diff --git a/docs/chat-foederation-konzept.md b/docs/chat-foederation-konzept.md
index 61602b55..fa7f7008 100644
--- a/docs/chat-foederation-konzept.md
+++ b/docs/chat-foederation-konzept.md
@@ -285,6 +285,8 @@ Als erster nutzbarer Meilenstein wird **ein abgegrenzter Projekt-Unterraum mit e
 
 Unter **Kommunikation → Zusammenarbeit** können interne Teilnehmer ihre eigenen Unterräume und Einladungen öffnen. Projektverantwortliche mit `organisation.projects.write` beziehungsweise Administratoren können Unterräume verwalten und freie Partnerchats anlegen. Der Projektkopf verlinkt ebenfalls auf die Zusammenarbeit. Die Verwaltung eines Raums berechtigt weiterhin nicht zum Lesen seiner Nachrichten; nur der Ersteller wird bei der Anlage automatisch Teilnehmer.
 
+Unter **Kommunikation → Chat** erscheinen diese Unterräume ebenfalls in der Raumgruppe **Zusammenarbeit**, einschließlich instanzübergreifender Räume und Ungelesen-Zähler. Einladungen lassen sich dort direkt annehmen oder ablehnen; nach der Annahme öffnet sich der Raum. Nachrichten, Teilnehmer und ausdrücklich freigegebene Projektinformationen werden direkt im normalen Chat angezeigt. **Details und Freigaben** öffnet die bestehende Verwaltungsansicht für den ausgewählten Raum; **Im Chat öffnen** führt zurück. Sichtbar sind ausschließlich Räume mit eigener Teilnahme, keine lediglich verwaltbaren Räume. Interne Chaträume und Unterräume behalten ihre getrennten Berechtigungsprüfungen.
+
 Einladungen unterstützen eigene Mitarbeiter, persönliche FEDEO-Kontaktadressen und Gäste ohne FEDEO-Konto. Eine Kontaktadresse bezeichnet immer genau einen Benutzer in einem Heimatmandanten. Sie lässt sich in der Zusammenarbeit kopieren. Kontakte anderer Mandanten derselben Instanz benötigen keinen Server-Server-Transport. Fremde Instanzen müssen auf beiden Seiten im jeweiligen Mandanten unter **Partnerinstanzen** freigegeben sein. Beim Empfänger erscheint ein Angebot, das ausdrücklich angenommen oder abgelehnt wird.
 
 Gäste erhalten ausschließlich einen Projekt-Unterraum. Der Link öffnet `/guest`; ein achtstelliger, zehn Minuten gültiger Einmalcode wird an die eingeladene Adresse geschickt. Nach fünf falschen Versuchen ist der Code gesperrt. Die eigene Gastsitzung gilt zwölf Stunden und wird nicht als reguläres FEDEO-Token akzeptiert. Für spätere Anmeldungen bietet die Gastoberfläche eine als Lesezeichen speicherbare Adresse mit öffentlicher Workspace-ID an. Ohne gewähltes Ablaufdatum gilt die Teilnahme eines Gasts 30 Tage. Der Einladungslink allein erlaubt keinen Inhaltszugriff.
