diff --git a/apps/README.md b/apps/README.md
index 259e2b8..d27d648 100644
--- a/apps/README.md
+++ b/apps/README.md
@@ -4,7 +4,7 @@
 |---|---|---|
 | [`ui/`](ui/README.md) | shared Vue 3 + Tailwind design system: tokens, fonts, icons, components | in use by admin and web |
 | [`admin/`](admin/README.md) | admin web panel, served by its own optional `warp-admin` binary | done for the current admin API |
-| [`web/`](web/README.md) | web client (Vue SPA on matrix-js-sdk), served by its own optional `warp-web` binary | rooms, timeline, sending text and files, invites; E2EE next |
+| [`web/`](web/README.md) | web client (Vue SPA on matrix-js-sdk), served by its own optional `warp-web` binary | rooms, timeline, files, replies, reactions, threads, spaces, E2EE with QR and emoji verification, calls |
 | `desktop/` | Wails v3 shell reusing the web frontend | next: web has a timeline |
 | [`mobile/`](mobile/README.md) | Flutter app on the famedly Matrix SDK, Android first | rooms, timeline, files, replies, reactions, threads, spaces, E2EE with QR and emoji verification, push, calls, sliding sync |
 
diff --git a/apps/ui/src/icons/index.ts b/apps/ui/src/icons/index.ts
index 7a108df..b929a1f 100644
--- a/apps/ui/src/icons/index.ts
+++ b/apps/ui/src/icons/index.ts
@@ -45,7 +45,9 @@ export type IconName =
   | 'settings'
   | 'shield-alert'
   | 'shield-check'
+  | 'space'
   | 'sun'
+  | 'thread'
   | 'trash'
   | 'user'
   | 'users'
diff --git a/apps/ui/src/icons/space.svg b/apps/ui/src/icons/space.svg
new file mode 100644
index 0000000..b30c951
--- /dev/null
+++ b/apps/ui/src/icons/space.svg
@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.75" stroke-linecap="round" stroke-linejoin="round"><title>space</title><rect x="4" y="4" width="7" height="7" rx="2"/><rect x="13" y="4" width="7" height="7" rx="2"/><rect x="4" y="13" width="7" height="7" rx="2"/><rect x="13" y="13" width="7" height="7" rx="2"/></svg>
\ No newline at end of file
diff --git a/apps/ui/src/icons/thread.svg b/apps/ui/src/icons/thread.svg
new file mode 100644
index 0000000..1463971
--- /dev/null
+++ b/apps/ui/src/icons/thread.svg
@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.75" stroke-linecap="round" stroke-linejoin="round"><title>thread</title><path d="M4.5 5.5h11a2 2 0 0 1 2 2v5a2 2 0 0 1-2 2H10l-3.5 3v-3h-2a2 2 0 0 1-2-2v-5a2 2 0 0 1 2-2z"/><path d="M20.5 9.5v6a2 2 0 0 1-2 2h-1v2.5l-3-2.5"/><path d="M6.5 9h7"/><path d="M6.5 11.5h4"/></svg>
\ No newline at end of file
diff --git a/apps/web/README.md b/apps/web/README.md
index b81b1d1..77f166a 100644
--- a/apps/web/README.md
+++ b/apps/web/README.md
@@ -46,6 +46,29 @@ that one origin.
   then *Read* when someone else has read it; a failed one says *Not sent*
   with Retry and Delete.
 - Read receipts, typing notifications, deleting messages.
+- Replies: *Reply* on a message puts it above the composer, and the
+  reply quotes it (sender and first line); the quote goes to the message.
+  Replies are sent without the old quoted fallback in their body
+  (Matrix 1.13), and the fallback in older ones is not shown.
+- Reactions: six quick ones, or any other emoji typed in, from a
+  message's *React* button; chips under the message with counts, yours
+  highlighted. A chip adds your reaction or takes it back, and says who
+  reacted.
+- Threads: *Reply in thread* opens the thread beside the room, its root
+  and replies, with a composer that writes into it. Replies stay out of
+  the room's timeline; the root shows "N replies", the last reply's
+  sender and time. *Threads* in the room header lists the room's threads
+  (`/threads`). Read receipts are per thread (MSC3771): the room is
+  marked read with `thread_id` "main", a thread with its root's ID, so
+  reading one leaves the others unread. Unread counts come per thread
+  from the homeserver (MSC3773) and threads with replies you haven't
+  read have a dot, on the root, in the list and on the *Threads* button.
+- Spaces: when you're in any, the top of the room list picks *All chats*
+  or one space, which narrows the list to its rooms and its subspaces'.
+  A space's page (`/hierarchy`) lists its rooms and subspaces with *Open*
+  or *Join*; its admins add rooms you're in, create new ones in it, and
+  take rooms out. *New space* in the + menu and in the picker. Invites to
+  spaces show with the others.
 - End-to-end encryption, with the Rust crypto stack matrix-js-sdk loads
   (`matrix-sdk-crypto-wasm`): reading and sending in encrypted rooms,
   files included (encrypted in the browser before they are uploaded, with
@@ -90,7 +113,7 @@ that one origin.
 - Settings: display name, avatar, theme, encryption.
 - Phone layout: one pane at a time, the room list is the home screen.
 
-**Not yet:** replies, reactions, threads, spaces, notifications.
+**Not yet:** editing messages, notifications.
 
 ### Encryption keys in the browser
 
diff --git a/apps/web/src/components/CreateRoomDialog.vue b/apps/web/src/components/CreateRoomDialog.vue
index 74a8a66..adcae32 100644
--- a/apps/web/src/components/CreateRoomDialog.vue
+++ b/apps/web/src/components/CreateRoomDialog.vue
@@ -4,8 +4,16 @@ import { WBanner, WButton, WDialog, WIcon, WSwitch, WTextArea, WTextField } from
 import { errorMessage } from '@/lib/format'
 import { useChat } from '@/stores/chat'
 
+// A room, a room in a space (`spaceId`), or a space (`kind` "space").
+const props = withDefaults(defineProps<{ kind?: 'room' | 'space'; spaceId?: string; spaceName?: string }>(), {
+  kind: 'room',
+  spaceId: undefined,
+  spaceName: undefined,
+})
 const open = defineModel<boolean>('open', { default: false })
 const emit = defineEmits<{ created: [roomId: string] }>()
+const isSpace = computed(() => props.kind === 'space')
+const noun = computed(() => (isSpace.value ? 'space' : 'room'))
 
 const chat = useChat()
 const name = ref('')
@@ -22,7 +30,8 @@ watch(open, (v) => {
   topic.value = ''
   isPublic.value = false
   alias.value = ''
-  encrypted.value = chat.canEncrypt
+  // A space holds no conversation to protect: never encrypted.
+  encrypted.value = chat.canEncrypt && !isSpace.value
   error.value = ''
 })
 
@@ -40,7 +49,7 @@ watch(name, (n, old) => {
 })
 
 const aliasError = computed(() =>
-  isPublic.value && alias.value && !/^[a-z0-9._=-]+$/.test(alias.value) ? 'Use lowercase letters, digits and . _ = -' : undefined,
+  isPublic.value && !isSpace.value && alias.value && !/^[a-z0-9._=-]+$/.test(alias.value) ? 'Use lowercase letters, digits and . _ = -' : undefined,
 )
 
 async function submit() {
@@ -48,13 +57,16 @@ async function submit() {
   busy.value = true
   error.value = ''
   try {
-    const roomId = await chat.createRoom({
-      name: name.value,
-      topic: topic.value,
-      isPublic: isPublic.value,
-      alias: alias.value,
-      encrypted: encrypted.value,
-    })
+    const roomId = isSpace.value
+      ? await chat.createSpace({ name: name.value, topic: topic.value, isPublic: isPublic.value })
+      : await chat.createRoom({
+          name: name.value,
+          topic: topic.value,
+          isPublic: isPublic.value,
+          alias: alias.value,
+          encrypted: encrypted.value,
+          spaceId: props.spaceId,
+        })
     open.value = false
     emit('created', roomId)
   } catch (e) {
@@ -66,19 +78,22 @@ async function submit() {
 </script>
 
 <template>
-  <WDialog v-model:open="open" title="Create room" :persistent="busy">
+  <WDialog v-model:open="open" :title="isSpace ? 'Create space' : spaceName ? `New room in ${spaceName}` : 'Create room'" :persistent="busy">
     <form id="create-room" class="flex flex-col gap-4" @submit.prevent="submit">
       <WTextField v-model="name" wide label="Name" maxlength="255" required autofocus />
       <WTextArea v-model="topic" label="Topic (optional)" rows="2" />
       <div class="flex items-start gap-3">
         <div class="flex-1">
-          <p class="body-strong m-0">Public room</p>
-          <p class="small m-0 text-ink-muted">Anyone can find it in the room directory and join. Otherwise people join by invite.</p>
+          <p class="body-strong m-0">Public {{ noun }}</p>
+          <p class="small m-0 text-ink-muted">
+            <template v-if="isSpace">Anyone can find it in the room directory, join it and see its public rooms. Otherwise people join by invite.</template>
+            <template v-else>Anyone can find it in the room directory and join. Otherwise people join by invite.</template>
+          </p>
         </div>
-        <WSwitch v-model="isPublic" label="Public room" />
+        <WSwitch v-model="isPublic" :label="`Public ${noun}`" />
       </div>
       <WTextField
-        v-if="isPublic"
+        v-if="isPublic && !isSpace"
         v-model="alias"
         wide
         mono
@@ -88,7 +103,7 @@ async function submit() {
         autocomplete="off"
         spellcheck="false"
       />
-      <div class="flex items-start gap-3">
+      <div v-if="!isSpace" class="flex items-start gap-3">
         <div class="flex-1">
           <p class="body-strong m-0 flex items-center gap-1">
             <WIcon :name="encrypted ? 'lock' : 'shield-alert'" :size="16" :class="encrypted ? 'text-teal-text' : 'text-ink-muted'" />
@@ -111,7 +126,7 @@ async function submit() {
     <template #actions>
       <WButton variant="ghost" :disabled="busy" @click="open = false">Cancel</WButton>
       <WButton type="submit" form="create-room" variant="primary" :loading="busy" :disabled="!name.trim() || !!aliasError">
-        Create room
+        Create {{ noun }}
       </WButton>
     </template>
   </WDialog>
diff --git a/apps/web/src/components/JoinRoomDialog.vue b/apps/web/src/components/JoinRoomDialog.vue
index 1056ec5..2147a54 100644
--- a/apps/web/src/components/JoinRoomDialog.vue
+++ b/apps/web/src/components/JoinRoomDialog.vue
@@ -20,7 +20,7 @@ const directory = ref<IPublicRoomsChunkRoom[]>()
 const directoryError = ref('')
 const search = ref('')
 
-const joined = computed(() => new Set(chat.rooms.map((r) => r.roomId)))
+const joined = computed(() => new Set([...chat.rooms, ...chat.spaces].map((r) => r.roomId)))
 
