                    <AfterItemDropTarget
-                      item={roomItem}
-                      nextRoomId={nextRoomId}
-                      canDrop={canDrop}
-                    />
-                  }
-                  data-dragging={roomDragging}
-                  onDragging={onDragging}
-                />
+                <React.Fragment key={roomItem.roomId}>
+                  <RoomItemCard
+                    item={roomItem}
+                    loading={fetching}
+                    error={error}
+                    summary={roomSummary}
+                    dm={mDirects.has(roomItem.roomId)}
+                    onOpen={onOpenRoom}
+                    getRoom={getRoom}
+                    canReorder={
+                      !!spacePermissions?.stateEvent(StateEvent.SpaceChild, mx.getSafeUserId()) &&
+                      !disabledReorder
+                    }
+                    options={
+                      <HierarchyItemMenu
+                        item={roomItem}
+                        powerLevels={roomPowerLevels}
+                        joined={allJoinedRooms.has(roomItem.roomId)}
+                        canEditChild={
+                          !!spacePermissions?.stateEvent(StateEvent.SpaceChild, mx.getSafeUserId())
+                        }
+                      />
+                    }
+                    after={
+                      <AfterItemDropTarget
+                        item={roomItem}
+                        nextRoomId={nextRoomId}
+                        canDrop={canDrop}
+                      />
+                    }
+                    data-dragging={roomDragging}
+                    onDragging={onDragging}
+                  />
+                  {/* Render sub-rooms nested under this room */}
+                  {subRoomIds.length > 0 && (
+                    <Box direction="Column" gap="100" style={{ marginLeft: config.space.S400 }}>
+                      {subRoomIds.map((subRoomId) => {
+                        const subRoom = mx.getRoom(subRoomId);
+                        const subRoomSummary = rooms.get(subRoomId);
+                        const subRoomPowerLevels = roomsPowerLevels.get(subRoomId) ?? {};
+                        const subRoomItem: HierarchyItemRoom = {
+                          roomId: subRoomId,
+                          parentId: roomItem.roomId,
+                          content: {},
+                        };
+                        const subRoomDragging =
+                          draggingItem?.roomId === subRoomId &&
+                          draggingItem.parentId === roomItem.roomId;
+
+                        // Skip sub-rooms the user hasn't joined yet if we have no summary
+                        if (!subRoom && !subRoomSummary) return null;
+
+                        return (
+                          <RoomItemCard
+                            key={subRoomId}
+                            item={subRoomItem}
+                            loading={fetching}
+                            error={error}
+                            summary={subRoomSummary}
+                            dm={mDirects.has(subRoomId)}
+                            onOpen={onOpenRoom}
+                            getRoom={getRoom}
+                            canReorder={false}
+                            options={
+                              <HierarchyItemMenu
+                                item={subRoomItem}
+                                powerLevels={subRoomPowerLevels}
+                                joined={allJoinedRooms.has(subRoomId)}
+                                canEditChild={false}
+                              />
+                            }
+                            data-dragging={subRoomDragging}
+                            onDragging={onDragging}
+                          />
+                        );
+                      })}
+                    </Box>
+                  )}
+                </React.Fragment>
               );
             })}
           </Box>

src/app/features/room-nav/RoomNavItem.tsx

@@ -60,6 +60,7 @@ import { CallType } from '../call/types';
 import { roomToParentsAtom } from '../../state/room/roomToParents';
 import { StateEvent } from '../../../types/matrix/room';
 import { mDirectAtom } from '../../state/mDirectList';
+import { useOpenCreateSubRoomModal } from '../../state/hooks/createRoomModal';
 
 type RoomNavItemMenuProps = {
   room: Room;
@@ -78,7 +79,9 @@ const RoomNavItemMenu = forwardRef<HTMLDivElement, RoomNavItemMenuProps>(
 
     const permissions = useRoomPermissions(creators, powerLevels);
     const canInvite = permissions.action('invite', mx.getSafeUserId());
+    const canAddSubRoom = permissions.stateEvent(StateEvent.PaarrotSubRooms, mx.getSafeUserId());
     const openRoomSettings = useOpenRoomSettings();
+    const openCreateSubRoom = useOpenCreateSubRoomModal();
     const space = useSpaceOptionally();
 
     const [invitePrompt, setInvitePrompt] = useState(false);
@@ -104,6 +107,11 @@ const RoomNavItemMenu = forwardRef<HTMLDivElement, RoomNavItemMenuProps>(
       requestClose();
     };
 