diff --git a/frontend/components/CollaborationRoom.vue b/frontend/components/CollaborationRoom.vue
index effbb3bd..9884e2b5 100644
--- a/frontend/components/CollaborationRoom.vue
+++ b/frontend/components/CollaborationRoom.vue
@@ -1,5 +1,5 @@
 <script setup>
-const props = defineProps({ roomId: { type: String, required: true }, api: { type: Function, required: true }, base: { type: String, default: '/api/collaboration' } })
+const props = defineProps({ roomId: { type: String, required: true }, api: { type: Function, required: true }, base: { type: String, default: '/api/collaboration' }, embedded: { type: Boolean, default: false } })
 const emit = defineEmits(['changed', 'unauthorized'])
 const room = ref(null), messages = ref([]), members = ref([]), draft = ref(''), error = ref('')
 const loading = ref(false), sending = ref(false), olderLoading = ref(false), hasMore = ref(false), viewport = ref(null)
@@ -95,9 +95,9 @@ onBeforeUnmount(() => { ++generation; clearInterval(timer) })
 </script>
 
 <template>
-  <section class="flex min-h-[32rem] min-w-0 flex-col rounded-xl border border-gray-200 bg-white dark:border-gray-700 dark:bg-gray-900">
+  <section class="flex min-w-0 flex-col bg-white dark:bg-gray-900" :class="embedded ? 'min-h-0 overflow-hidden' : 'min-h-[32rem] rounded-xl border border-gray-200 dark:border-gray-700'">
     <header class="border-b border-gray-200 p-4 dark:border-gray-700">
-      <h2 class="text-lg font-semibold">{{ room?.name || 'Unterraum' }}</h2>
+      <div class="flex flex-wrap items-center justify-between gap-2"><h2 class="text-lg font-semibold">{{ room?.name || 'Unterraum' }}</h2><slot name="actions" /></div>
       <p v-if="room?.topic" class="mt-1 text-sm text-gray-500">{{ room.topic }}</p>
       <p v-if="members.length" class="mt-2 text-xs text-gray-500">{{ members.map(m => `${m.name}${m.kind === 'guest' ? ' (Gast)' : ''}`).join(' · ') }}</p>
     </header>
@@ -110,7 +110,7 @@ onBeforeUnmount(() => { ++generation; clearInterval(timer) })
       <p v-if="room.overview.date">Termin: {{ room.overview.date }}</p>
       <p v-if="room.overview.description" class="mt-1 whitespace-pre-wrap">{{ room.overview.description }}</p>
     </div>
-    <div ref="viewport" class="max-h-[60vh] min-h-[18rem] flex-1 space-y-3 overflow-y-auto p-4" aria-live="polite">
+    <div ref="viewport" class="flex-1 space-y-3 overflow-y-auto p-4" :class="embedded ? 'min-h-0' : 'max-h-[60vh] min-h-[18rem]'" aria-live="polite">
       <p v-if="loading" class="text-sm text-gray-500">Nachrichten werden geladen …</p>
       <button v-if="hasMore" class="block w-full text-center text-sm text-green-700 underline" :disabled="olderLoading" @click="older">{{ olderLoading ? 'Wird geladen …' : 'Ältere Nachrichten laden' }}</button>
       <p v-if="room && !loading && !messages.length" class="py-12 text-center text-sm text-gray-500">Hier beginnt eure Unterhaltung. Du siehst Nachrichten ab deinem Beitritt.</p>
diff --git a/frontend/pages/communication/chat.vue b/frontend/pages/communication/chat.vue
index 69ad2503..d6775188 100644
--- a/frontend/pages/communication/chat.vue
+++ b/frontend/pages/communication/chat.vue
@@ -1,219 +1,231 @@
 <script setup>
 const toast = useToast()
 const { $api } = useNuxtApp()
+const auth = useAuthStore()
 const route = useRoute()
 const router = useRouter()
 
-const baseRooms = ref([])
-const projectRooms = ref([])
-const directRooms = ref([])
-const unreadRooms = ref({})
-const messages = ref([])
-const members = ref([])
-const draft = ref("")
-const activeRoomKey = ref(typeof route.query.room === "string" ? route.query.room : "allgemein")