 async function loadDirectory() {
   directoryError.value = ''
diff --git a/apps/web/src/components/ReactionPicker.vue b/apps/web/src/components/ReactionPicker.vue
new file mode 100644
index 0000000..11123fd
--- /dev/null
+++ b/apps/web/src/components/ReactionPicker.vue
@@ -0,0 +1,91 @@
+<script setup lang="ts">
+import { nextTick, onBeforeUnmount, ref, useId, watch } from 'vue'
+import { WIconButton } from '@warp/ui'
+
+// Reacting to a message: the six quick reactions of the Android app, or
+// any other typed (or picked with the system's emoji keyboard) into the
+// field. Closes on a choice, Escape or a click outside.
+
+const props = withDefaults(defineProps<{ align?: 'left' | 'right' }>(), { align: 'left' })
+const emit = defineEmits<{ pick: [key: string] }>()
+
+const quickReactions = ['👍', '❤️', '😂', '😮', '😢', '🙏']
+
+const id = useId()
+const open = ref(false)
+const other = ref('')
+const root = ref<HTMLElement>()
+const panel = ref<HTMLElement>()
+
+function onDocClick(e: MouseEvent) {
+  if (root.value && !root.value.contains(e.target as Node)) open.value = false
+}
+
+watch(open, async (v) => {
+  if (v) {
+    other.value = ''
+    document.addEventListener('mousedown', onDocClick)
+    await nextTick()
+    panel.value?.querySelector<HTMLButtonElement>('button')?.focus()
+  } else {
+    document.removeEventListener('mousedown', onDocClick)
+  }
+})
+onBeforeUnmount(() => document.removeEventListener('mousedown', onDocClick))
+
+function pick(key: string) {
+  const k = key.trim()
+  if (!k) return
+  open.value = false
+  emit('pick', k)
+}
+
+function onKey(e: KeyboardEvent) {
+  if (e.key === 'Escape' && open.value) {
+    e.stopPropagation()
+    open.value = false
+    root.value?.querySelector<HTMLButtonElement>('button')?.focus()
+  }
+}
+</script>
+
+<template>
+  <div ref="root" class="relative" @keydown="onKey">
+    <WIconButton icon="react" label="React" :aria-expanded="open" aria-haspopup="dialog" @click="open = !open" />
+    <div
+      v-if="open"
+      ref="panel"
+      role="dialog"
+      aria-label="React to the message"
+      :class="[
+        'absolute bottom-full z-30 mb-1 flex flex-col gap-2 rounded-md border border-line bg-surface-raised p-2 shadow-float',
+        props.align === 'right' ? 'right-0' : 'left-0',
+      ]"
+    >
+      <div class="flex gap-1">
+        <button
+          v-for="key in quickReactions"
+          :key="key"
+          type="button"
+          class="flex h-10 w-10 items-center justify-center rounded-sm text-[22px] hover:bg-surface-sunken focus-visible:bg-surface-sunken"
+          :aria-label="`React with ${key}`"
+          @click="pick(key)"
+        >
+          {{ key }}
+        </button>
+      </div>
+      <form class="flex gap-1" @submit.prevent="pick(other)">
+        <label class="wp-sr" :for="id">Another reaction</label>
+        <input
+          :id="id"
+          v-model="other"
+          maxlength="64"
+          placeholder="Another emoji"
+          autocomplete="off"
+          class="h-9 min-w-0 flex-1 rounded-sm border border-line-strong bg-surface px-2 text-ink outline-none focus:border-teal-strong"
+        />
+        <button type="submit" class="h-9 rounded-sm px-3 font-semibold text-teal-text hover:bg-surface-sunken" :disabled="!other.trim()">React</button>
+      </form>
+    </div>
+  </div>
+</template>
diff --git a/apps/web/src/components/RoomComposer.vue b/apps/web/src/components/RoomComposer.vue
new file mode 100644
index 0000000..7c30565
--- /dev/null
+++ b/apps/web/src/components/RoomComposer.vue
@@ -0,0 +1,156 @@
+<script setup lang="ts">
+import type { Room } from 'matrix-js-sdk'
+import { computed, nextTick, onBeforeUnmount, ref, watch } from 'vue'
+import { WBanner, WComposer, WIcon, WIconButton, WSpinner } from '@warp/ui'
+import { drafts } from '@/lib/drafts'
+import { errorMessage, formatBytes } from '@/lib/format'
+import { client, cryptoError } from '@/matrix/client'
+import { sendFile, sendText } from '@/matrix/send'
+import type { MessageItem } from '@/matrix/timeline'
+import { useChat } from '@/stores/chat'
+import { useToasts } from '@/stores/toasts'
+
+// Writing to a room, or to one of its threads: the text field with its
+// draft, typing notifications, uploads, and the message being replied to
+// above it. The room view and the thread pane each have one.
+
+const props = defineProps<{ room: Room; threadId?: string; placeholder: string }>()
+const replyTo = defineModel<MessageItem | null>('replyTo', { default: null })
+
+const chat = useChat()
+const toasts = useToasts()
+const me = client().getUserId() ?? ''
+const composer = ref<InstanceType<typeof WComposer>>()
+
+const encrypted = computed(() => {
+  void chat.version
+  return props.room.hasEncryptionStateEvent()
+})
+// An encrypted room can't be written to without the crypto stack.
+const cryptoMissing = computed(() => encrypted.value && !chat.canEncrypt)
+const canSend = computed(() => {
+  void chat.version
+  return props.room.currentState.maySendMessage(me)
+})
+/** Whether files dropped on the room can be sent. */
+const canWrite = computed(() => canSend.value && !cryptoMissing.value)
+
+const draftKey = computed(() => (props.threadId ? `${props.room.roomId}/${props.threadId}` : props.room.roomId))
+const text = ref(drafts.get(draftKey.value) ?? '')
+watch(text, (v) => (v ? drafts.set(draftKey.value, v) : drafts.delete(draftKey.value)))
+
+// Typing notifications: on while there is text, renewed every few seconds,
+// off when it's sent or cleared. Matrix has them per room, threads or not.
+let typingUntil = 0
+function setTyping(on: boolean) {
+  const now = Date.now()
+  if (on && now < typingUntil) return
+  if (!on && typingUntil === 0) return
+  typingUntil = on ? now + 4000 : 0
+  client()
+    .sendTyping(props.room.roomId, on, on ? 6000 : 0)
+    .catch(() => undefined)
+}
+watch(text, (v) => setTyping(Boolean(v.trim())))
+onBeforeUnmount(() => setTyping(false))
+
+// Choosing a message to reply to puts the cursor in the field.
+watch(replyTo, (v) => v && nextTick(() => composer.value?.focus()))
+
+function target() {
+  return { threadId: props.threadId, replyTo: replyTo.value?.eventId }
+}
+
+function send(body: string) {
+  setTyping(false)
+  const to = target()
+  replyTo.value = null
+  sendText(client(), props.room.roomId, body, to).catch((e) => {
+    // A failed send stays in the timeline with Retry; say why once.
+    toasts.error(errorMessage(e))
+  })
+}
+
+// Uploads.
+const uploads = ref<{ id: number; name: string; loaded: number; total: number }[]>([])
+let uploadId = 0
+let maxUpload: number | null | undefined
+
+async function sendFiles(list: FileList | File[]) {
+  // Copied first: the composer clears its file input (and so the live
+  // FileList) as soon as it has handed the files over.
+  const files = Array.from(list)
+  if (!canWrite.value) return
+  if (maxUpload === undefined) {
+    maxUpload = await client()
+      .getMediaConfig(true)
+      .then((c) => c['m.upload.size'] ?? null)
+      .catch(() => null)
+  }
+  const to = target()
+  replyTo.value = null
+  for (const file of files) {
+    if (maxUpload && file.size > maxUpload) {
+      toasts.error(`${file.name} is ${formatBytes(file.size)}; this homeserver takes files up to ${formatBytes(maxUpload)}.`)
+      continue
+    }
+    const id = ++uploadId
+    uploads.value.push({ id, name: file.name, loaded: 0, total: file.size })
+    sendFile(
+      client(),
+      props.room,
+      file,
+      (p) => {
+        const u = uploads.value.find((x) => x.id === id)
+        if (u) Object.assign(u, { loaded: p.loaded, total: p.total })
+      },
+      to,
+    )
+      .catch((e) => toasts.error(`Can’t send ${file.name}: ${errorMessage(e)}`))
+      .finally(() => (uploads.value = uploads.value.filter((x) => x.id !== id)))
+  }
+}
+
+function onPaste(e: ClipboardEvent) {
+  const files = e.clipboardData?.files
+  if (files && files.length) {
+    e.preventDefault()
+    sendFiles(files)
+  }
+}
+
+function onKey(e: KeyboardEvent) {
+  if (e.key === 'Escape' && replyTo.value) {
+    e.stopPropagation()
+    replyTo.value = null
+  }
+}
+
+defineExpose({ sendFiles, canWrite, focus: () => composer.value?.focus() })
+</script>
+
+<template>
+  <div class="flex flex-col gap-2">
+    <div v-for="u in uploads" :key="u.id" class="flex items-center gap-3 rounded-md bg-surface-raised px-3 py-2">
+      <WSpinner :size="16" />
+      <span class="small min-w-0 flex-1 truncate">Uploading {{ u.name }}</span>
+      <span class="caption text-ink-muted tabular-nums">{{ u.total ? Math.round((u.loaded / u.total) * 100) : 0 }}%</span>
+    </div>
+
+    <WBanner v-if="cryptoMissing" tone="warning" title="End-to-end encryption didn’t start">
+      This browser can’t read or send messages in this room. Reload the page to try again{{ cryptoError() ? ` (${cryptoError()})` : '' }}.
+    </WBanner>
+    <WBanner v-else-if="!canSend" tone="info">You don’t have permission to post in this room.</WBanner>
+    <div v-else class="flex flex-col gap-1" @paste="onPaste" @keydown="onKey">
+      <div v-if="replyTo" class="flex items-center gap-2 rounded-md border-l-2 border-teal-strong bg-surface-raised py-1 pr-1 pl-3">
+        <WIcon name="reply" :size="16" class="shrink-0 text-ink-muted" />
+        <p class="small m-0 min-w-0 flex-1 truncate">
+          Replying to <span class="font-semibold">{{ replyTo.own ? 'yourself' : replyTo.senderName }}</span>
+          <span class="text-ink-muted"> · {{ replyTo.media ? replyTo.media.filename : replyTo.body }}</span>
+        </p>
+        <WIconButton icon="close" label="Cancel reply" @click="replyTo = null" />
+      </div>
+      <WComposer ref="composer" v-model="text" :placeholder="placeholder" :encrypted="encrypted" @send="send" @attach="sendFiles" />
+    </div>
+  </div>
+</template>
diff --git a/apps/web/src/components/RoomInfoPane.vue b/apps/web/src/components/RoomInfoPane.vue
index 2dcdcce..666e0d4 100644
--- a/apps/web/src/components/RoomInfoPane.vue
+++ b/apps/web/src/components/RoomInfoPane.vue
@@ -35,6 +35,14 @@ const alias = computed(() => {
   void chat.version
   return props.room.getCanonicalAlias()
 })
+// "room", or what else it is: a direct chat or a space.
+const noun = computed(() => {
+  void chat.version
+  if (props.isDirect) return 'chat'
+  return props.room.isSpaceRoom() ? 'space' : 'room'
+})
+const Noun = computed(() => noun.value[0]!.toUpperCase() + noun.value.slice(1))
+
 const encrypted = computed(() => {
   void chat.version
   return props.room.hasEncryptionStateEvent()
@@ -149,8 +157,8 @@ function copy(text: string) {
     aria-label="Room info"
   >
     <div class="flex h-16 shrink-0 items-center gap-2 border-b border-line px-4">
-      <h2 class="subheading m-0 flex-1">{{ isDirect ? 'Chat info' : 'Room info' }}</h2>
-      <WIconButton icon="close" label="Close room info" @click="emit('close')" />
+      <h2 class="subheading m-0 flex-1">{{ Noun }} info</h2>
+      <WIconButton icon="close" :label="`Close ${noun} info`" @click="emit('close')" />
     </div>
 
     <div class="flex-1 overflow-y-auto">
@@ -161,7 +169,7 @@ function copy(text: string) {
           {{ alias }}
         </button>
         <p v-if="topic" class="body m-0 whitespace-pre-wrap text-ink-muted">{{ topic }}</p>
-        <p class="caption m-0 flex items-center gap-1" :class="encrypted ? 'text-teal-text' : 'text-ink-muted'">
+        <p v-if="noun !== 'space'" class="caption m-0 flex items-center gap-1" :class="encrypted ? 'text-teal-text' : 'text-ink-muted'">
           <WIcon :name="encrypted ? 'lock' : 'shield-alert'" :size="14" />
           {{ encrypted ? 'End-to-end encrypted' : 'Not encrypted' }}
         </p>
@@ -207,11 +215,11 @@ function copy(text: string) {
         <button type="button" class="caption flex items-center gap-1 text-left text-ink-muted hover:text-ink" title="Copy room ID" @click="copy(room.roomId)">
           <WIcon name="copy" :size="14" /> <span class="mxid break-all">{{ room.roomId }}</span>
         </button>
-        <WButton variant="danger" icon="logout" @click="leaveOpen = true">{{ isDirect ? 'Leave chat' : 'Leave room' }}</WButton>
+        <WButton variant="danger" icon="logout" @click="leaveOpen = true">Leave {{ noun }}</WButton>
       </section>
     </div>
 
-    <WDialog v-model:open="editing" title="Edit room" :persistent="saving">
+    <WDialog v-model:open="editing" :title="`Edit ${noun}`" :persistent="saving">
       <form id="edit-room" class="flex flex-col gap-4" @submit.prevent="save">
         <WTextField v-if="canEditName" v-model="draftName" wide label="Name" maxlength="255" />
         <WTextArea v-if="canEditTopic" v-model="draftTopic" label="Topic" rows="3" />
@@ -223,7 +231,7 @@ function copy(text: string) {
       </template>
     </WDialog>
 
-    <WDialog v-model:open="inviteOpen" title="Invite to room" :persistent="inviting">
+    <WDialog v-model:open="inviteOpen" :title="`Invite to ${noun}`" :persistent="inviting">
       <form id="invite-form" class="flex flex-col gap-4" @submit.prevent="invite">
         <WTextField
           v-model="inviteId"
@@ -252,14 +260,15 @@ function copy(text: string) {
 
     <ConfirmDialog
       v-model:open="leaveOpen"
-      :title="isDirect ? 'Leave chat' : 'Leave room'"
-      :confirm-label="isDirect ? 'Leave chat' : 'Leave room'"
+      :title="`Leave ${noun}`"
+      :confirm-label="`Leave ${noun}`"
       :busy="leaving"
       :error="leaveError"
       @confirm="leave"
     >
       <p>
-        You stop getting messages from <span class="body-strong">{{ name }}</span>.
+        <template v-if="noun === 'space'">You leave <span class="body-strong">{{ name }}</span>, not the rooms in it.</template>
+        <template v-else>You stop getting messages from <span class="body-strong">{{ name }}</span>.</template>
         <template v-if="room.getJoinRule() !== 'public'"> To come back, someone has to invite you again.</template>
       </p>
     </ConfirmDialog>
diff --git a/apps/web/src/components/RoomTimeline.vue b/apps/web/src/components/RoomTimeline.vue
index 19c32a3..713f09f 100644
--- a/apps/web/src/components/RoomTimeline.vue
+++ b/apps/web/src/components/RoomTimeline.vue
@@ -1,26 +1,30 @@
 <script setup lang="ts">
-import type { Room } from 'matrix-js-sdk'
+import { EventType, type Room } from 'matrix-js-sdk'
 import { computed, onBeforeUnmount, onMounted, ref, watch } from 'vue'
-import { WButton, WIcon, WMessageBubble, WSpinner } from '@warp/ui'
+import { WButton, WIcon, WIconButton, WMessageBubble, WSpinner } from '@warp/ui'
 import ConfirmDialog from '@/components/ConfirmDialog.vue'
 import ImageViewer from '@/components/ImageViewer.vue'
 import MessageBody from '@/components/MessageBody.vue'
 import MxcAvatar from '@/components/MxcAvatar.vue'
 import PopMenu from '@/components/PopMenu.vue'
+import ReactionPicker from '@/components/ReactionPicker.vue'
 import type { MenuItem } from '@/components/menu'
-import { errorMessage, formatDateTime, formatDay, formatTime } from '@/lib/format'
+import { errorMessage, formatDateTime, formatDay, formatListTime, formatTime } from '@/lib/format'
 import { client } from '@/matrix/client'
 import type { MediaSource } from '@/matrix/media'
-import { buildTimeline, canPaginateBack, lastSyncedEvent, type MessageItem } from '@/matrix/timeline'
+import { sendReaction } from '@/matrix/send'
+import { buildTimeline, canPaginateBack, inThread, lastSyncedEvent, type MessageItem, type ReactionChip } from '@/matrix/timeline'
 import { useChat } from '@/stores/chat'
 import { useToasts } from '@/stores/toasts'
 
 // The scrolling list of a room's messages. The scroller is laid out bottom
 // up (column-reverse), so it starts at the newest message, stays there as
 // messages arrive, and older history loading above doesn't move what you
-// are reading.
+// are reading. Given a thread's root ID it shows that thread instead: the
+// root, then its replies.
 