+    const handleAddSubRoom = () => {
+      openCreateSubRoom(room.roomId);
+      requestClose();
+    };
+
     const handleJoinVoice = async () => {
       try {
         // If already in a call, leave it first
@@ -211,6 +219,17 @@ const RoomNavItemMenu = forwardRef<HTMLDivElement, RoomNavItemMenuProps>(
               Room Settings
             </Text>
           </MenuItem>
+          <MenuItem
+            onClick={handleAddSubRoom}
+            size="300"
+            after={<Icon size="100" src={Icons.Plus} />}
+            radii="300"
+            disabled={!canAddSubRoom}
+          >
+            <Text style={{ flexGrow: 1 }} as="span" size="T300" truncate>
+              Add Sub-Room
+            </Text>
+          </MenuItem>
         </Box>
         <Line variant="Surface" size="300" />
         <Box direction="Column" gap="100" style={{ padding: config.space.S100 }}>

src/app/features/room-nav/UnjoinedSubRoomItem.tsx

@@ -0,0 +1,99 @@
+import React, { useCallback } from 'react';
+import {
+  Avatar,
+  Box,
+  Chip,
+  Icon,
+  Icons,
+  Spinner,
+  Text,
+  Tooltip,
+  TooltipProvider,
+  color,
+  config,
+  toRem,
+} from 'folds';
+import { MatrixError, Room } from 'matrix-js-sdk';
+import { useMatrixClient } from '../../hooks/useMatrixClient';
+import { AsyncStatus, useAsyncCallback } from '../../hooks/useAsyncCallback';
+
+/**
+ * Extract server name from a room ID (!abc:server.com => server.com)
+ */
+function getServerFromRoomId(roomId: string): string {
+  const parts = roomId.split(':');
+  return parts.length > 1 ? parts.slice(1).join(':') : '';
+}
+
+type UnjoinedSubRoomItemProps = {
+  roomId: string;
+  depth: number;
+};
+
+/**
+ * Displays an unjoined sub-room with a Join button.
+ * Used when a parent room has sub-rooms that the user hasn't joined yet.
+ */
+export function UnjoinedSubRoomItem({ roomId, depth }: UnjoinedSubRoomItemProps) {
+  const mx = useMatrixClient();
+  const viaServers = [getServerFromRoomId(roomId)].filter(Boolean);
+
+  const [joinState, join] = useAsyncCallback<Room, MatrixError, []>(
+    useCallback(() => mx.joinRoom(roomId, { viaServers }), [mx, roomId, viaServers])
+  );
+
+  const canJoin = joinState.status === AsyncStatus.Idle || joinState.status === AsyncStatus.Error;
+  const paddingLeft = depth > 0 ? `${depth * 24}px` : undefined;
+
+  return (
+    <Box
+      style={{ paddingLeft, padding: `${config.space.S100} ${config.space.S200}` }}
+      alignItems="Center"
+      gap="200"
+    >
+      <Avatar size="200" radii="400">
+        <Icon src={Icons.Hash} size="100" />
+      </Avatar>
+      <Box grow="Yes" alignItems="Center" gap="100">
+        <Text size="T300" truncate style={{ opacity: 0.7 }}>
+          {roomId}
+        </Text>
+      </Box>
+      <Box shrink="No" gap="100" alignItems="Center">
+        {joinState.status === AsyncStatus.Error && (
+          <TooltipProvider
+            tooltip={
+              <Tooltip variant="Critical" style={{ maxWidth: toRem(200) }}>
+                <Text style={{ wordBreak: 'break-word' }} size="T300">
+                  {joinState.error.data?.error || joinState.error.message}
+                </Text>
+              </Tooltip>
+            }
+          >
+            {(triggerRef) => (
+              <Icon
+                ref={triggerRef}
+                style={{ color: color.Critical.Main, cursor: 'pointer' }}
+                src={Icons.Warning}
+                size="200"
+              />
+            )}
+          </TooltipProvider>
+        )}
+        <Chip
+          variant="Secondary"
+          fill="Soft"
+          size="400"
+          radii="Pill"
+          before={
+            canJoin ? <Icon src={Icons.Plus} size="50" /> : <Spinner variant="Secondary" size="50" />
+          }
+          onClick={join}
+          disabled={!canJoin}
+        >
+          <Text size="B300">Join</Text>
+        </Chip>
+      </Box>
+    </Box>
+  );
+}