-const loading = ref(true)
-const roomLoading = ref(false)
-const sending = ref(false)
-const creating = ref(false)
-const showRoomForm = ref(false)
-const messagesViewport = ref(null)
-const roomForm = reactive({ name: "", key: "", topic: "" })
-let pollTimer = null
-let pollBusy = false
-let syncAfterId = 0
-
-const normalizeRoomKey = (value) => String(value || "")
-  .toLowerCase()
-  .normalize("NFKD")
-  .replace(/[\u0300-\u036f]/g, "")
-  .replace(/ß/g, "ss")
-  .replace(/[^a-z0-9._=-]+/g, "_")
-  .replace(/_+/g, "_")
-  .replace(/^[._=-]+|[._=-]+$/g, "")
+const baseRooms = ref([]), projectRooms = ref([]), directRooms = ref([]), collaborationRooms = ref([])
+const invitations = ref([]), invitationBusy = ref(''), collaborationError = ref(''), listError = ref('')
+const unreadRooms = ref({}), messages = ref([]), members = ref([]), draft = ref('')
+const requestedRoom = () => typeof route.query.room === 'string' ? route.query.room : 'allgemein'
+const activeRoomKey = ref(requestedRoom())
+const loading = ref(true), roomLoading = ref(false), sending = ref(false), creating = ref(false)
+const showRoomForm = ref(false), messagesViewport = ref(null)
+const roomForm = reactive({ name: '', key: '', topic: '' })
+const session = computed(() => `${auth.user?.id || auth.user?.user_id || ''}:${auth.activeTenant || ''}`)
+const hasSession = computed(() => !!auth.user && !!auth.activeTenant)
+let pollTimer, listTimer, pollBusy = false, mounted = false, disposed = false
+let scopeVersion = 0, roomVersion = 0, collaborationRequest = 0, nativeListRequest = 0, syncAfterId = 0
+const currentScope = version => !disposed && version === scopeVersion
+const selection = () => ({ scope: scopeVersion, version: roomVersion, key: activeRoomKey.value })
+const currentSelection = request => currentScope(request.scope) && request.version === roomVersion && request.key === activeRoomKey.value
+const errorMessage = error => error?.data?.message || error?.data?.error || 'Die Anfrage konnte nicht ausgeführt werden.'
+// Colons cannot occur in normalized internal room keys, so both room types stay unambiguous.
+const collaborationKey = id => `collaboration:${id}`
+const nativeEndpoint = (key, suffix = '') => `/api/communication/chat/rooms/${encodeURIComponent(key)}${suffix}`
 
+const normalizeRoomKey = value => String(value || '')
+  .toLowerCase().normalize('NFKD').replace(/[\u0300-\u036f]/g, '').replace(/ß/g, 'ss')
+  .replace(/[^a-z0-9._=-]+/g, '_').replace(/_+/g, '_').replace(/^[._=-]+|[._=-]+$/g, '')
 const roomKeyPreview = computed(() => normalizeRoomKey(roomForm.key || roomForm.name))
 const decoratedRooms = computed(() => [
-  ...baseRooms.value.map((room) => ({ ...room, group: "Räume", icon: "i-heroicons-chat-bubble-left-right" })),
-  ...projectRooms.value.map((room) => ({ ...room, group: "Projekte", icon: "i-heroicons-briefcase" })),
-  ...directRooms.value.map((room) => ({ ...room, group: "Direktnachrichten", icon: "i-heroicons-user-circle" }))
-].map((room) => ({ ...room, unread: unreadRooms.value[room.key]?.count || 0 })))
-const groupedRooms = computed(() => ["Räume", "Projekte", "Direktnachrichten"]
-  .map((label) => ({ label, rooms: decoratedRooms.value.filter((room) => room.group === label) }))
-  .filter((group) => group.rooms.length))
-const activeRoom = computed(() => decoratedRooms.value.find((room) => room.key === activeRoomKey.value) || null)
-const roomEndpoint = computed(() => `/api/communication/chat/rooms/${encodeURIComponent(activeRoomKey.value)}`)
+  ...[
+    ...baseRooms.value.map(room => ({ ...room, group: 'Räume', icon: 'i-heroicons-chat-bubble-left-right' })),