-const props = defineProps<{ room: Room; isDirect: boolean }>()
+const props = defineProps<{ room: Room; isDirect: boolean; threadId?: string }>()
+const emit = defineEmits<{ reply: [item: MessageItem]; openThread: [eventId: string] }>()
 
 const chat = useChat()
 const toasts = useToasts()
@@ -37,11 +41,18 @@ const deleteError = ref('')
 
 const items = computed(() => {
   void chat.version
-  return buildTimeline(client(), props.room)
+  return buildTimeline(client(), props.room, props.threadId)
 })
 const hasMore = computed(() => {
   void chat.version
-  return canPaginateBack(props.room)
+  return canPaginateBack(props.room, props.threadId)
+})
+// A thread's first replies come from the homeserver when the SDK first
+// sees it; until then there is only the root.
+const threadLoading = computed(() => {
+  void chat.version
+  const thread = props.threadId ? props.room.getThread(props.threadId) : null
+  return Boolean(thread && !thread.initialEventsFetched)
 })
 
 // Distance from the oldest loaded message, given column-reverse's
@@ -55,7 +66,9 @@ async function paginate() {
   paginating.value = true
   paginateError.value = false
   try {
-    await client().scrollback(props.room, 30)
+    const thread = props.threadId ? props.room.getThread(props.threadId) : null
+    if (thread) await client().paginateEventTimeline(thread.liveTimeline, { backwards: true, limit: 30 })
+    else if (!props.threadId) await client().scrollback(props.room, 30)
   } catch {
     paginateError.value = true
   } finally {
@@ -65,7 +78,7 @@ async function paginate() {
   // Keep going until the view is full or history runs out.
   requestAnimationFrame(() => {
     const el = scroller.value
-    if (el && !paginateError.value && distanceFromTop(el) < 400 && canPaginateBack(props.room)) paginate()
+    if (el && !paginateError.value && distanceFromTop(el) < 400 && canPaginateBack(props.room, props.threadId)) paginate()
   })
 }
 
@@ -82,21 +95,28 @@ function jumpToLatest() {
 }
 
 // Read receipts: sent when you are looking at the newest message, with the
-// tab in front.
+// tab in front. With threads on, the SDK gives each receipt the thread it
+// belongs to (MSC3771): "main" for the room, the root's ID in a thread. A
+// thread has no read marker of its own, only the receipt.
 let lastMarked = ''
 function markRead() {
   if (!atBottom.value || document.visibilityState !== 'visible' || !document.hasFocus()) return
-  const ev = lastSyncedEvent(props.room)
+  const ev = lastSyncedEvent(props.room, props.threadId)
   const id = ev?.getId()
   if (!ev || !id || id === lastMarked) return
-  if (ev.getSender() === me && props.room.getUnreadNotificationCount() === 0) {
+  const unread = props.threadId
+    ? props.room.getThreadUnreadNotificationCount(props.threadId)
+    : props.room.getRoomUnreadNotificationCount()
+  if (ev.getSender() === me && unread === 0) {
     lastMarked = id
     return
   }
   lastMarked = id
-  client()
-    .setRoomReadMarkers(props.room.roomId, id, ev)
-    .catch(() => (lastMarked = ''))
+  // The receipt goes on its own: one sent with the read marker (POST
+  // /read_markers) has no thread_id, so it would read every thread too.
+  const c = client()
+  const sent = props.threadId ? c.sendReadReceipt(ev) : Promise.all([c.setRoomReadMarkers(props.room.roomId, id), c.sendReadReceipt(ev)])
+  sent.catch(() => (lastMarked = ''))
 }
 
 watch(items, () => requestAnimationFrame(markRead))
@@ -139,8 +159,29 @@ const redactLevel = computed(() => {
   return typeof pl.redact === 'number' ? pl.redact : 50
 })
 
+const canSend = computed(() => {
+  void chat.version
+  return props.room.currentState.maySendMessage(me)
+})
+const canReact = computed(() => {
+  void chat.version
+  return props.room.currentState.maySendEvent(EventType.Reaction, me)
+})
+
+/** Whether a message can be answered or reacted to: it is on the server, not deleted. */
+function answerable(item: MessageItem): boolean {
+  return Boolean(item.eventId && !item.eventId.startsWith('~')) && !item.redacted && !item.status?.match(/sending|failed/)
+}
+
+/** Starting a thread on a message: not from inside one, nor on a reply that is in one. */
+function canThread(item: MessageItem): boolean {
+  return !props.threadId && canSend.value && answerable(item) && !inThread(item.event)
+}
+
 function actions(item: MessageItem): MenuItem[] {
   const out: MenuItem[] = []
+  if (canSend.value && answerable(item)) out.push({ label: 'Reply', icon: 'reply', action: () => emit('reply', item) })
+  if (canThread(item)) out.push({ label: 'Reply in thread', icon: 'thread', action: () => emit('openThread', item.eventId!) })
   if (item.body && !item.media && !item.encrypted) {
     out.push({
       label: 'Copy text',
@@ -154,6 +195,50 @@ function actions(item: MessageItem): MenuItem[] {
   return out
 }
 
+// Reactions: a chip adds yours, or takes it back.
+async function react(item: MessageItem, key: string) {
+  const mine = item.reactions.find((r) => r.key === key)?.mine
+  try {
+    if (mine) {
+      // Yours, still on its way: nothing to take back yet.
+      if (mine.status || !mine.getId()) return
+      await client().redactEvent(props.room.roomId, mine.getId()!)
+    } else {
+      await sendReaction(client(), item.event, key)
+    }
+  } catch (e) {
+    toasts.error(errorMessage(e))
+  }
+}
+
+function chipLabel(chip: ReactionChip): string {
+  const names = chip.names.length > 8 ? [...chip.names.slice(0, 8), `${chip.names.length - 8} more`] : chip.names
+  return `${names.join(', ')} reacted with ${chip.key}`
+}
+
+// The avatar sits by the bubble's last line, above any reactions and thread summary.
+function avatarLift(item: MessageItem): number {
+  return 20 + (item.reactions.length ? 32 : 0) + (item.thread ? 34 : 0)
+}
+
+function threadLabel(item: MessageItem): string {
+  const n = item.thread?.count ?? 0
+  return n === 1 ? '1 reply' : `${n} replies`
+}
+
+// A quote goes to the message it quotes, when that is loaded here.
+const flashed = ref('')
+function jumpTo(eventId: string) {
+  const el = scroller.value?.querySelector<HTMLElement>(`[data-event-id="${CSS.escape(eventId)}"]`)
+  if (!el) {
+    toasts.info('That message isn’t loaded here.')
+    return
+  }
+  el.scrollIntoView({ block: 'center', behavior: 'smooth' })
+  flashed.value = eventId
+  window.setTimeout(() => flashed.value === eventId && (flashed.value = ''), 1500)
+}
+
 function askDelete(item: MessageItem) {
   deleteError.value = ''
   deleting.value = item
@@ -180,9 +265,9 @@ async function confirmDelete() {
     <div ref="scroller" class="flex h-full flex-col-reverse overflow-y-auto" @scroll.passive="onScroll">
       <div class="mx-auto flex w-full max-w-4xl flex-col px-4 pt-4 pb-2" role="log" aria-live="polite" aria-relevant="additions">
         <div class="flex min-h-10 items-center justify-center py-2">
-          <WSpinner v-if="paginating" :size="18" label="Loading older messages" />
+          <WSpinner v-if="paginating || threadLoading" :size="18" label="Loading older messages" />
           <WButton v-else-if="paginateError" size="sm" icon="refresh" @click="paginate">Load older messages</WButton>
-          <p v-else-if="!hasMore" class="caption text-ink-muted">This is the beginning of the room.</p>
+          <p v-else-if="!hasMore && !threadId" class="caption text-ink-muted">This is the beginning of the room.</p>
         </div>
 
         <template v-for="item in items" :key="item.key">
@@ -196,48 +281,114 @@ async function confirmDelete() {
 
           <div
             v-else
-            :class="['group flex items-end gap-2', item.own ? 'flex-row-reverse' : '', item.first ? 'mt-4' : 'mt-2']"
+            :data-event-id="item.eventId"
+            :class="[
+              'group flex items-end gap-2 rounded-md transition-colors',
+              item.own ? 'flex-row-reverse' : '',
+              item.first ? 'mt-4' : 'mt-2',
+              flashed && flashed === item.eventId ? 'bg-teal-soft' : '',
+            ]"
           >
             <template v-if="!item.own && !isDirect">
-              <MxcAvatar v-if="item.last" :name="item.senderName" :mxc="item.avatarMxc" :size="32" class="mb-5" />
+              <MxcAvatar
+                v-if="item.last"
+                :name="item.senderName"
+                :mxc="item.avatarMxc"
+                :size="32"
+                :style="{ marginBottom: `${avatarLift(item)}px` }"
+              />
               <span v-else class="w-8 shrink-0" />
             </template>
 
-            <WMessageBubble
-              class="max-w-[min(520px,80%)] min-w-0"
-              :own="item.own"
-              :sender="item.first && !isDirect ? item.senderName : undefined"
-              :time="item.last || item.status === 'failed' ? formatTime(item.ts) : ''"
-              :edited="item.edited"
-              :status="item.status"
-              :title="formatDateTime(item.ts)"
-            >
-              <MessageBody :item="item" @view="(source, name) => (viewing = { source, name })" />
-              <template v-if="item.status === 'failed' || item.shield" #meta>
-                <span
-                  v-if="item.shield"
-                  :class="['inline-flex items-center gap-1', item.shield.tone === 'danger' ? 'text-danger' : 'text-warning']"
-                  :title="item.shield.text"
+            <div :class="['flex max-w-[min(520px,80%)] min-w-0 flex-col gap-1', item.own ? 'items-end' : 'items-start']">
+              <WMessageBubble
+                class="max-w-full min-w-0"
+                :own="item.own"
+                :sender="item.first && !isDirect ? item.senderName : undefined"
+                :time="item.last || item.status === 'failed' ? formatTime(item.ts) : ''"
+                :edited="item.edited"
+                :status="item.status"
+                :title="formatDateTime(item.ts)"
+              >
+                <button
+                  v-if="item.reply"
+                  type="button"
+                  class="mb-1 flex w-full min-w-0 flex-col rounded-sm border-l-2 border-teal-strong bg-surface px-2 py-1 text-left"
+                  :aria-label="`Go to the message this replies to: ${item.reply.text}`"
+                  @click="jumpTo(item.reply.eventId)"
                 >
-                  <WIcon name="shield-alert" :size="14" />
-                  <span>{{ item.shield.tone === 'danger' ? item.shield.text : 'Not verified' }}</span>
-                  <span v-if="item.shield.tone !== 'danger'" class="wp-sr">: {{ item.shield.text }}</span>
-                </span>
-                <template v-if="item.status === 'failed'">
-                  <button type="button" class="font-semibold text-teal-text hover:underline" @click="retry(item)">Retry</button>
-                  <button type="button" class="font-semibold text-ink-muted hover:underline" @click="discard(item)">Delete</button>
+                  <span v-if="item.reply.senderName" class="caption truncate font-semibold text-teal-text">{{ item.reply.senderName }}</span>
+                  <span class="small block w-full truncate text-ink-muted">{{ item.reply.text }}</span>
+                </button>
+                <MessageBody :item="item" @view="(source, name) => (viewing = { source, name })" />
+                <template v-if="item.status === 'failed' || item.shield" #meta>
+                  <span
+                    v-if="item.shield"
+                    :class="['inline-flex items-center gap-1', item.shield.tone === 'danger' ? 'text-danger' : 'text-warning']"
+                    :title="item.shield.text"
+                  >
+                    <WIcon name="shield-alert" :size="14" />
+                    <span>{{ item.shield.tone === 'danger' ? item.shield.text : 'Not verified' }}</span>
+                    <span v-if="item.shield.tone !== 'danger'" class="wp-sr">: {{ item.shield.text }}</span>
+                  </span>
+                  <template v-if="item.status === 'failed'">
+                    <button type="button" class="font-semibold text-teal-text hover:underline" @click="retry(item)">Retry</button>
+                    <button type="button" class="font-semibold text-ink-muted hover:underline" @click="discard(item)">Delete</button>
+                  </template>
                 </template>
-              </template>
-            </WMessageBubble>
+              </WMessageBubble>
 