src/app/features/room-nav/index.ts

@@ -1,3 +1,4 @@
 export * from './RoomNavItem';
 export * from './RoomNavCategoryButton';
 export * from './CallParticipantsIndicator';
+export * from './UnjoinedSubRoomItem';

src/app/features/room-settings/RoomSettings.tsx

@@ -17,6 +17,7 @@ import { Permissions } from './permissions';
 import { RoomSettingsPage } from '../../state/roomSettings';
 import { useRoom } from '../../hooks/useRoom';
 import { DeveloperTools } from '../common-settings/developer-tools';
+import { SubRooms } from './sub-rooms';
 
 type RoomSettingsMenuItem = {
   page: RoomSettingsPage;
@@ -42,6 +43,11 @@ const useRoomSettingsMenuItems = (): RoomSettingsMenuItem[] =>
         name: 'Permissions',
         icon: Icons.Lock,
       },
+      {
+        page: RoomSettingsPage.SubRoomsPage,
+        name: 'Sub-Rooms',
+        icon: Icons.Hash,
+      },
       {
         page: RoomSettingsPage.EmojisStickersPage,
         name: 'Emojis & Stickers',
@@ -161,6 +167,9 @@ export function RoomSettings({ initialPage, requestClose }: RoomSettingsProps) {
       {activePage === RoomSettingsPage.PermissionsPage && (
         <Permissions requestClose={handlePageRequestClose} />
       )}
+      {activePage === RoomSettingsPage.SubRoomsPage && (
+        <SubRooms />
+      )}
       {activePage === RoomSettingsPage.EmojisStickersPage && (
         <EmojisStickers requestClose={handlePageRequestClose} />
       )}