+    ...projectRooms.value.map(room => ({ ...room, group: 'Projekte', icon: 'i-heroicons-briefcase' })),
+    ...directRooms.value.map(room => ({ ...room, group: 'Direktnachrichten', icon: 'i-heroicons-user-circle' }))
+  ].map(room => ({ ...room, source: 'internal', unread: unreadRooms.value[room.key]?.count || 0 })),
+  ...collaborationRooms.value.map(room => ({ ...room, key: collaborationKey(room.id), source: 'collaboration', exists: true,
+    group: 'Zusammenarbeit', icon: 'i-heroicons-user-group', unread: room.unread || 0 }))
+])
+const groupedRooms = computed(() => ['Räume', 'Projekte', 'Direktnachrichten', 'Zusammenarbeit']
+  .map(label => ({ label, rooms: decoratedRooms.value.filter(room => room.group === label) }))
+  .filter(group => group.rooms.length))
+const activeRoom = computed(() => decoratedRooms.value.find(room => room.key === activeRoomKey.value) || null)
+const isCollaboration = computed(() => activeRoom.value?.source === 'collaboration')
+const isInternal = computed(() => activeRoom.value?.source === 'internal')
 
-const mergeMessages = (incoming) => {
-  const byId = new Map(messages.value.map((message) => [message.id, message]))
+function clearMessages() { messages.value = []; members.value = []; draft.value = ''; syncAfterId = 0; roomLoading.value = false; sending.value = false }
+function mergeMessages(incoming) {
+  const byId = new Map(messages.value.map(message => [message.id, message]))
   for (const message of incoming || []) byId.set(message.id, message)
-  messages.value = Array.from(byId.values()).sort((a, b) => Number(a.id) - Number(b.id))
-  syncAfterId = Number(messages.value.at(-1)?.id || syncAfterId)
+  messages.value = [...byId.values()].sort((a, b) => Number(a.id) - Number(b.id))
+  syncAfterId = Number(messages.value.at(-1)?.id || 0)
 }
-
-const scrollToBottom = async () => {
+const scrollToBottom = async request => {
   await nextTick()
-  if (messagesViewport.value) messagesViewport.value.scrollTop = messagesViewport.value.scrollHeight
+  if (currentSelection(request) && messagesViewport.value) messagesViewport.value.scrollTop = messagesViewport.value.scrollHeight
 }
 
-const loadUnread = async () => {
+async function loadUnread(scope = scopeVersion) {
+  if (!hasSession.value || !currentScope(scope)) return
   try {
-    unreadRooms.value = (await $api("/api/communication/chat/unread")).rooms || {}
-  } catch {
-    unreadRooms.value = {}
-  }
+    const result = await $api('/api/communication/chat/unread')
+    if (currentScope(scope)) unreadRooms.value = result.rooms || {}
+  } catch { /* A count failure must not interrupt an open conversation. */ }
 }
-
-const loadRoomLists = async () => {
-  const [rooms, projects, directs] = await Promise.all([
-    $api("/api/communication/chat/rooms"),
-    $api("/api/communication/chat/project-rooms"),
-    $api("/api/communication/chat/direct-rooms")
-  ])
-  baseRooms.value = rooms.rooms || []
-  projectRooms.value = projects.rooms || []
-  directRooms.value = directs.rooms || []
-}
-
-const markRead = async () => {
-  if (!activeRoom.value?.exists) return
+async function loadRoomLists(scope = scopeVersion) {
+  if (!hasSession.value || !currentScope(scope)) return
+  const request = ++nativeListRequest
   try {
-    await $api(`${roomEndpoint.value}/read`, {
-      method: "POST",
-      body: { messageId: Number(messages.value.at(-1)?.id || 0) }
-    })
-    unreadRooms.value = { ...unreadRooms.value, [activeRoomKey.value]: { count: 0, mentions: 0 } }
-  } catch {
-    // Der Lesestatus darf die Unterhaltung nicht blockieren.