-            <PopMenu
-              v-if="actions(item).length && item.status !== 'sending' && item.status !== 'failed'"
-              class="-my-2 self-center opacity-0 group-hover:opacity-100 focus-within:opacity-100 [@media(hover:none)]:opacity-100"
-              icon="more"
-              label="Message actions"
-              :align="item.own ? 'right' : 'left'"
-              :items="actions(item)"
-            />
+              <div v-if="item.reactions.length" :class="['flex flex-wrap gap-1', item.own ? 'justify-end' : '']">
+                <button
+                  v-for="chip in item.reactions"
+                  :key="chip.key"
+                  type="button"
+                  :class="[
+                    'flex h-7 items-center gap-1 rounded-full border px-2 text-sm tabular-nums',
+                    chip.mine ? 'border-teal-strong bg-teal-soft text-teal-text' : 'border-line bg-surface-raised text-ink hover:bg-surface-sunken',
+                  ]"
+                  :title="chipLabel(chip)"
+                  :aria-label="`${chipLabel(chip)}. ${chip.mine ? 'Take yours back' : 'React too'}`"
+                  :aria-pressed="Boolean(chip.mine)"
+                  :disabled="!canReact"
+                  @click="react(item, chip.key)"
+                >
+                  <span class="text-base leading-none">{{ chip.key }}</span>
+                  <span class="font-semibold">{{ chip.count }}</span>
+                </button>
+              </div>
+
+              <button
+                v-if="item.thread && item.eventId"
+                type="button"
+                class="flex max-w-full items-center gap-2 rounded-md border border-line bg-surface-raised px-3 py-1 text-left hover:bg-surface-sunken"
+                @click="emit('openThread', item.eventId)"
+              >
+                <WIcon name="thread" :size="16" class="shrink-0 text-teal-text" />
+                <span class="small shrink-0 font-semibold text-teal-text">{{ threadLabel(item) }}</span>
+                <span class="caption truncate text-ink-muted">{{ item.thread.lastSenderName }} · {{ formatListTime(item.thread.lastTs) }}</span>
+                <span v-if="item.thread.unread" class="h-2 w-2 shrink-0 rounded-full bg-teal-strong" role="img" aria-label="Unread replies" />
+              </button>
+            </div>
+
+            <div
+              v-if="item.status !== 'sending' && item.status !== 'failed' && (actions(item).length || (canReact && answerable(item)))"
+              :class="[
+                '-my-2 flex items-center self-center opacity-0 group-hover:opacity-100 focus-within:opacity-100 [@media(hover:none)]:opacity-100',
+                item.own ? 'flex-row-reverse' : '',
+              ]"
+            >
+              <ReactionPicker v-if="canReact && answerable(item)" :align="item.own ? 'right' : 'left'" @pick="(key) => react(item, key)" />
+              <WIconButton v-if="canSend && answerable(item)" class="hidden sm:inline-flex" icon="reply" label="Reply" @click="emit('reply', item)" />
+              <WIconButton
+                v-if="canThread(item)"
+                class="hidden sm:inline-flex"
+                icon="thread"
+                label="Reply in thread"
+                @click="emit('openThread', item.eventId!)"
+              />
+              <PopMenu v-if="actions(item).length" icon="more" label="Message actions" :align="item.own ? 'right' : 'left'" :items="actions(item)" />
+            </div>
           </div>
         </template>
       </div>
diff --git a/apps/web/src/components/SpacePicker.vue b/apps/web/src/components/SpacePicker.vue
new file mode 100644
index 0000000..d8ebf29
--- /dev/null
+++ b/apps/web/src/components/SpacePicker.vue
@@ -0,0 +1,114 @@
+<script setup lang="ts">
+import { computed, nextTick, onBeforeUnmount, ref, watch } from 'vue'
+import { WIcon } from '@warp/ui'
+import MxcAvatar from '@/components/MxcAvatar.vue'
+import type { SpaceSummary } from '@/stores/chat'
+
+// Which rooms the list shows: all of them, or one space's. A space with
+// unread messages in its rooms has a dot.
+
+const props = defineProps<{ spaces: SpaceSummary[]; unread: Set<string> }>()
+const selected = defineModel<string>({ default: '' })
+const emit = defineEmits<{ newSpace: [] }>()
+
+const open = ref(false)
+const root = ref<HTMLElement>()
+const list = ref<HTMLElement>()
+
+const current = computed(() => props.spaces.find((s) => s.roomId === selected.value))
+
+function onDocClick(e: MouseEvent) {
+  if (root.value && !root.value.contains(e.target as Node)) open.value = false
+}
+watch(open, async (v) => {
+  if (v) {
+    document.addEventListener('mousedown', onDocClick)
+    await nextTick()
+    list.value?.querySelector<HTMLButtonElement>('[aria-checked="true"]')?.focus()
+  } else {
+    document.removeEventListener('mousedown', onDocClick)
+  }
+})
+onBeforeUnmount(() => document.removeEventListener('mousedown', onDocClick))
+
+function choose(id: string) {
+  selected.value = id
+  open.value = false
+}
+
+function onKey(e: KeyboardEvent) {
+  const buttons = Array.from(list.value?.querySelectorAll<HTMLButtonElement>('button') ?? [])
+  const i = buttons.indexOf(document.activeElement as HTMLButtonElement)
+  if (e.key === 'Escape') {
+    open.value = false
+    root.value?.querySelector<HTMLButtonElement>('button')?.focus()
+  } else if (e.key === 'ArrowDown') {
+    e.preventDefault()
+    buttons[(i + 1) % buttons.length]?.focus()
+  } else if (e.key === 'ArrowUp') {
+    e.preventDefault()
+    buttons[(i - 1 + buttons.length) % buttons.length]?.focus()
+  }
+}
+</script>
+
+<template>
+  <div ref="root" class="relative px-3 pb-2" @keydown="onKey">
+    <button
+      type="button"
+      class="flex h-10 w-full items-center gap-2 rounded-sm px-2 text-left font-semibold text-ink hover:bg-surface-raised"
+      aria-haspopup="menu"
+      :aria-expanded="open"
+      @click="open = !open"
+    >
+      <MxcAvatar v-if="current" :name="current.name" :mxc="current.avatarMxc" :size="24" />
+      <WIcon v-else name="space" :size="20" class="text-ink-muted" />
+      <span class="min-w-0 flex-1 truncate">{{ current?.name ?? 'All chats' }}</span>
+      <span v-if="!open && [...unread].some((id) => id !== selected)" class="h-2 w-2 rounded-full bg-teal-strong" role="img" aria-label="Unread in other spaces" />
+      <WIcon name="chevron-right" :size="16" :class="['text-ink-muted transition-transform', open ? '-rotate-90' : 'rotate-90']" />
+    </button>
+    <div
+      v-if="open"
+      ref="list"
+      role="menu"
+      aria-label="Show rooms from"
+      class="absolute inset-x-3 top-full z-30 mt-1 flex max-h-96 flex-col overflow-y-auto rounded-md border border-line bg-surface-raised p-1 shadow-float"
+    >
+      <button
+        type="button"
+        role="menuitemradio"
+        :aria-checked="selected === ''"
+        :class="['flex h-10 items-center gap-3 rounded-sm px-3 text-left font-semibold hover:bg-surface-sunken focus-visible:bg-surface-sunken', selected === '' && 'text-teal-text']"
+        @click="choose('')"
+      >
+        <WIcon name="space" class="text-ink-muted" />
+        <span class="flex-1">All chats</span>
+        <WIcon v-if="selected === ''" name="check" :size="16" />
+      </button>
+      <button
+        v-for="s in spaces"
+        :key="s.roomId"
+        type="button"
+        role="menuitemradio"
+        :aria-checked="selected === s.roomId"
+        :class="['flex h-10 items-center gap-3 rounded-sm px-3 text-left font-semibold hover:bg-surface-sunken focus-visible:bg-surface-sunken', selected === s.roomId && 'text-teal-text']"
+        @click="choose(s.roomId)"
+      >
+        <MxcAvatar :name="s.name" :mxc="s.avatarMxc" :size="24" />
+        <span class="min-w-0 flex-1 truncate">{{ s.name }}</span>
+        <span v-if="unread.has(s.roomId)" class="h-2 w-2 rounded-full bg-teal-strong" role="img" aria-label="Unread" />
+        <WIcon v-if="selected === s.roomId" name="check" :size="16" />
+      </button>
+      <div class="my-1 h-px bg-line" role="separator" />
+      <button
+        type="button"
+        role="menuitem"
+        class="flex h-10 items-center gap-3 rounded-sm px-3 text-left font-semibold text-ink hover:bg-surface-sunken focus-visible:bg-surface-sunken"
+        @click="(open = false), emit('newSpace')"
+      >
+        <WIcon name="plus" class="text-ink-muted" />
+        New space
+      </button>
+    </div>
+  </div>
+</template>
diff --git a/apps/web/src/components/SpaceView.vue b/apps/web/src/components/SpaceView.vue
new file mode 100644
index 0000000..58d4c3f
--- /dev/null
+++ b/apps/web/src/components/SpaceView.vue
@@ -0,0 +1,278 @@
+<script setup lang="ts">
+import { EventType, KnownMembership, RoomType, type HierarchyRoom, type Room } from 'matrix-js-sdk'
+import { computed, ref, watch } from 'vue'
+import { useRouter } from 'vue-router'
+import { WBadge, WBanner, WButton, WCheckbox, WDialog, WIcon, WIconButton, WSpinner } from '@warp/ui'
+import ConfirmDialog from '@/components/ConfirmDialog.vue'
+import CreateRoomDialog from '@/components/CreateRoomDialog.vue'
+import MxcAvatar from '@/components/MxcAvatar.vue'
+import PopMenu from '@/components/PopMenu.vue'
+import RoomInfoPane from '@/components/RoomInfoPane.vue'
+import { errorMessage } from '@/lib/format'
+import { client } from '@/matrix/client'
+import { spaceChildren, useChat } from '@/stores/chat'
+import { useToasts } from '@/stores/toasts'
+
+// A space's home: the rooms and subspaces it lists (GET /hierarchy), the
+// ones you're in and the ones you can join. Its admins add rooms to it,
+// existing or new, and take them out.
+
+const props = defineProps<{ space: Room }>()
+
+const chat = useChat()
+const toasts = useToasts()
+const router = useRouter()
+const me = client().getUserId() ?? ''
+
+const name = computed(() => {
+  void chat.version
+  return props.space.name
+})
+const topic = computed(() => {
+  void chat.version
+  return String(props.space.currentState.getStateEvents(EventType.RoomTopic, '')?.getContent().topic ?? '')
+})
+const memberCount = computed(() => {
+  void chat.version
+  return props.space.getJoinedMemberCount()
+})
+const canManage = computed(() => {
+  void chat.version
+  return props.space.currentState.maySendStateEvent(EventType.SpaceChild, me)
+})
+const childIds = computed(() => {
+  void chat.version
+  return spaceChildren(props.space)
+})
+
+function joinedHere(roomId: string): boolean {
+  void chat.version
+  return client().getRoom(roomId)?.getMyMembership() === KnownMembership.Join
+}
+
+// The hierarchy, a page at a time. Its first room is the space itself.
+const rooms = ref<HierarchyRoom[]>([])
+const nextBatch = ref<string>()
+const loading = ref(false)
+const loadError = ref('')
+// Where to join each room through: the via of the m.space.child that lists it.
+const vias = new Map<string, string[]>()
+
+async function load(more = false) {
+  loading.value = true
+  loadError.value = ''
+  try {
+    const res = await client().getRoomHierarchy(props.space.roomId, 50, undefined, false, more ? nextBatch.value : undefined)
+    for (const r of res.rooms) {
+      for (const child of r.children_state ?? []) {
+        if (child.content.via?.length) vias.set(child.state_key, child.content.via)
+      }
+    }
+    const found = res.rooms.filter((r) => r.room_id !== props.space.roomId)
+    rooms.value = more ? [...rooms.value, ...found] : found
+    nextBatch.value = res.next_batch
+  } catch (e) {
+    loadError.value = errorMessage(e)
+  } finally {
+    loading.value = false
+  }
+}
+// Again whenever the space's own list of rooms changes.
+watch(() => childIds.value.join(','), () => load(), { immediate: true })
+
+// The rooms the space lists directly first, each subspace's after.
+const direct = computed(() => new Set(childIds.value))
+const shown = computed(() =>
+  [...rooms.value].sort((a, b) => Number(direct.value.has(b.room_id)) - Number(direct.value.has(a.room_id))),
+)
+
+function roomName(r: HierarchyRoom): string {
+  return r.name ?? r.canonical_alias ?? r.room_id
+}
+
+const joining = ref('')
+async function join(r: HierarchyRoom) {
+  joining.value = r.room_id
+  try {
+    await client().joinRoom(r.room_id, { viaServers: vias.get(r.room_id) })
+    chat.bump()
+  } catch (e) {
+    toasts.error(`Can’t join ${roomName(r)}: ${errorMessage(e)}`)
+  } finally {
+    joining.value = ''
+  }
+}
+
+function open(roomId: string) {
+  router.push({ name: 'room', params: { roomId } })
+}
+
+// Taking a room out of the space.
+const removing = ref<HierarchyRoom | null>(null)
+const removeBusy = ref(false)
+const removeError = ref('')
+async function remove() {
+  const r = removing.value
+  if (!r) return
+  removeBusy.value = true
+  removeError.value = ''
+  try {
+    await chat.removeFromSpace(props.space.roomId, r.room_id)
+    removing.value = null
+  } catch (e) {
+    removeError.value = errorMessage(e)
+  } finally {
+    removeBusy.value = false
+  }
+}
+
+// Adding rooms you're in that the space doesn't list yet.
+const addOpen = ref(false)
+const picked = ref<string[]>([])
+const adding = ref(false)
+const addError = ref('')
+const addable = computed(() => [
+  ...chat.spaces.filter((s) => s.roomId !== props.space.roomId),
+  ...chat.rooms,
+].filter((r) => !direct.value.has(r.roomId)))
+watch(addOpen, (v) => {
+  if (v) {
+    picked.value = []
+    addError.value = ''
+  }
+})
+function togglePick(roomId: string, on: boolean) {
+  picked.value = on ? [...picked.value, roomId] : picked.value.filter((id) => id !== roomId)
+}
+async function addPicked() {
+  adding.value = true
+  addError.value = ''
+  try {
+    for (const roomId of picked.value) await chat.addToSpace(props.space.roomId, roomId)
+    addOpen.value = false
+  } catch (e) {
+    addError.value = errorMessage(e)
+  } finally {
+    adding.value = false
+  }
+}
+
+const createOpen = ref(false)
+const infoOpen = ref(false)
+</script>
+
+<template>
+  <div class="flex min-h-0 flex-1">
+    <div class="flex min-w-0 flex-1 flex-col">
+      <header class="flex h-16 shrink-0 items-center gap-3 border-b border-line bg-surface px-2 md:px-4">
+        <WIconButton class="md:hidden" icon="chevron-left" label="Back to rooms" @click="router.push('/')" />
+        <MxcAvatar :name="name" :mxc="space.getMxcAvatarUrl() ?? undefined" :size="36" />
+        <button type="button" class="min-w-0 flex-1 cursor-pointer border-0 bg-transparent p-0 text-left" @click="infoOpen = !infoOpen">
+          <span class="flex items-center gap-2">
+            <h1 class="heading m-0 truncate">{{ name }}</h1>
+            <WBadge tone="neutral">Space</WBadge>
+          </span>
+          <span class="small block truncate text-ink-muted">
+            <template v-if="topic">{{ topic }}</template>
+            <template v-else>{{ memberCount }} {{ memberCount === 1 ? 'member' : 'members' }}</template>
+          </span>
+        </button>
+        <WIconButton icon="users" :label="infoOpen ? 'Hide space info' : 'Space info'" :aria-pressed="infoOpen" @click="infoOpen = !infoOpen" />
+      </header>
+
+      <div class="flex-1 overflow-y-auto">
+        <div class="mx-auto flex w-full max-w-3xl flex-col gap-3 px-4 py-6">
+          <div class="flex flex-wrap items-center gap-2">
+            <h2 class="subheading m-0 flex-1">Rooms</h2>
+            <template v-if="canManage">
+              <WButton size="sm" icon="plus" @click="addOpen = true">Add existing room</WButton>
+              <WButton size="sm" variant="primary" icon="plus" @click="createOpen = true">New room</WButton>
+            </template>
+          </div>
+
+          <WBanner v-if="loadError" tone="warning">
+            Can’t load the space’s rooms: {{ loadError }}
+            <button type="button" class="font-semibold text-teal-text hover:underline" @click="load()">Try again</button>
+          </WBanner>
+          <div v-if="loading && !rooms.length" class="flex justify-center py-8"><WSpinner /></div>
+          <div v-else-if="!rooms.length && !loadError" class="flex flex-col items-center gap-2 py-10 text-center">
+            <WIcon name="space" :size="32" class="text-ink-muted" />
+            <p class="body-strong m-0">No rooms in this space yet</p>
+            <p class="small m-0 text-ink-muted">
+              {{ canManage ? 'Add a room you’re in, or create a new one here.' : 'Its admins haven’t added any rooms yet.' }}
+            </p>
+          </div>
+
+          <ul v-else class="m-0 flex list-none flex-col gap-1 p-0">
+            <li v-for="r in shown" :key="r.room_id" class="flex items-center gap-3 rounded-md px-2 py-2 hover:bg-surface-sunken">
+              <MxcAvatar :name="roomName(r)" :mxc="r.avatar_url" :size="40" />
+              <div class="min-w-0 flex-1">
+                <p class="body-strong m-0 flex items-center gap-2">
+                  <span class="truncate">{{ roomName(r) }}</span>
+                  <WBadge v-if="r.room_type === RoomType.Space" tone="neutral">Space</WBadge>
+                  <WIcon v-if="!direct.has(r.room_id)" name="chevron-right" :size="14" class="shrink-0 text-ink-muted" label="In a subspace" />
+                </p>
+                <p class="small m-0 truncate text-ink-muted">
+                  {{ r.num_joined_members }} {{ r.num_joined_members === 1 ? 'member' : 'members' }}<template v-if="r.topic"> · {{ r.topic }}</template>
+                </p>
+              </div>
+              <WButton v-if="joinedHere(r.room_id)" size="sm" variant="ghost" @click="open(r.room_id)">Open</WButton>
+              <!-- The homeserver lists the rooms you may join or look into; joining says if it can't. -->
+              <WButton v-else size="sm" :loading="joining === r.room_id" @click="join(r)">Join</WButton>
+              <PopMenu
+                v-if="canManage && direct.has(r.room_id)"
+                icon="more"
+                :label="`Actions for ${roomName(r)}`"
+                :items="[{ label: 'Remove from space', icon: 'trash', danger: true, action: () => ((removeError = ''), (removing = r)) }]"
+              />
+            </li>
+          </ul>
+          <div v-if="nextBatch" class="flex justify-center py-2">
+            <WButton size="sm" :loading="loading" @click="load(true)">More rooms</WButton>
+          </div>
+        </div>
+      </div>
+    </div>
+
+    <RoomInfoPane v-if="infoOpen" :room="space" :is-direct="false" @close="infoOpen = false" />
+
+    <CreateRoomDialog v-model:open="createOpen" :space-id="space.roomId" :space-name="name" @created="open" />
+
+    <WDialog v-model:open="addOpen" :title="`Add rooms to ${name}`" :persistent="adding">
+      <p v-if="!addable.length" class="body m-0 text-ink-muted">Every room you’re in is in this space already.</p>
+      <ul v-else class="m-0 flex max-h-80 list-none flex-col overflow-y-auto p-0">
+        <li v-for="r in addable" :key="r.roomId" class="flex items-center gap-3 rounded-md px-2 py-1.5">
+          <MxcAvatar :name="r.name" :mxc="r.avatarMxc" :size="32" />
+          <WCheckbox
+            class="min-w-0 flex-1"
+            :model-value="picked.includes(r.roomId)"
+            :label="r.name"
+            @update:model-value="(v: boolean) => togglePick(r.roomId, v)"
+          />
+        </li>
+      </ul>
+      <WBanner v-if="addError" tone="danger">{{ addError }}</WBanner>
+      <template #actions>
+        <WButton variant="ghost" :disabled="adding" @click="addOpen = false">Cancel</WButton>
+        <WButton variant="primary" :loading="adding" :disabled="!picked.length" @click="addPicked">
+          {{ picked.length > 1 ? `Add ${picked.length} rooms` : 'Add room' }}
+        </WButton>
+      </template>
+    </WDialog>
+
+    <ConfirmDialog
+      :open="removing !== null"
+      title="Remove from space"
+      confirm-label="Remove"
+      :busy="removeBusy"
+      :error="removeError"
+      @update:open="(v) => !v && (removing = null)"
+      @confirm="remove"
+    >
+      <p>
+        <span class="body-strong">{{ removing ? roomName(removing) : '' }}</span> is no longer listed in {{ name }}. The room itself stays, and
+        its members stay in it.
+      </p>
+    </ConfirmDialog>
+  </div>
+</template>
diff --git a/apps/web/src/components/ThreadPane.vue b/apps/web/src/components/ThreadPane.vue
new file mode 100644
index 0000000..e422e50
--- /dev/null
+++ b/apps/web/src/components/ThreadPane.vue
@@ -0,0 +1,55 @@
+<script setup lang="ts">
+import type { Room } from 'matrix-js-sdk'
+import { computed, shallowRef } from 'vue'
+import { WIconButton } from '@warp/ui'
+import RoomComposer from '@/components/RoomComposer.vue'
+import RoomTimeline from '@/components/RoomTimeline.vue'
+import { threadCount, threadRoot, type MessageItem } from '@/matrix/timeline'
+import { useChat } from '@/stores/chat'
+
+// One thread beside its room: the root, its replies, and a composer that
+// writes into it. On a phone it covers the room.
+
+const props = defineProps<{ room: Room; threadId: string; isDirect: boolean }>()
+const emit = defineEmits<{ close: []; back: [] }>()
+
+const chat = useChat()
+const replyTo = shallowRef<MessageItem | null>(null)
+
+// A thread starting here has no thread object yet, and the SDK only files
+// a reply's remote echo into one that exists: without this the first
+// reply would vanish until the next reload. Element's thread view does
+// the same.
+if (!props.room.getThread(props.threadId)) {
+  const root = threadRoot(props.room, props.threadId)
+  if (root) props.room.createThread(props.threadId, root, [], true)
+}
+
+const count = computed(() => {
+  void chat.version
+  return threadCount(props.room, props.threadId)
+})
+</script>
+
+<template>
+  <aside
+    class="fixed inset-0 z-30 flex flex-col bg-surface-sunken md:static md:z-auto md:w-[400px] md:shrink-0 md:border-l md:border-line"
+    aria-label="Thread"
+    @keydown.esc="emit('close')"
+  >
+    <div class="flex h-16 shrink-0 items-center gap-2 border-b border-line bg-surface px-2 md:px-4">
+      <WIconButton icon="chevron-left" label="All threads" @click="emit('back')" />
+      <div class="min-w-0 flex-1">
+        <h2 class="subheading m-0">Thread</h2>
+        <p class="caption m-0 truncate text-ink-muted">{{ room.name }} · {{ count === 1 ? '1 reply' : `${count} replies` }}</p>
+      </div>
+      <WIconButton icon="close" label="Close thread" @click="emit('close')" />
+    </div>
+
+    <RoomTimeline :key="threadId" :room="room" :is-direct="isDirect" :thread-id="threadId" @reply="(item) => (replyTo = item)" />
+
+    <div class="shrink-0 px-4 pt-2 pb-4">
+      <RoomComposer v-model:reply-to="replyTo" :room="room" :thread-id="threadId" placeholder="Reply in thread" />
+    </div>
+  </aside>
+</template>
diff --git a/apps/web/src/components/ThreadsPane.vue b/apps/web/src/components/ThreadsPane.vue
new file mode 100644
index 0000000..65776a9
--- /dev/null
+++ b/apps/web/src/components/ThreadsPane.vue
@@ -0,0 +1,127 @@
+<script setup lang="ts">
+import { Direction, type Room } from 'matrix-js-sdk'
+import { computed, onMounted, ref } from 'vue'
+import { WBanner, WButton, WIcon, WIconButton, WSpinner } from '@warp/ui'
+import MxcAvatar from '@/components/MxcAvatar.vue'
+import { errorMessage, formatListTime } from '@/lib/format'
+import { client } from '@/matrix/client'
+import { quoteText, threadCount, threadUnread } from '@/matrix/timeline'
+import { useChat } from '@/stores/chat'
+
+// A room's threads, the latest reply first (GET /threads). The ones with
+// replies you haven't read have a dot.
+
+const props = defineProps<{ room: Room }>()
+const emit = defineEmits<{ open: [threadId: string]; close: [] }>()
+
+const chat = useChat()
+const me = client().getUserId() ?? ''
+const loading = ref(true)
+const loadingMore = ref(false)
+const error = ref('')
+
+const threads = computed(() => {
+  void chat.version
+  return props.room
+    .getThreads()
+    .filter((t) => threadCount(props.room, t.id) > 0)
+    .map((t) => {
+      const root = t.rootEvent
+      const sender = root?.getSender() ?? ''
+      const last = t.replyToEvent
+      return {
+        id: t.id,
+        senderName: props.room.getMember(sender)?.name ?? sender,
+        avatarMxc: props.room.getMember(sender)?.getMxcAvatarUrl() ?? undefined,
+        text: root ? quoteText(root) : '',
+        count: threadCount(props.room, t.id),
+        ts: last?.getTs() ?? root?.getTs() ?? 0,
+        unread: threadUnread(props.room, t.id, me),
+      }
+    })
+    .sort((a, b) => b.ts - a.ts)
+})
+
+const timeline = computed(() => {
+  void chat.version
+  return props.room.threadsTimelineSets[0]?.getLiveTimeline()
+})
+const hasMore = computed(() => {
+  void chat.version
+  return Boolean(timeline.value?.getPaginationToken(Direction.Backward))
+})
+
+onMounted(async () => {
+  try {
+    await props.room.createThreadsTimelineSets()
+    await props.room.fetchRoomThreads()
+  } catch (e) {
+    error.value = errorMessage(e)
+  } finally {
+    loading.value = false
+    chat.bump()
+  }
+})
+
+async function more() {
+  const tl = timeline.value
+  if (!tl) return
+  loadingMore.value = true
+  try {
+    await client().paginateEventTimeline(tl, { backwards: true, limit: 20 })
+  } catch (e) {
+    error.value = errorMessage(e)
+  } finally {
+    loadingMore.value = false
+    chat.bump()
+  }
+}
+</script>
+
+<template>
+  <aside
+    class="fixed inset-0 z-30 flex flex-col bg-surface-sunken md:static md:z-auto md:w-80 md:shrink-0 md:border-l md:border-line"
+    aria-label="Threads"
+  >
+    <div class="flex h-16 shrink-0 items-center gap-2 border-b border-line px-4">
+      <h2 class="subheading m-0 flex-1">Threads</h2>
+      <WIconButton icon="close" label="Close threads" @click="emit('close')" />
+    </div>
+
+    <div class="flex-1 overflow-y-auto p-2">
+      <WBanner v-if="error" tone="warning" class="m-2">{{ error }}</WBanner>
+      <div v-if="loading && !threads.length" class="flex justify-center py-8"><WSpinner /></div>
+      <div v-else-if="!threads.length" class="flex flex-col items-center gap-2 px-4 py-10 text-center">
+        <WIcon name="thread" :size="32" class="text-ink-muted" />
+        <p class="body-strong m-0">No threads yet</p>
+        <p class="small m-0 text-ink-muted">Reply in thread on a message to start one. Its replies stay out of the room’s timeline.</p>
+      </div>
+      <ul v-else class="m-0 flex list-none flex-col gap-1 p-0">
+        <li v-for="t in threads" :key="t.id">
+          <button
+            type="button"
+            class="flex w-full items-start gap-3 rounded-md px-2 py-2 text-left hover:bg-surface-raised focus-visible:bg-surface-raised"
+            @click="emit('open', t.id)"
+          >
+            <MxcAvatar :name="t.senderName" :mxc="t.avatarMxc" :size="32" />
+            <span class="min-w-0 flex-1">
+              <span class="flex items-center gap-2">
+                <span class="body-strong min-w-0 flex-1 truncate">{{ t.senderName }}</span>
+                <span class="caption shrink-0 text-ink-muted">{{ formatListTime(t.ts) }}</span>
+              </span>
+              <span class="small block truncate text-ink-muted">{{ t.text }}</span>
+              <span class="caption flex items-center gap-1 text-teal-text">
+                <WIcon name="thread" :size="14" />
+                {{ t.count === 1 ? '1 reply' : `${t.count} replies` }}
+                <span v-if="t.unread" class="ml-1 h-2 w-2 rounded-full bg-teal-strong" role="img" aria-label="Unread replies" />
+              </span>
+            </span>
+          </button>
+        </li>
+      </ul>
+      <div v-if="hasMore && threads.length" class="flex justify-center py-3">
+        <WButton size="sm" :loading="loadingMore" @click="more">Older threads</WButton>
+      </div>
+    </div>
+  </aside>
+</template>
diff --git a/apps/web/src/layouts/ClientShell.vue b/apps/web/src/layouts/ClientShell.vue
index becf360..f2de6a5 100644
--- a/apps/web/src/layouts/ClientShell.vue
+++ b/apps/web/src/layouts/ClientShell.vue
@@ -1,5 +1,5 @@
 <script setup lang="ts">
-import { computed, onMounted, ref, watchEffect } from 'vue'
+import { computed, onMounted, ref, watch, watchEffect } from 'vue'
 import { useRoute, useRouter } from 'vue-router'
 import { WBanner, WButton, WIcon, WIconButton, WLogo, WSpinner } from '@warp/ui'
 import CallView from '@/components/CallView.vue'
@@ -11,6 +11,7 @@ import PopMenu from '@/components/PopMenu.vue'
 import RoomRow from '@/components/RoomRow.vue'
 import type { MenuItem } from '@/components/menu'
 import SettingsDialog from '@/components/SettingsDialog.vue'
+import SpacePicker from '@/components/SpacePicker.vue'
 import StartChatDialog from '@/components/StartChatDialog.vue'
 import VerificationDialog from '@/components/VerificationDialog.vue'
 import { formatListTime, localpart } from '@/lib/format'
@@ -30,7 +31,41 @@ const router = useRouter()
 