src/app/features/room-settings/sub-rooms/SubRooms.tsx

@@ -0,0 +1,243 @@
+import React, { useState, useCallback } from 'react';
+import {
+  Box,
+  Button,
+  Text,
+  Icon,
+  Icons,
+  config,
+  IconButton,
+  Chip,
+  Line,
+} from 'folds';
+import { useAtomValue } from 'jotai';
+import { useMatrixClient } from '../../../hooks/useMatrixClient';
+import { useRoom } from '../../../hooks/useRoom';
+import { useRoomSubRooms, PaarrotSubRoomsContent } from '../../../hooks/useRoomSubRooms';
+import { useAllJoinedRoomsSet, useGetRoom } from '../../../hooks/useGetRoom';
+import { AsyncStatus, useAsyncCallback } from '../../../hooks/useAsyncCallback';
+import { SettingTile } from '../../../components/setting-tile';
+import { StateEvent } from '../../../../types/matrix/room';
+import { usePowerLevels } from '../../../hooks/usePowerLevels';
+import { useRoomCreators } from '../../../hooks/useRoomCreators';
+import { useRoomPermissions } from '../../../hooks/useRoomPermissions';
+import { roomToParentsAtom } from '../../../state/room/roomToParents';
+import { isSpace } from '../../../utils/room';
+
+function SubRoomItem({ roomId, onRemove, canEdit }: { roomId: string; onRemove: (roomId: string) => void; canEdit: boolean }) {
+  const allJoinedRooms = useAllJoinedRoomsSet();
+  const getRoom = useGetRoom(allJoinedRooms);
+  const room = getRoom(roomId);
+
+  if (!room) {
+    return (
+      <Chip variant="Secondary" radii="400">
+        <Text size="T300">Unknown Room ({roomId})</Text>
+        {canEdit && (
+          <IconButton
+            variant="SurfaceVariant"
+            size="300"
+            radii="Pill"
+            onClick={() => onRemove(roomId)}
+          >
+            <Icon size="50" src={Icons.Cross} />
+          </IconButton>
+        )}
+      </Chip>
+    );
+  }
+
+  return (
+    <Chip variant="Secondary" radii="400">
+      <Text size="T300">{room.name}</Text>
+      {canEdit && (
+        <IconButton
+          variant="SurfaceVariant"
+          size="300"
+          radii="Pill"
+          onClick={() => onRemove(roomId)}
+        >
+          <Icon size="50" src={Icons.Cross} />
+        </IconButton>
+      )}
+    </Chip>
+  );
+}
+
+export function SubRooms() {
+  const mx = useMatrixClient();
+  const room = useRoom();
+  const subRooms = useRoomSubRooms(room);
+  const [addRoomId, setAddRoomId] = useState('');
+  const roomToParents = useAtomValue(roomToParentsAtom);
+
+  const powerLevels = usePowerLevels(room);
+  const creators = useRoomCreators(room);
+  const permissions = useRoomPermissions(creators, powerLevels);
+  const canEdit = permissions.stateEvent(StateEvent.PaarrotSubRooms, mx.getSafeUserId());
+
+  const [updateState, updateSubRooms] = useAsyncCallback(
+    useCallback(
+      async (newSubRooms: string[]) => {
+        const content: PaarrotSubRoomsContent = {
+          children: newSubRooms,
+        };
+        // Custom state event type - need to cast
+        // eslint-disable-next-line @typescript-eslint/no-explicit-any
+        await mx.sendStateEvent(room.roomId, StateEvent.PaarrotSubRooms as any, content, '');
+      },
+      [mx, room]
+    )
+  );
+
+  const handleRemoveSubRoom = useCallback(
+    (roomId: string) => {
+      const newSubRooms = subRooms.filter(id => id !== roomId);
+      updateSubRooms(newSubRooms);
+    },
+    [subRooms, updateSubRooms]
+  );
+
+  const handleAddRoom = useCallback(async () => {
+    const trimmedId = addRoomId.trim();
+    if (!trimmedId) return;
+    
+    // Try to resolve room - if it's already joined, use the actual room ID
+    const targetRoom = mx.getRoom(trimmedId);
+    const roomIdToAdd = targetRoom ? targetRoom.roomId : trimmedId;
+    
+    // Allow adding any room ID (even if not joined)
+    if (!subRooms.includes(roomIdToAdd)) {
+      const newSubRooms = [...subRooms, roomIdToAdd];
+      await updateSubRooms(newSubRooms);
+
+      // Also add sub-room to this room's parent space(s) for joining/permissions
+      // The UI will hide it from the top-level display and only show it nested
+      const parentSpaces = roomToParents.get(room.roomId);
+      if (parentSpaces) {
+        const spaceIds = Array.from(parentSpaces);
+        await Promise.all(
+          spaceIds.map(async (parentSpaceId) => {
+            const parentSpaceRoom = mx.getRoom(parentSpaceId);
+            // Only add to actual spaces (not other sub-room parents)
+            if (parentSpaceRoom && isSpace(parentSpaceRoom)) {
+              try {
+                const spaceServerName = parentSpaceId.split(':')[1] || '';
+                const spaceViaServers = spaceServerName ? [spaceServerName] : [];
+                // Add as m.space.child to the space
+                await mx.sendStateEvent(
+                  parentSpaceId,
+                  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+                  StateEvent.SpaceChild as any,
+                  { via: spaceViaServers },