-  }
-}
-
-const loadActiveRoom = async () => {
-  if (!activeRoom.value?.exists) return
-  roomLoading.value = true
-  syncAfterId = 0
-  try {
-    const [messageResult, memberResult] = await Promise.all([
-      $api(`${roomEndpoint.value}/messages`),
-      $api(`${roomEndpoint.value}/members`)
+    const [rooms, projects, directs] = await Promise.all([
+      $api('/api/communication/chat/rooms'), $api('/api/communication/chat/project-rooms'), $api('/api/communication/chat/direct-rooms')
     ])
-    messages.value = []
-    mergeMessages(messageResult.messages || [])
-    members.value = memberResult.members || []
-    await markRead()
-    await scrollToBottom()
-  } catch (error) {
-    toast.add({ title: error?.data?.error || "Chatraum konnte nicht geladen werden", color: "error" })
-  } finally {
-    roomLoading.value = false
-  }
+    if (!currentScope(scope) || request !== nativeListRequest) return
+    baseRooms.value = rooms.rooms || []; projectRooms.value = projects.rooms || []; directRooms.value = directs.rooms || []
+    listError.value = ''
+  } catch (error) { if (currentScope(scope) && request === nativeListRequest) listError.value = errorMessage(error) }
 }
-
-const provisionRoom = async (room) => {
-  const endpoint = room.type === "project"
-    ? `/api/communication/chat/project-rooms/${room.projectId}/provision`
-    : `/api/communication/chat/direct-rooms/${encodeURIComponent(room.userId)}/provision`
-  const created = await $api(endpoint, { method: "POST" })
-  await loadRoomLists()
-  activeRoomKey.value = created.key
-  await router.replace({ query: { ...route.query, room: created.key } })
-}
-
-const selectRoom = async (room) => {
+async function loadCollaboration(scope = scopeVersion) {
+  if (!hasSession.value || !currentScope(scope)) return
+  const request = ++collaborationRequest
   try {
-    if (!room.exists) await provisionRoom(room)
-    else {
-      activeRoomKey.value = room.key
-      await router.replace({ query: { ...route.query, room: room.key } })
-    }
-    messages.value = []
-    members.value = []
-    await loadActiveRoom()
+    const result = await $api('/api/collaboration')
+    if (!currentScope(scope) || request !== collaborationRequest) return
+    // Managed rooms alone never grant chat access. Only explicit memberships enter this list.
+    collaborationRooms.value = result.rooms || []; invitations.value = result.invitations || []
+    collaborationError.value = ''
   } catch (error) {
-    toast.add({ title: error?.data?.error || "Chatraum konnte nicht geöffnet werden", color: "error" })
+    if (!currentScope(scope) || request !== collaborationRequest) return
+    collaborationError.value = errorMessage(error)
+    if ([401, 403].includes(error?.statusCode || error?.status)) { collaborationRooms.value = []; invitations.value = [] }
   }
 }
+async function refreshLists() {
+  const scope = scopeVersion
+  await Promise.all([loadRoomLists(scope), loadUnread(scope), loadCollaboration(scope)])
+}
 
-const createRoom = async () => {
-  if (!roomForm.name.trim()) return
+async function markRead(request) {
+  if (!currentSelection(request) || !isInternal.value || !activeRoom.value?.exists) return
+  const messageId = Number(messages.value.at(-1)?.id || 0)
+  try {
+    await $api(nativeEndpoint(request.key, '/read'), { method: 'POST', body: { messageId } })
+    if (currentSelection(request)) unreadRooms.value = { ...unreadRooms.value, [request.key]: { count: 0, mentions: 0 } }
+  } catch { /* Read receipts must not interrupt the conversation. */ }
+}
+async function loadActiveRoom(request = selection()) {
+  if (!currentSelection(request) || !isInternal.value || !activeRoom.value?.exists) return
+  roomLoading.value = true
+  try {
+    const [result, people] = await Promise.all([
+      $api(nativeEndpoint(request.key, '/messages')), $api(nativeEndpoint(request.key, '/members'))
+    ])
+    if (!currentSelection(request)) return
+    messages.value = []; mergeMessages(result.messages); members.value = people.members || []
+    await markRead(request); await scrollToBottom(request)
+  } catch (error) {
+    if (currentSelection(request)) toast.add({ title: errorMessage(error), color: 'error' })
+  } finally { if (currentSelection(request)) roomLoading.value = false }
+}
+async function selectRoom(room) {
+  ++roomVersion; activeRoomKey.value = room.key; clearMessages()
+  let request = selection()
+  try {
+    if (room.source !== 'collaboration' && !room.exists) {
+      const endpoint = room.type === 'project'