 const startError = ref('')
 const filter = ref('')
-const dialog = ref<'create' | 'chat' | 'join' | 'settings' | 'verify' | null>(null)
+const dialog = ref<'create' | 'space' | 'chat' | 'join' | 'settings' | 'verify' | null>(null)
+
+// The space the room list is narrowed to, '' for all chats; kept for the
+// next visit in this browser.
+const spaceKey = 'warp.web.space'
+function savedSpace(): string {
+  try {
+    return localStorage.getItem(spaceKey) ?? ''
+  } catch {
+    return ''
+  }
+}
+const space = ref(savedSpace())
+watch(space, (v) => {
+  try {
+    if (v) localStorage.setItem(spaceKey, v)
+    else localStorage.removeItem(spaceKey)
+  } catch {
+    // Not kept: the list starts at all chats next time.
+  }
+})
+// A space you left (or never were in, here) shows all chats.
+watch(
+  () => chat.ready && chat.spaces.map((s) => s.roomId).join(','),
+  () => {
+    if (chat.ready && space.value && !chat.spaces.some((s) => s.roomId === space.value)) space.value = ''
+  },
+)
+const spaceRooms = computed(() => (space.value ? chat.roomsInSpace(space.value) : null))
+const currentSpace = computed(() => chat.spaces.find((s) => s.roomId === space.value))
+// Spaces with something unread in their rooms.
+const unreadSpaces = computed(() => {
+  const unread = new Set(chat.rooms.filter((r) => r.unread > 0).map((r) => r.roomId))
+  return new Set(chat.spaces.filter((s) => [...chat.roomsInSpace(s.roomId)].some((id) => unread.has(id))).map((s) => s.roomId))
+})
 
 // The account has encryption keys but this session isn't signed by them:
 // it can't read older messages and others see it as unverified.
@@ -56,8 +91,10 @@ const activeRoomId = computed(() => (route.name === 'room' ? String(route.params
 
 const shownRooms = computed(() => {
   const q = filter.value.trim().toLowerCase()
-  if (!q) return chat.rooms
-  return chat.rooms.filter((r) => r.name.toLowerCase().includes(q) || r.roomId.toLowerCase().includes(q))
+  const inSpace = spaceRooms.value
+  const rooms = inSpace ? chat.rooms.filter((r) => inSpace.has(r.roomId)) : chat.rooms
+  if (!q) return rooms
+  return rooms.filter((r) => r.name.toLowerCase().includes(q) || r.roomId.toLowerCase().includes(q))
 })
 const shownInvites = computed(() => {
   const q = filter.value.trim().toLowerCase()
@@ -69,8 +106,14 @@ const menu: MenuItem[] = [
   { label: 'New room', icon: 'plus', action: () => (dialog.value = 'create') },
   { label: 'Start chat', icon: 'user', action: () => (dialog.value = 'chat') },
   { label: 'Join room', icon: 'room', action: () => (dialog.value = 'join') },
+  { label: 'New space', icon: 'space', action: () => (dialog.value = 'space') },
 ]
 
+function spaceCreated(roomId: string) {
+  space.value = roomId
+  open(roomId)
+}
+
 function open(roomId: string) {
   router.push({ name: 'room', params: { roomId } })
 }
@@ -78,7 +121,7 @@ function open(roomId: string) {
 // The tab title counts rooms with unread messages, and invites.
 watchEffect(() => {
   const n = chat.totalUnread
-  const room = chat.rooms.find((r) => r.roomId === activeRoomId.value)
+  const room = chat.rooms.find((r) => r.roomId === activeRoomId.value) ?? chat.spaces.find((r) => r.roomId === activeRoomId.value)
   const name = room?.name ?? chat.invites.find((r) => r.roomId === activeRoomId.value)?.name
   document.title = `${n ? `(${n}) ` : ''}${name ? `${name} · ` : ''}Warp`
 })
@@ -128,6 +171,8 @@ const ownName = computed(() => profile.displayName || localpart(session.userId))
         <PopMenu icon="plus" label="New" :items="menu" />
       </div>
 
+      <SpacePicker v-if="chat.spaces.length" v-model="space" :spaces="chat.spaces" :unread="unreadSpaces" @new-space="dialog = 'space'" />
+
       <div class="px-3 pb-2">
         <label class="flex h-10 items-center gap-2 rounded-sm border border-line-strong bg-surface-raised px-3 focus-within:border-teal-strong focus-within:shadow-focus">
           <WIcon name="search" :size="18" class="text-ink-muted" />
@@ -168,6 +213,20 @@ const ownName = computed(() => profile.displayName || localpart(session.userId))
           <h2 class="caption px-3 pt-3 pb-1 text-ink-muted">Rooms</h2>
         </template>
 