+                  roomIdToAdd
+                );
+              } catch (spaceError) {
+                // eslint-disable-next-line no-console
+                console.warn(`Failed to add sub-room to space ${parentSpaceId}:`, spaceError);
+              }
+            }
+          })
+        );
+      }
+
+      setAddRoomId('');
+    }
+  }, [mx, addRoomId, subRooms, updateSubRooms, room.roomId, roomToParents]);
+
+  return (
+    <Box direction="Column" gap="400">
+      <SettingTile>
+        <Box direction="Column" gap="200">
+          <Text size="H5" priority="400">
+            Sub-Rooms
+          </Text>
+          <Text size="T300" priority="300">
+            Sub-rooms are nested rooms that appear indented under this room in the room list.
+            They work like sub-spaces but for regular rooms.
+          </Text>
+        </Box>
+      </SettingTile>
+
+      <Line variant="Surface" size="300" />
+
+      <SettingTile>
+        <Box direction="Column" gap="300">
+          <Box justifyContent="SpaceBetween" alignItems="Center">
+            <Text size="T400" priority="400">
+              Current Sub-Rooms
+            </Text>
+          </Box>
+
+          <Box gap="200" alignItems="Center">
+            <input
+              type="text"
+              placeholder="Enter room ID (!abc:server.com) or alias (#room:server.com)"
+              value={addRoomId}
+              onChange={(e) => setAddRoomId(e.target.value)}
+              onKeyDown={(e) => {
+                if (e.key === 'Enter') {
+                  handleAddRoom();
+                }
+              }}
+              style={{
+                flexGrow: 1,
+                padding: `${config.space.S200} ${config.space.S300}`,
+                borderRadius: config.radii.R400,
+                border: '1px solid var(--border-surface)',
+                backgroundColor: 'var(--background-surface)',
+                color: 'var(--text-primary)',
+                fontSize: config.fontSize.T300,
+              }}
+            />
+            <Button
+              variant="Primary"
+              size="300"
+              radii="Pill"
+              onClick={handleAddRoom}
+              disabled={!canEdit || updateState.status === AsyncStatus.Loading || !addRoomId.trim()}
+              before={<Icon size="100" src={Icons.Plus} />}
+            >
+              <Text size="B300">Add</Text>
+            </Button>
+          </Box>
+
+          {!canEdit && (
+            <Text size="T300" priority="300" style={{ fontStyle: 'italic' }}>
+              You need power level 50 or higher to manage sub-rooms.
+            </Text>
+          )}
+
+          {subRooms.length === 0 ? (
+            <Box
+              direction="Column"
+              alignItems="Center"
+              justifyContent="Center"
+              gap="200"
+              style={{ padding: `${config.space.S400} 0` }}
+            >
+              <Icon size="400" src={Icons.Hash} style={{ opacity: config.opacity.P300 }} />
+              <Text size="T300" priority="300" align="Center">
+                No sub-rooms added yet.
+                <br />
+                Add a room to nest it under this room.
+              </Text>
+            </Box>
+          ) : (
+            <Box wrap="Wrap" gap="200">
+              {subRooms.map((roomId) => (
+                <SubRoomItem key={roomId} roomId={roomId} onRemove={handleRemoveSubRoom} canEdit={canEdit} />
+              ))}
+            </Box>
+          )}
+
+          {updateState.status === AsyncStatus.Error && (
+            <Text size="T300" priority="400" style={{ color: 'var(--color-critical)' }}>
+              Failed to update sub-rooms: {String((updateState.error as Error)?.message || updateState.error)}
+            </Text>
+          )}
+        </Box>
+      </SettingTile>
+    </Box>
+  );
+}

src/app/features/room-settings/sub-rooms/index.ts

@@ -0,0 +1 @@
+export * from './SubRooms';

src/app/features/settings/account/Profile.tsx

@@ -494,9 +494,16 @@ function ProfileBanner({ avatarRef, nameRef }: { avatarRef: React.RefObject<HTML
         if (!httpUrl) throw new Error('Could not resolve uploaded avatar URL');
 
         const accessToken = mx.getAccessToken();
-        const response = await fetch(httpUrl, {
+        let response = await fetch(httpUrl, {
           headers: accessToken && useAuthentication ? { Authorization: `Bearer ${accessToken}` } : undefined,
         });
+        
+        // If we got a 401 and we tried with auth, fallback to unauthenticated request
+        if (!response.ok && response.status === 401 && accessToken && useAuthentication) {
+          console.warn('[Profile] Auth failed (401), attempting unauthenticated fallback for avatar fetch');
+          response = await fetch(httpUrl);
+        }
+        
         if (!response.ok) throw new Error('Failed to fetch uploaded avatar');
         const avatarData = await response.arrayBuffer();