+        ? `/api/communication/chat/project-rooms/${room.projectId}/provision`
+        : `/api/communication/chat/direct-rooms/${encodeURIComponent(room.userId)}/provision`
+      const created = await $api(endpoint, { method: 'POST' })
+      if (!currentSelection(request)) return
+      await loadRoomLists(request.scope)
+      if (!currentSelection(request)) return
+      activeRoomKey.value = created.key; request = selection()
+    }
+    await router.replace({ query: { ...route.query, room: request.key } })
+    if (currentSelection(request)) await loadActiveRoom(request)
+  } catch (error) { if (currentSelection(request)) toast.add({ title: errorMessage(error), color: 'error' }) }
+}
+async function createRoom() {
+  if (!roomForm.name.trim() || creating.value) return
+  const scope = scopeVersion
   creating.value = true
   try {
-    const created = await $api("/api/communication/chat/rooms", {
-      method: "POST",
-      body: { name: roomForm.name, key: roomKeyPreview.value, topic: roomForm.topic }
+    const created = await $api('/api/communication/chat/rooms', {
+      method: 'POST', body: { name: roomForm.name, key: roomKeyPreview.value, topic: roomForm.topic }
     })
-    Object.assign(roomForm, { name: "", key: "", topic: "" })
-    showRoomForm.value = false
-    await loadRoomLists()
-    await selectRoom(created)
-  } catch (error) {
-    toast.add({ title: error?.data?.error || "Chatraum konnte nicht erstellt werden", color: "error" })
-  } finally {
-    creating.value = false
-  }
+    if (!currentScope(scope)) return
+    Object.assign(roomForm, { name: '', key: '', topic: '' }); showRoomForm.value = false
+    await loadRoomLists(scope)
+    if (currentScope(scope)) await selectRoom(created)
+  } catch (error) { if (currentScope(scope)) toast.add({ title: errorMessage(error), color: 'error' }) }
+  finally { if (currentScope(scope)) creating.value = false }
 }
-
-const sendMessage = async () => {
-  const text = draft.value.trim()
-  if (!text || sending.value || !activeRoom.value?.exists) return
-  sending.value = true
-  draft.value = ""
+async function respondToInvitation(id, accept) {
+  if (invitationBusy.value) return
+  const scope = scopeVersion
+  invitationBusy.value = id
   try {
-    const message = await $api(`${roomEndpoint.value}/messages`, { method: "POST", body: { text } })
-    mergeMessages([message])
-    await markRead()
-    await scrollToBottom()
-  } catch (error) {
-    draft.value = text
-    toast.add({ title: error?.data?.error || "Nachricht konnte nicht gesendet werden", color: "error" })
-  } finally {
-    sending.value = false
-  }
+    const result = await $api(`/api/collaboration/invitations/${encodeURIComponent(id)}/${accept ? 'accept' : 'decline'}`, { method: 'POST' })
+    if (!currentScope(scope)) return
+    await loadCollaboration(scope)
+    if (!currentScope(scope)) return
+    const room = decoratedRooms.value.find(room => room.key === collaborationKey(result.roomId))
+    if (accept && room) await selectRoom(room)
+  } catch (error) { if (currentScope(scope)) toast.add({ title: errorMessage(error), color: 'error' }) }
+  finally { if (currentScope(scope)) invitationBusy.value = '' }
 }
-
-const pollMessages = async () => {
-  if (pollBusy || document.hidden || !activeRoom.value?.exists) return
+async function sendMessage() {
+  const text = draft.value.trim(), request = selection()
+  if (!text || sending.value || !isInternal.value || !activeRoom.value?.exists) return
+  sending.value = true; draft.value = ''
+  try {
+    const message = await $api(nativeEndpoint(request.key, '/messages'), { method: 'POST', body: { text } })
+    if (!currentSelection(request)) return
+    mergeMessages([message]); await markRead(request); await scrollToBottom(request)
+  } catch (error) {
+    if (currentSelection(request)) { draft.value = text; toast.add({ title: errorMessage(error), color: 'error' }) }
+  } finally { if (currentSelection(request)) sending.value = false }
+}
+async function pollMessages() {
+  if (pollBusy || loading.value || roomLoading.value || document.hidden || !isInternal.value || !activeRoom.value?.exists) return
+  const request = selection()
   pollBusy = true
   try {
-    const result = await $api(`${roomEndpoint.value}/sync?afterId=${syncAfterId}`)
-    if (result.messages?.length) {
-      mergeMessages(result.messages)
-      await markRead()
-      await scrollToBottom()
-    }
-    await loadUnread()
-  } catch {
-    // Beim nächsten Intervall erneut versuchen.