+        <button
+          v-if="currentSpace"
+          type="button"
+          :class="[
+            'mb-1 flex w-full items-center gap-3 rounded-md px-3 py-2 text-left hover:bg-surface-raised',
+            activeRoomId === currentSpace.roomId && 'bg-surface-raised',
+          ]"
+          @click="open(currentSpace.roomId)"
+        >
+          <WIcon name="space" class="text-ink-muted" />
+          <span class="body-strong min-w-0 flex-1 truncate">Browse {{ currentSpace.name }}</span>
+          <WIcon name="chevron-right" :size="16" class="text-ink-muted" />
+        </button>
+
         <RoomRow
           v-for="r in shownRooms"
           :key="r.roomId"
@@ -187,6 +246,11 @@ const ownName = computed(() => profile.displayName || localpart(session.userId))
           <template v-if="filter">
             <p class="body text-ink-muted">No rooms match “{{ filter }}”.</p>
           </template>
+          <template v-else-if="currentSpace">
+            <p class="body-strong">You’re in no rooms of {{ currentSpace.name }}</p>
+            <p class="body text-ink-muted">Browse the space to join its rooms.</p>
+            <WButton variant="primary" icon="space" @click="open(currentSpace.roomId)">Browse space</WButton>
+          </template>
           <template v-else>
             <p class="body-strong">No rooms yet</p>
             <p class="body text-ink-muted">Create a room, start a chat with someone, or join a room by its address.</p>
@@ -209,7 +273,14 @@ const ownName = computed(() => profile.displayName || localpart(session.userId))
       <RouterView :key="activeRoomId" />
     </main>
 
-    <CreateRoomDialog :open="dialog === 'create'" @update:open="dialog = $event ? 'create' : null" @created="open" />
+    <CreateRoomDialog
+      :open="dialog === 'create'"
+      :space-id="currentSpace?.roomId"
+      :space-name="currentSpace?.name"
+      @update:open="dialog = $event ? 'create' : null"
+      @created="open"
+    />
+    <CreateRoomDialog kind="space" :open="dialog === 'space'" @update:open="dialog = $event ? 'space' : null" @created="spaceCreated" />
     <StartChatDialog :open="dialog === 'chat'" @update:open="dialog = $event ? 'chat' : null" @started="open" />
     <JoinRoomDialog :open="dialog === 'join'" @update:open="dialog = $event ? 'join' : null" @joined="open" />
     <SettingsDialog :open="dialog === 'settings'" @update:open="dialog = $event ? 'settings' : null" />
diff --git a/apps/web/src/matrix/client.ts b/apps/web/src/matrix/client.ts
index 954114a..369271e 100644
--- a/apps/web/src/matrix/client.ts
+++ b/apps/web/src/matrix/client.ts
@@ -100,7 +100,10 @@ export async function startClient(creds: Credentials): Promise<MatrixClient> {
     // Your own messages that are still sending (or failed) are kept apart
     // from the timeline, so a failed one doesn't sit between others' replies.
     pendingEventOrdering: PendingEventOrdering.Detached,
-    threadSupport: false,
+    // Threads: their replies leave the room's timeline for one of their
+    // own, read receipts carry a thread_id (MSC3771) and the unread counts
+    // come per thread (MSC3773).
+    threadSupport: true,
   })
   return c
 }
diff --git a/apps/web/src/matrix/send.ts b/apps/web/src/matrix/send.ts
index db9e70c..8515a2b 100644
--- a/apps/web/src/matrix/send.ts
+++ b/apps/web/src/matrix/send.ts
@@ -1,4 +1,4 @@
-import { MsgType, type MatrixClient, type Room } from 'matrix-js-sdk'
+import { EventType, MsgType, RelationType, type MatrixClient, type MatrixEvent, type Room } from 'matrix-js-sdk'
 import type { RoomMessageEventContent } from 'matrix-js-sdk/lib/types'
 import { encryptAttachment } from './attachments'
 
@@ -8,13 +8,38 @@ import { encryptAttachment } from './attachments'
 // In an end-to-end encrypted room the SDK encrypts the message itself, and
 // a file is encrypted here before it is uploaded.
 
-export function sendText(c: MatrixClient, roomId: string, text: string) {
+/**
+ * Where a message goes: into a thread (by its root's ID), and as a reply
+ * to a message. A thread reply that answers no message in particular is
+ * marked as falling back, which the SDK does.
+ */
+export interface SendTarget {
+  threadId?: string
+  replyTo?: string
+}
+
+// Replies carry no quoted fallback in their body: Matrix 1.13 dropped it
+// (MSC2781), and the quote is drawn from the event replied to.
+function withTarget<T extends object>(content: T, target: SendTarget): T {
+  if (!target.replyTo) return content
+  return { ...content, 'm.relates_to': { 'm.in_reply_to': { event_id: target.replyTo } } }
+}
+
+export function sendText(c: MatrixClient, roomId: string, text: string, target: SendTarget = {}) {
   const body = text.replace(/\s+$/, '')
   const emote = body.match(/^\/me\s+([\s\S]+)$/)
   const content: RoomMessageEventContent = emote
     ? { msgtype: MsgType.Emote, body: emote[1]! }
     : { msgtype: MsgType.Text, body: body.replace(/^\/\//, '/') }
-  return c.sendMessage(roomId, content)
+  return c.sendMessage(roomId, target.threadId ?? null, withTarget(content, target))
+}
+
+/** Reacts to a message; in a thread, the reaction goes with the thread. */
+export function sendReaction(c: MatrixClient, ev: MatrixEvent, key: string) {
+  const threadId = ev.threadRootId && !ev.isThreadRoot ? ev.threadRootId : null
+  return c.sendEvent(ev.getRoomId()!, threadId, EventType.Reaction, {
+    'm.relates_to': { rel_type: RelationType.Annotation, event_id: ev.getId()!, key },
+  })
 }
 
 function imageSize(file: File): Promise<{ w: number; h: number } | undefined> {
@@ -85,6 +110,7 @@ export async function sendFile(
   room: Room,
   file: File,
   onProgress?: (p: UploadProgress) => void,
+  target: SendTarget = {},
 ) {
   const msgtype = msgtypeFor(file)
   const info: Record<string, unknown> = { mimetype: file.type || 'application/octet-stream', size: file.size }
@@ -125,5 +151,5 @@ export async function sendFile(
     ...source,
     info,
   } as RoomMessageEventContent
-  return c.sendMessage(room.roomId, content)
+  return c.sendMessage(room.roomId, target.threadId ?? null, withTarget(content, target))
 }
diff --git a/apps/web/src/matrix/timeline.ts b/apps/web/src/matrix/timeline.ts
index f5faa79..8f300dd 100644
--- a/apps/web/src/matrix/timeline.ts
+++ b/apps/web/src/matrix/timeline.ts
@@ -3,6 +3,9 @@ import {
   EventStatus,
   EventType,
   MsgType,
+  RelationType,
+  THREAD_RELATION_TYPE,
+  type EventTimeline,
   type MatrixClient,
   type MatrixEvent,
   type Room,
@@ -17,7 +20,9 @@ import { shieldOf, type Shield } from './trust'
 
 // Turns a room's live timeline into what the timeline view draws: messages
 // grouped by sender, day separators and one-line notices for state changes.
-// The SDK keeps the events; this is a pure projection of them.
+// The SDK keeps the events; this is a pure projection of them. A thread is
+// projected the same way, from its own timeline: with thread support on,
+// the SDK keeps a thread's replies out of the room's.
 
 export interface MediaInfo {
   mxc?: string
@@ -34,6 +39,32 @@ export interface MediaInfo {
   filename: string
 }
 
+/** The message a reply answers, quoted above it. */
+export interface ReplyQuote {
+  eventId: string
+  /** Unset while it loads, or when it can't be found. */
+  senderName?: string
+  text: string
+}
+
+/** One reaction under a message: a key and who reacted with it. */
+export interface ReactionChip {
+  key: string
+  count: number
+  /** Your own reaction with this key, to take back. */
+  mine?: MatrixEvent
+  names: string[]
+}
+
+/** What a thread's root shows under it. */
+export interface ThreadSummary {
+  count: number
+  lastTs: number
+  lastSenderName: string
+  /** Has replies you haven't read. */
+  unread: boolean
+}
+
 export interface MessageItem {
   kind: 'message'
   key: string
@@ -61,6 +92,10 @@ export interface MessageItem {
   first: boolean
   /** Last of a run: shows the time and status. */
   last: boolean
+  reply?: ReplyQuote
+  reactions: ReactionChip[]
+  /** Set on a thread's root in the room's timeline. */
+  thread?: ThreadSummary
   event: MatrixEvent
 }
 
@@ -296,17 +331,196 @@ function mediaOf(content: RoomMessageEventContent): MediaInfo | undefined {
   }
 }
 
-/** Every event the view shows, oldest first: the live timeline then your pending ones. */
-function displayEvents(room: Room): MatrixEvent[] {
-  return [...room.getLiveTimeline().getEvents(), ...room.getPendingEvents()]
+/** Whether an event is a reply in a thread rather than in the room. */
+export function inThread(ev: MatrixEvent): boolean {
+  return Boolean(ev.threadRootId) && !ev.isThreadRoot
 }
 
-export function buildTimeline(c: MatrixClient, room: Room): TimelineItem[] {
+/** The thread's own timeline, or the room's. */
+function liveTimelineOf(room: Room, threadId?: string): EventTimeline | undefined {
+  if (!threadId) return room.getLiveTimeline()
+  return room.getThread(threadId)?.liveTimeline
+}
+
+/** A thread's root event, wherever the SDK has it. */
+export function threadRoot(room: Room, threadId: string): MatrixEvent | undefined {
+  return room.getThread(threadId)?.rootEvent ?? room.findEventById(threadId)
+}
+
+/**
+ * Every event the view shows, oldest first: the live timeline then your
+ * pending ones. A thread starts with its root, which its own timeline
+ * only has once it is paged back to the start.
+ */
+function displayEvents(room: Room, threadId?: string): MatrixEvent[] {
+  const pending = room.getPendingEvents()
+  if (!threadId) return [...room.getLiveTimeline().getEvents(), ...pending.filter((e) => !inThread(e))]
+  const events = liveTimelineOf(room, threadId)?.getEvents() ?? []
+  const root = threadRoot(room, threadId)
+  const head = root && !events.some((e) => e.getId() === threadId) ? [root] : []
+  return [...head, ...events, ...pending.filter((e) => e.threadRootId === threadId && inThread(e))]
+}
+
+// Replies sent with the old fallback start with the quoted message, a
+// "> " line each, then an empty line.
+function stripReplyFallback(body: string): string {
+  return body.replace(/^(>[^\n]*\n)+\n?/, '')
+}
+
+/** The message a reply answers, unless it is only a thread's fallback. */
+function replyTarget(ev: MatrixEvent): string | undefined {
+  const rel = ev.getWireContent()['m.relates_to']
+  const id = rel?.['m.in_reply_to']?.event_id
+  if (!rel || typeof id !== 'string') return undefined
+  if (rel.rel_type === THREAD_RELATION_TYPE.name && rel.is_falling_back) return undefined
+  return id
+}
+
+/** One line for a quoted message. */
+export function quoteText(ev: MatrixEvent): string {
+  if (ev.isRedacted()) return 'Message deleted'
+  if (ev.isDecryptionFailure() || ev.getType() === EventType.RoomMessageEncrypted) return 'Encrypted message'
+  if (ev.getType() === EventType.Sticker) return 'Sticker'
+  const content = ev.getContent()
+  switch (content.msgtype) {
+    case MsgType.Image:
+      return 'Image'
+    case MsgType.Video:
+      return 'Video'
+    case MsgType.Audio:
+      return 'Audio'
+    case MsgType.File:
+      return `File: ${content.body ?? ''}`
+  }
+  const body = String(content.body ?? '')
+  const text = replyTarget(ev) ? stripReplyFallback(body) : body
+  return text.split('\n').find((l) => l.trim()) ?? ''
+}
+
+function reactionsOf(room: Room, ev: MatrixEvent, me: string): ReactionChip[] {
+  const id = ev.getId()
+  if (!id || ev.isRedacted()) return []
+  const sorted = room.relations.getChildEventsForEvent(id, RelationType.Annotation, EventType.Reaction)?.getSortedAnnotationsByKey()
+  if (!sorted) return []
+  const out: ReactionChip[] = []
+  for (const [key, set] of sorted) {
+    const events = [...set].filter((r) => !r.isRedacted() && r.status !== EventStatus.CANCELLED)
+    if (!events.length) continue
+    out.push({
+      key,
+      count: events.length,
+      mine: events.find((r) => r.getSender() === me),
+      names: events.map((r) => (r.getSender() === me ? 'You' : nameOf(room, r.getSender() ?? ''))),
+    })
+  }
+  return out
+}
+
+// The SDK takes a thread's reply count from the server once, then adds
+// one for each reply that comes in; a reply that lands between the two is
+// counted twice. So after replies come in the count is asked for again
+// (the root's bundled m.thread count), and used while the newest reply it
+// counted is still the newest.
+const recounted = new Map<string, { count: number; lastId: string }>()
+const recountTimers = new Map<string, number>()
+
+/** A thread's number of replies. */
+export function threadCount(room: Room, threadId: string): number {
+  const thread = room.getThread(threadId)
+  if (!thread) return 0
+  const known = recounted.get(`${room.roomId}|${threadId}`)
+  const lastId = thread.replyToEvent?.getId()
+  return known && lastId && known.lastId === lastId ? known.count : thread.length
+}
+
+/** Asks the homeserver for a thread's reply count, a second after the last call. */
+export function recountThread(c: MatrixClient, room: Room, threadId: string) {
+  const key = `${room.roomId}|${threadId}`
+  window.clearTimeout(recountTimers.get(key))
+  recountTimers.set(
+    key,
+    window.setTimeout(async () => {
+      recountTimers.delete(key)
+      try {
+        const root = await c.fetchRoomEvent(room.roomId, threadId)
+        const bundled = root.unsigned?.['m.relations']?.[THREAD_RELATION_TYPE.name] as
+          | { count?: number; latest_event?: { event_id?: string } }
+          | undefined
+        if (typeof bundled?.count === 'number' && bundled.latest_event?.event_id) {
+          recounted.set(key, { count: bundled.count, lastId: bundled.latest_event.event_id })
+          quoteLoaded()
+        }
+      } catch {
+        // The SDK's count stands.
+      }
+    }, 1000),
+  )
+}
+
+function threadSummary(room: Room, ev: MatrixEvent, me: string): ThreadSummary | undefined {
+  const id = ev.getId()
+  const thread = id ? room.getThread(id) : null
+  if (!thread || thread.length === 0) return undefined
+  return {
+    count: threadCount(room, thread.id),
+    lastTs: thread.replyToEvent?.getTs() ?? ev.getTs(),
+    lastSenderName: thread.replyToEvent ? nameOf(room, thread.replyToEvent.getSender() ?? '') : '',
+    unread: threadUnread(room, thread.id, me),
+  }
+}
+
+/**
+ * Whether a thread has replies you haven't read: the homeserver's count
+ * for it (MSC3773), or a reply from someone else past your receipt in it.
+ */
+export function threadUnread(room: Room, threadId: string, me: string): boolean {
+  if (room.getThreadUnreadNotificationCount(threadId) > 0) return true
+  const thread = room.getThread(threadId)
+  const last = thread?.replyToEvent
+  const id = last?.getId()
+  if (!thread || !last || !id || last.status || last.getSender() === me) return false
+  return !thread.hasUserReadEvent(me, id)
+}
+
+// Replied-to messages the SDK doesn't have, fetched once each: the event,
+// null when it can't be had, or absent while the fetch runs.
+const fetchedQuotes = new Map<string, MatrixEvent | null>()
+const fetchingQuotes = new Set<string>()
+let quoteLoaded: () => void = () => undefined
+
+/** Runs when something fetched for the timeline lands: a quoted message, a thread's count. */
+export function onQuoteLoaded(fn: () => void) {
+  quoteLoaded = fn
+}
+
+function quoteOf(c: MatrixClient, room: Room, eventId: string): ReplyQuote {
+  const key = `${room.roomId}|${eventId}`
+  const ev = room.findEventById(eventId) ?? fetchedQuotes.get(key)
+  if (ev) return { eventId, senderName: nameOf(room, ev.getSender() ?? ''), text: quoteText(ev) }
+  if (ev === null) return { eventId, text: 'The message can’t be found' }
+  if (!fetchingQuotes.has(key)) {
+    fetchingQuotes.add(key)
+    c.fetchRoomEvent(room.roomId, eventId)
+      .then(async (raw) => {
+        const found = c.getEventMapper()(raw)
+        await c.decryptEventIfNeeded(found)
+        fetchedQuotes.set(key, found)
+      })
+      .catch(() => fetchedQuotes.set(key, null))
+      .finally(() => {
+        fetchingQuotes.delete(key)
+        quoteLoaded()
+      })
+  }
+  return { eventId, text: 'Loading…' }
+}
+
+export function buildTimeline(c: MatrixClient, room: Room, threadId?: string): TimelineItem[] {
   const me = c.getUserId() ?? ''
   const cryptoRunning = Boolean(c.getCrypto())
   // Messages after this were meant to be end-to-end encrypted.
   const encryptionSince = room.currentState.getStateEvents(EventType.RoomEncryption, '')?.getTs() ?? Infinity
-  const events = displayEvents(room)
+  const events = displayEvents(room, threadId)
 
   // An event is read when someone else's receipt is on it or on anything
   // after it, or someone else has written after it.
@@ -377,6 +591,8 @@ export function buildTimeline(c: MatrixClient, room: Room): TimelineItem[] {
     const encrypted = type === EventType.RoomMessageEncrypted || ev.isDecryptionFailure()
     const redacted = ev.isRedacted()
     const status = own ? statusOf(ev) : undefined
+    const replyId = redacted || encrypted ? undefined : replyTarget(ev)
+    const body = redacted ? '' : String(content.body ?? '')
     const item: MessageItem = {
       kind: 'message',
       key,
@@ -387,7 +603,7 @@ export function buildTimeline(c: MatrixClient, room: Room): TimelineItem[] {
       own,
       ts,
       msgtype: type === EventType.Sticker ? 'm.sticker' : String(content.msgtype ?? ''),
-      body: redacted ? '' : String(content.body ?? ''),
+      body: replyId ? stripReplyFallback(body) : body,
       media: redacted || encrypted ? undefined : type === EventType.Sticker ? stickerMedia(ev) : mediaOf(content),
       edited: Boolean(ev.replacingEventId()),
       redacted,
@@ -398,6 +614,9 @@ export function buildTimeline(c: MatrixClient, room: Room): TimelineItem[] {
       status: status === 'sent' && ev.getId() && readByOthers.has(ev.getId()!) ? 'read' : status,
       first: true,
       last: true,
+      reply: replyId ? quoteOf(c, room, replyId) : undefined,
+      reactions: reactionsOf(room, ev, me),
+      thread: threadId ? undefined : threadSummary(room, ev, me),
       event: ev,
     }
     if (prev && prev.sender === sender && ts - prev.ts < groupGapMs) {
@@ -424,15 +643,17 @@ function stickerMedia(ev: MatrixEvent): MediaInfo {
 }
 
 /** Whether there is older history to page back to. */
-export function canPaginateBack(room: Room): boolean {
-  const tl = room.getLiveTimeline()
+export function canPaginateBack(room: Room, threadId?: string): boolean {
+  const tl = liveTimelineOf(room, threadId)
+  if (!tl) return false
+  if (threadId) return tl.getPaginationToken(Direction.Backward) !== null
   if (tl.getEvents().some((e) => e.getType() === EventType.RoomCreate)) return false
   return tl.getPaginationToken(Direction.Backward) !== null
 }
 
 /** The newest event that came from the server, for read receipts. */
-export function lastSyncedEvent(room: Room): MatrixEvent | undefined {
-  const events = room.getLiveTimeline().getEvents()
+export function lastSyncedEvent(room: Room, threadId?: string): MatrixEvent | undefined {
+  const events = liveTimelineOf(room, threadId)?.getEvents() ?? []
   for (let i = events.length - 1; i >= 0; i--) {
     const ev = events[i]!
     if (ev.getId() && !ev.status && !ev.getId()!.startsWith('~')) return ev
diff --git a/apps/web/src/stores/chat.ts b/apps/web/src/stores/chat.ts
index a9b835c..c70a7d4 100644
--- a/apps/web/src/stores/chat.ts
+++ b/apps/web/src/stores/chat.ts
@@ -7,9 +7,11 @@ import {
   NotificationCountType,
   Preset,
   RoomEvent,
+  RoomType,
   RoomMemberEvent,
   RoomStateEvent,
   SyncState,
+  ThreadEvent,
   UserEvent,
   Visibility,
   type MatrixClient,
@@ -19,7 +21,7 @@ import {
 import { defineStore } from 'pinia'
 import { computed, ref, shallowRef, watch } from 'vue'
 import { client } from '@/matrix/client'
-import { isVideoCallInvite } from '@/matrix/timeline'
+import { isVideoCallInvite, onQuoteLoaded, recountThread } from '@/matrix/timeline'
 import { onShieldsChanged } from '@/matrix/trust'
 
 // The room list and the client's connection state, as plain reactive data
@@ -49,9 +51,20 @@ export interface InviteSummary {
   inviter?: string
   inviterName?: string
   isDirect: boolean
+  isSpace: boolean
   ts: number
 }
 
+/** A space you're in: a room that groups other rooms (m.space). */
+export interface SpaceSummary {
+  roomId: string
+  name: string
+  avatarMxc?: string
+  topic: string
+  /** The rooms and subspaces it lists (m.space.child). */
+  children: string[]
+}
+
 /** The text a room list shows for an event, or null when it shows nothing. */
 export function describeEvent(ev: MatrixEvent, me: string, room: Room, withSender: boolean): string | null {
   if (ev.isRedacted()) return 'Message deleted'
@@ -158,10 +171,30 @@ function summarizeInvite(c: MatrixClient, room: Room): InviteSummary {
     inviter,
     inviterName: inviter ? (room.getMember(inviter)?.name ?? inviter) : undefined,
     isDirect: Boolean(memberEvent?.getContent().is_direct),
+    isSpace: room.isSpaceRoom(),
     ts: memberEvent?.getTs() ?? 0,
   }
 }
 
+/** The rooms a space lists: m.space.child events with a non-empty via. */
+export function spaceChildren(space: Room): string[] {
+  return space.currentState
+    .getStateEvents(EventType.SpaceChild)
+    .filter((ev) => Array.isArray(ev.getContent().via) && ev.getContent().via.length > 0)
+    .map((ev) => ev.getStateKey() ?? '')
+    .filter(Boolean)
+}
+
+function summarizeSpace(space: Room): SpaceSummary {
+  return {
+    roomId: space.roomId,
+    name: space.name,
+    avatarMxc: space.getMxcAvatarUrl() ?? undefined,
+    topic: String(space.currentState.getStateEvents(EventType.RoomTopic, '')?.getContent().topic ?? ''),
+    children: spaceChildren(space),
+  }
+}
+
 const encryptionState = {
   type: EventType.RoomEncryption,
   state_key: '',
@@ -190,6 +223,20 @@ export const useChat = defineStore('chat', () => {
     const timer = window.setTimeout(fire, 250)
   }
 
+  // Threads and per-thread counts are only emitted on their room, not
+  // re-emitted by the client: each room is listened to once.
+  const watched = new WeakSet<Room>()
+  function watchRoom(room: Room) {
+    if (watched.has(room)) return
+    watched.add(room)
+    for (const e of [ThreadEvent.New, ThreadEvent.Update, ThreadEvent.NewReply, ThreadEvent.Delete, RoomEvent.UnreadNotifications] as const) {
+      room.on(e as ThreadEvent.Update, bump)
+    }
+    room.on(ThreadEvent.NewReply, (thread, ev) => {
+      if (!ev.status && attached) recountThread(attached, room, thread.id)
+    })
+  }
+
   function onSync(state: SyncState) {
     syncState.value = state
     if (state === SyncState.Prepared || state === SyncState.Syncing) ready.value = true
@@ -205,6 +252,9 @@ export const useChat = defineStore('chat', () => {
     syncState.value = c.getSyncState()
     c.on(ClientEvent.Sync, onSync)
     onShieldsChanged(bump)
+    onQuoteLoaded(bump)
+    for (const room of c.getRooms()) watchRoom(room)
+    c.on(ClientEvent.Room, watchRoom)
     for (const e of [
       ClientEvent.Room,
       ClientEvent.DeleteRoom,
@@ -242,11 +292,38 @@ export const useChat = defineStore('chat', () => {
     const direct = directRooms(c)
     return c
       .getRooms()
-      .filter((r) => r.getMyMembership() === KnownMembership.Join && !isReplaced(r))
+      .filter((r) => r.getMyMembership() === KnownMembership.Join && !isReplaced(r) && !r.isSpaceRoom())
       .map((r) => summarize(c, r, direct))
       .sort((a, b) => b.ts - a.ts)
   })
 
+  const spaces = computed<SpaceSummary[]>(() => {
+    void version.value
+    if (!attached) return []
+    return attached
+      .getRooms()
+      .filter((r) => r.getMyMembership() === KnownMembership.Join && r.isSpaceRoom() && !isReplaced(r))
+      .map(summarizeSpace)
+      .sort((a, b) => a.name.localeCompare(b.name))
+  })
+
+  /** Every room in a space and in its subspaces you're in. */
+  function roomsInSpace(spaceId: string): Set<string> {
+    const bySpace = new Map(spaces.value.map((s) => [s.roomId, s]))
+    const out = new Set<string>()
+    const seen = new Set<string>()
+    const walk = (id: string) => {
+      if (seen.has(id)) return
+      seen.add(id)
+      for (const child of bySpace.get(id)?.children ?? []) {
+        if (bySpace.has(child)) walk(child)
+        else out.add(child)
+      }
+    }
+    walk(spaceId)
+    return out
+  }
+
   const invites = computed<InviteSummary[]>(() => {
     void version.value
     if (!attached) return []
@@ -278,24 +355,63 @@ export const useChat = defineStore('chat', () => {
     else if (!offline.value) offlineTimer = window.setTimeout(() => (offline.value = true), 5000)
   })
 
+  /** The servers to reach a room through: this homeserver. */
+  function via(): string[] {
+    const server = client().getDomain()
+    return server ? [server] : []
+  }
+
   async function createRoom(opts: {
     name: string
     topic?: string
     isPublic: boolean
     alias?: string
     encrypted: boolean
+    /** A space to create it in: the room names it as its parent, the space lists it. */
+    spaceId?: string
   }): Promise<string> {
+    const initialState: { type: string; state_key: string; content: Record<string, unknown> }[] = []
+    if (opts.encrypted) initialState.push(encryptionState)
+    if (opts.spaceId) initialState.push({ type: EventType.SpaceParent, state_key: opts.spaceId, content: { via: via(), canonical: true } })
     const res = await client().createRoom({
       name: opts.name.trim(),
       topic: opts.topic?.trim() || undefined,
       preset: opts.isPublic ? Preset.PublicChat : Preset.PrivateChat,
       visibility: opts.isPublic ? Visibility.Public : Visibility.Private,
       room_alias_name: opts.isPublic && opts.alias ? opts.alias.trim() : undefined,
-      initial_state: opts.encrypted ? [encryptionState] : undefined,
+      initial_state: initialState.length ? initialState : undefined,
+    })
+    if (opts.spaceId) await addToSpace(opts.spaceId, res.room_id)
+    return res.room_id
+  }
+
+  /**
+   * A new space. A public one is in the room directory and anyone can
+   * join; a private one is by invite. Only admins post in it: a space's
+   * timeline holds its settings, not a conversation.
+   */
+  async function createSpace(opts: { name: string; topic?: string; isPublic: boolean }): Promise<string> {
+    const res = await client().createRoom({
+      name: opts.name.trim(),
+      topic: opts.topic?.trim() || undefined,
+      preset: opts.isPublic ? Preset.PublicChat : Preset.PrivateChat,
+      visibility: opts.isPublic ? Visibility.Public : Visibility.Private,
+      creation_content: { type: RoomType.Space },
+      power_level_content_override: { events_default: 100, invite: opts.isPublic ? 0 : 50 },
     })
     return res.room_id
   }
 
+  /** Lists a room in a space. */
+  async function addToSpace(spaceId: string, roomId: string) {
+    await client().sendStateEvent(spaceId, EventType.SpaceChild, { via: via() }, roomId)
+  }
+
+  /** Takes a room out of a space: an empty m.space.child. */
+  async function removeFromSpace(spaceId: string, roomId: string) {
+    await client().sendStateEvent(spaceId, EventType.SpaceChild, {}, roomId)
+  }
+
   /** Whether this session can encrypt a new room: the crypto stack is running. */
   const canEncrypt = computed(() => {
     void version.value
@@ -374,13 +490,18 @@ export const useChat = defineStore('chat', () => {
     syncState,
     offline,
     rooms,
+    spaces,
     invites,
     totalUnread,
+    roomsInSpace,
     canEncrypt,
     attach,
     detach,
     bump,
     createRoom,
+    createSpace,
+    addToSpace,
+    removeFromSpace,
     startDirect,
     sharedRoomWith,
     join,
diff --git a/apps/web/src/views/RoomView.vue b/apps/web/src/views/RoomView.vue
index 8ec017d..92dc351 100644
--- a/apps/web/src/views/RoomView.vue
+++ b/apps/web/src/views/RoomView.vue
@@ -1,16 +1,19 @@
 <script setup lang="ts">
 import { KnownMembership } from 'matrix-js-sdk'
-import { computed, onBeforeUnmount, ref, watch } from 'vue'
+import { computed, ref, shallowRef, watch } from 'vue'
 import { useRouter } from 'vue-router'
-import { WBanner, WButton, WComposer, WIcon, WIconButton, WSpinner } from '@warp/ui'
+import { WBanner, WButton, WIcon, WIconButton } from '@warp/ui'
 import MxcAvatar from '@/components/MxcAvatar.vue'
+import RoomComposer from '@/components/RoomComposer.vue'
 import RoomInfoPane from '@/components/RoomInfoPane.vue'
 import RoomTimeline from '@/components/RoomTimeline.vue'
-import { drafts } from '@/lib/drafts'
-import { errorMessage, formatBytes } from '@/lib/format'
-import { client, cryptoError } from '@/matrix/client'
+import SpaceView from '@/components/SpaceView.vue'
+import ThreadPane from '@/components/ThreadPane.vue'
+import ThreadsPane from '@/components/ThreadsPane.vue'
+import { errorMessage } from '@/lib/format'
+import { client } from '@/matrix/client'
 import { acceptIdentity, identityWarnings, withdrawVerification, type IdentityWarning } from '@/matrix/crypto'
-import { sendFile, sendText } from '@/matrix/send'
+import type { MessageItem } from '@/matrix/timeline'
 import { useCall } from '@/stores/call'
 import { useChat } from '@/stores/chat'
 import { useSecurity } from '@/stores/security'
@@ -46,13 +49,26 @@ const memberCount = computed(() => {
   void chat.version
   return room.value?.getJoinedMemberCount() ?? 0
 })
-const canSend = computed(() => {
+const isSpace = computed(() => {
   void chat.version
-  const r = room.value
-  return r ? r.currentState.maySendMessage(me) : false
+  return Boolean(room.value?.isSpaceRoom())
 })
 
-const infoOpen = ref(false)
+// The side pane: room info, the room's threads, or one thread.
+const pane = ref<'info' | 'threads' | 'thread' | null>(null)
+const infoOpen = computed(() => pane.value === 'info')
+const threadId = ref('')
+function toggle(which: 'info' | 'threads') {
+  pane.value = pane.value === which ? null : which
+}
+function openThread(id: string) {
+  threadId.value = id
+  pane.value = 'thread'
+}
+const threadsUnread = computed(() => {
+  void chat.version
+  return Boolean(room.value?.hasThreadUnreadNotification())
+})
 
 // Calls, in any room with someone else in it: starting one in a room of
 // two rings the other person. One call at a time, wherever it is.
@@ -113,24 +129,6 @@ async function ackWarning(verifyAgain = false) {
   }
 }
 
-const text = ref(drafts.get(props.roomId) ?? '')
-watch(text, (v) => (v ? drafts.set(props.roomId, v) : drafts.delete(props.roomId)))
-
-// Typing notifications: on while there is text, renewed every few seconds,
-// off when it's sent or cleared.
-let typingUntil = 0
-function setTyping(on: boolean) {
-  const now = Date.now()
-  if (on && now < typingUntil) return
-  if (!on && typingUntil === 0) return
-  typingUntil = on ? now + 4000 : 0
-  client()
-    .sendTyping(props.roomId, on, on ? 6000 : 0)
-    .catch(() => undefined)
-}
-watch(text, (v) => setTyping(Boolean(v.trim())))
-onBeforeUnmount(() => setTyping(false))
-
 const typingNames = computed(() => {
   void chat.version
   return (room.value?.getMembers() ?? []).filter((m) => m.typing && m.userId !== me).map((m) => m.name)
@@ -143,60 +141,14 @@ const typingText = computed(() => {
   return `${n.length} people are typing…`
 })
 
-function send(body: string) {
-  setTyping(false)
-  sendText(client(), props.roomId, body).catch((e) => {
-    // A failed send stays in the timeline with Retry; say why once.
-    toasts.error(errorMessage(e))
-  })
-}
-
-// Uploads.
-const uploads = ref<{ id: number; name: string; loaded: number; total: number }[]>([])
-let uploadId = 0
-let maxUpload: number | null | undefined
-
-async function sendFiles(list: FileList | File[]) {
-  // Copied first: the composer clears its file input (and so the live
-  // FileList) as soon as it has handed the files over.
-  const files = Array.from(list)
-  const r = room.value
-  if (!r) return
-  if (maxUpload === undefined) {
-    maxUpload = await client()
-      .getMediaConfig(true)
-      .then((c) => c['m.upload.size'] ?? null)
-      .catch(() => null)
-  }
-  for (const file of Array.from(files)) {
-    if (maxUpload && file.size > maxUpload) {
-      toasts.error(`${file.name} is ${formatBytes(file.size)}; this homeserver takes files up to ${formatBytes(maxUpload)}.`)
-      continue
-    }
-    const id = ++uploadId
-    uploads.value.push({ id, name: file.name, loaded: 0, total: file.size })
-    sendFile(client(), r, file, (p) => {
-      const u = uploads.value.find((x) => x.id === id)
-      if (u) Object.assign(u, { loaded: p.loaded, total: p.total })
-    })
-      .catch((e) => toasts.error(`Can’t send ${file.name}: ${errorMessage(e)}`))
-      .finally(() => (uploads.value = uploads.value.filter((x) => x.id !== id)))
-  }
-}
-
-function onPaste(e: ClipboardEvent) {
-  const files = e.clipboardData?.files
-  if (files && files.length) {
-    e.preventDefault()
-    sendFiles(files)
-  }
-}
+const replyTo = shallowRef<MessageItem | null>(null)
+const composer = ref<InstanceType<typeof RoomComposer>>()
 
 const dragging = ref(false)
 function onDrop(e: DragEvent) {
   dragging.value = false
   const files = e.dataTransfer?.files
-  if (files && files.length && canSend.value && !cryptoMissing.value) sendFiles(files)
+  if (files && files.length && composer.value?.canWrite) composer.value.sendFiles(files)
 }
 
 // Invites and rooms you're not in.
@@ -237,6 +189,10 @@ async function joinHere() {
 }
 
 const placeholder = computed(() => `Message ${summary.value?.name ?? 'this room'}`)
+const invitedTo = computed(() => {
+  if (invite.value?.isDirect) return 'to chat'
+  return invite.value?.isSpace ? 'to this space' : 'to this room'
+})
 </script>
 
 <template>
@@ -250,7 +206,7 @@ const placeholder = computed(() => `Message ${summary.value?.name ?? 'this room'
           <template v-if="invite.inviter">
             <span class="body-strong text-ink">{{ invite.inviterName }}</span>
             <span v-if="invite.inviterName !== invite.inviter" class="mxid"> ({{ invite.inviter }})</span>
-            invited you {{ invite.isDirect ? 'to chat' : 'to this room' }}.
+            invited you {{ invitedTo }}.
           </template>
           <template v-else>You’re invited to this room.</template>
         </p>
@@ -277,17 +233,19 @@ const placeholder = computed(() => `Message ${summary.value?.name ?? 'this room'
     </div>
   </div>
 
+  <SpaceView v-else-if="isSpace" :space="room" />
+
   <div v-else class="flex min-h-0 flex-1">
     <div
       class="relative flex min-w-0 flex-1 flex-col"
-      @dragover.prevent="dragging = canSend && !cryptoMissing"
+      @dragover.prevent="dragging = Boolean(composer?.canWrite)"
       @dragleave.self="dragging = false"
       @drop.prevent="onDrop"
     >
       <header class="flex h-16 shrink-0 items-center gap-3 border-b border-line bg-surface px-2 md:px-4">
         <WIconButton class="md:hidden" icon="chevron-left" label="Back to rooms" @click="router.push('/')" />
         <MxcAvatar :name="summary?.name ?? room.name" :mxc="summary?.avatarMxc" :size="36" :presence="summary?.presence" />
-        <button type="button" class="min-w-0 flex-1 cursor-pointer border-0 bg-transparent p-0 text-left" @click="infoOpen = !infoOpen">
+        <button type="button" class="min-w-0 flex-1 cursor-pointer border-0 bg-transparent p-0 text-left" @click="toggle('info')">
           <span class="flex items-center gap-2">
             <h1 class="heading m-0 truncate">{{ summary?.name ?? room.name }}</h1>
             <WIcon v-if="encrypted" name="lock" :size="16" class="shrink-0 text-teal-text" label="End-to-end encrypted" />
@@ -304,7 +262,12 @@ const placeholder = computed(() => `Message ${summary.value?.name ?? 'this room'
           <WIconButton icon="phone" label="Voice call" :disabled="inCall" @click="joinCall(false)" />
           <WIconButton icon="video" label="Video call" :disabled="inCall" @click="joinCall(true)" />
         </template>
-        <WIconButton icon="users" :label="infoOpen ? 'Hide room info' : 'Room info'" :aria-pressed="infoOpen" @click="infoOpen = !infoOpen" />
+        <span class="relative">
+          <WIconButton icon="thread" label="Threads" :aria-pressed="pane === 'threads' || pane === 'thread'" @click="toggle('threads')" />
+          <span v-if="threadsUnread" class="pointer-events-none absolute top-2 right-2 h-2 w-2 rounded-full bg-teal-strong" />
+          <span v-if="threadsUnread" class="wp-sr">: unread replies</span>
+        </span>
+        <WIconButton icon="users" :label="infoOpen ? 'Hide room info' : 'Room info'" :aria-pressed="infoOpen" @click="toggle('info')" />
       </header>
 
       <div v-if="callHere && canCall && !inThisCall" class="shrink-0 border-b border-line bg-surface px-4 py-2">
@@ -317,18 +280,12 @@ const placeholder = computed(() => `Message ${summary.value?.name ?? 'this room'
         </WBanner>
       </div>
 
-      <RoomTimeline :room="room" :is-direct="summary?.isDirect ?? false" />
+      <RoomTimeline :room="room" :is-direct="summary?.isDirect ?? false" @reply="(item) => (replyTo = item)" @open-thread="openThread" />
 
       <div class="shrink-0 bg-surface-sunken px-4 pt-2 pb-4">
         <div class="mx-auto flex max-w-4xl flex-col gap-2">
           <p class="caption m-0 h-4 text-ink-muted" aria-live="polite">{{ typingText }}</p>
 
-          <div v-for="u in uploads" :key="u.id" class="flex items-center gap-3 rounded-md bg-surface-raised px-3 py-2">
-            <WSpinner :size="16" />
-            <span class="small min-w-0 flex-1 truncate">Uploading {{ u.name }}</span>
-            <span class="caption text-ink-muted tabular-nums">{{ u.total ? Math.round((u.loaded / u.total) * 100) : 0 }}%</span>
-          </div>
-
           <WBanner
             v-if="warning && !cryptoMissing"
             :tone="warning.kind === 'verifiedReset' ? 'danger' : 'warning'"
@@ -346,13 +303,7 @@ const placeholder = computed(() => `Message ${summary.value?.name ?? 'this room'
               <WButton size="sm" :loading="acking" @click="ackWarning()">{{ warning.kind === 'verifiedReset' ? 'Withdraw verification' : 'OK' }}</WButton>
             </span>
           </WBanner>
-          <WBanner v-if="cryptoMissing" tone="warning" title="End-to-end encryption didn’t start">
-            This browser can’t read or send messages in this room. Reload the page to try again{{ cryptoError() ? ` (${cryptoError()})` : '' }}.
-          </WBanner>
-          <WBanner v-else-if="!canSend" tone="info">You don’t have permission to post in this room.</WBanner>
-          <div v-else @paste="onPaste">
-            <WComposer v-model="text" :placeholder="placeholder" :encrypted="encrypted" @send="send" @attach="sendFiles" />
-          </div>
+          <RoomComposer ref="composer" v-model:reply-to="replyTo" :room="room" :placeholder="placeholder" />
         </div>
       </div>
 
@@ -364,6 +315,16 @@ const placeholder = computed(() => `Message ${summary.value?.name ?? 'this room'
       </div>
     </div>
 
-    <RoomInfoPane v-if="infoOpen" :room="room" :is-direct="summary?.isDirect ?? false" @close="infoOpen = false" />
+    <RoomInfoPane v-if="pane === 'info'" :room="room" :is-direct="summary?.isDirect ?? false" @close="pane = null" />
+    <ThreadsPane v-else-if="pane === 'threads'" :room="room" @open="openThread" @close="pane = null" />
+    <ThreadPane
+      v-else-if="pane === 'thread' && threadId"
+      :key="threadId"
+      :room="room"
+      :thread-id="threadId"
+      :is-direct="summary?.isDirect ?? false"
+      @back="pane = 'threads'"
+      @close="pane = null"
+    />
   </div>
 </template>
diff --git a/apps/web/vite.config.ts b/apps/web/vite.config.ts
index 1804c63..f54a3dd 100644
--- a/apps/web/vite.config.ts
+++ b/apps/web/vite.config.ts
@@ -1,3 +1,6 @@
+import { existsSync } from 'node:fs'
+import { createRequire } from 'node:module'
+import { dirname, join } from 'node:path'
 import { fileURLToPath } from 'node:url'
 import tailwindcss from '@tailwindcss/vite'
 import vue from '@vitejs/plugin-vue'
@@ -8,10 +11,33 @@ import { defineConfig } from 'vite'
 // know where the homeserver is. WARP_WEB_HOMESERVER points it elsewhere.
 const homeserver = process.env.WARP_WEB_HOMESERVER ?? 'http://localhost:8008'
 
-export default defineConfig({
+// The Rust crypto loads its .wasm relative to its own module
+// (new URL(..., import.meta.url)). Vite's dependency pre-bundling moves the
+// module into node_modules/.vite/deps without the .wasm, so the dev server
+// answered with index.html and initRustCrypto failed. The dev server serves
+// the package as it is instead. It is only matrix-js-sdk's dependency, which
+// pnpm doesn't make reachable from the pre-bundled matrix-js-sdk, so it is
+// aliased to the very copy matrix-js-sdk resolves. A production build is
+// bundled by Rollup and needs neither.
+const cryptoWasm = '@matrix-org/matrix-sdk-crypto-wasm'
+
+function cryptoWasmDir(): string {
+  const sdk = createRequire(import.meta.url).resolve('matrix-js-sdk')
+  let dir = dirname(createRequire(sdk).resolve(cryptoWasm))
+  while (!existsSync(join(dir, 'package.json'))) dir = dirname(dir)
+  return dir
+}
+
+export default defineConfig(({ command }) => ({
   plugins: [vue(), tailwindcss()],
   resolve: {
-    alias: { '@': fileURLToPath(new URL('./src', import.meta.url)) },
+    alias: {
+      '@': fileURLToPath(new URL('./src', import.meta.url)),
+      ...(command === 'serve' ? { [cryptoWasm]: cryptoWasmDir() } : {}),
+    },
+  },
+  optimizeDeps: {
+    exclude: [cryptoWasm],
   },
   build: {
     // Embedded into the warp-web binary by client/ (and the desktop app).
@@ -30,4 +56,4 @@ export default defineConfig({
       '/.well-known/matrix/client': { target: homeserver, changeOrigin: true },
     },
   },
-})
+}))
diff --git a/server/internal/setup/routes.go b/server/internal/setup/routes.go
index 25b5760..0f804b9 100644
--- a/server/internal/setup/routes.go
+++ b/server/internal/setup/routes.go
@@ -20,7 +20,16 @@ import (
 // client to be used. Advertising v1.11 or later is also what makes a client
 // use the authenticated media endpoints, the only download endpoints Warp
 // serves by default.
-var SpecVersions = []string{"v1.13", "v1.14"}
+//
+// The earlier v1.x versions are listed too: each is a subset of the
+// later ones, and clients test for the exact version a feature came in.
+// matrix-js-sdk only turns on threads (v1.3), unread counts per thread
+// (v1.4), recursive relations (v1.10) and intentional mentions (v1.7)
+// when that version is in the list.
+var SpecVersions = []string{
+	"v1.1", "v1.2", "v1.3", "v1.4", "v1.5", "v1.6", "v1.7",
+	"v1.8", "v1.9", "v1.10", "v1.11", "v1.12", "v1.13", "v1.14",
+}
 
 // UnstableFeatures advertises MSC support. Simplified Sliding Sync
 // (MSC4186) is how Element X and the matrix-rust-sdk find out they can