-  } finally {
-    pollBusy = false
-  }
-}
-
-const formatMessageTime = (timestamp) => new Intl.DateTimeFormat("de-DE", {
-  day: "2-digit", month: "2-digit", hour: "2-digit", minute: "2-digit"
-}).format(new Date(timestamp))
-
-onMounted(async () => {
-  try {
-    await Promise.all([loadRoomLists(), loadUnread()])
-    if (!activeRoom.value) activeRoomKey.value = "allgemein"
-    await loadActiveRoom()
-    pollTimer = window.setInterval(pollMessages, 2500)
+    const result = await $api(nativeEndpoint(request.key, `/sync?afterId=${syncAfterId}`))
+    if (!currentSelection(request)) return
+    if (result.messages?.length) { mergeMessages(result.messages); await markRead(request); await scrollToBottom(request) }
+    await loadUnread(request.scope)
   } catch (error) {
-    toast.add({ title: error?.data?.error || "Chat konnte nicht geladen werden", color: "error" })
-  } finally {
-    loading.value = false
-  }
-})
+    if (currentSelection(request) && [401, 403, 404, 410].includes(error?.statusCode || error?.status)) {
+      ++roomVersion; clearMessages(); void loadRoomLists(request.scope)
+    }
+  } finally { pollBusy = false }
+}
+const formatMessageTime = timestamp => new Intl.DateTimeFormat('de-DE', { day: '2-digit', month: '2-digit', hour: '2-digit', minute: '2-digit' }).format(new Date(timestamp))
 
-onBeforeUnmount(() => {
-  if (pollTimer) window.clearInterval(pollTimer)
+async function initialize() {
+  const scope = ++scopeVersion
+  ++roomVersion; clearMessages()
+  baseRooms.value = []; projectRooms.value = []; directRooms.value = []; collaborationRooms.value = []; invitations.value = []
+  unreadRooms.value = {}; invitationBusy.value = ''; collaborationError.value = ''; listError.value = ''; creating.value = false
+  showRoomForm.value = false; Object.assign(roomForm, { name: '', key: '', topic: '' })
+  activeRoomKey.value = requestedRoom(); loading.value = true
+  await refreshLists()
+  if (!currentScope(scope)) return
+  loading.value = false
+  const room = activeRoom.value || (activeRoomKey.value === 'allgemein' ? decoratedRooms.value[0] : null)
+  if (room) await selectRoom(room)
+}
+watch(session, () => { if (mounted) void initialize() }, { flush: 'sync' })
+watch(() => route.query.room, () => {
+  if (!mounted || loading.value || requestedRoom() === activeRoomKey.value) return
+  const room = decoratedRooms.value.find(room => room.key === requestedRoom())
+  if (room) void selectRoom(room)
+  else { ++roomVersion; activeRoomKey.value = requestedRoom(); clearMessages() }
 })
+onMounted(() => {
+  mounted = true; void initialize()
+  pollTimer = window.setInterval(pollMessages, 2500)
+  listTimer = window.setInterval(() => { if (!document.hidden && !loading.value) void refreshLists() }, 15000)
+})
+onBeforeUnmount(() => { disposed = true; ++scopeVersion; ++roomVersion; clearInterval(pollTimer); clearInterval(listTimer) })
 </script>
 
 <template>
   <div class="flex min-h-0 flex-1 bg-gray-50 p-4 sm:p-6">
-    <div class="mx-auto grid h-[calc(100vh-8rem)] w-full max-w-7xl grid-cols-1 overflow-hidden rounded-xl border border-gray-200 bg-white shadow-sm lg:grid-cols-[19rem_minmax(0,1fr)_15rem]">
+    <div class="mx-auto grid h-[calc(100vh-8rem)] w-full max-w-7xl grid-cols-1 grid-rows-[minmax(8rem,30%)_minmax(0,1fr)] overflow-hidden rounded-xl border border-gray-200 bg-white shadow-sm lg:grid-rows-1"
+      :class="isCollaboration ? 'lg:grid-cols-[19rem_minmax(0,1fr)]' : 'lg:grid-cols-[19rem_minmax(0,1fr)_15rem]'">
       <aside class="min-h-0 overflow-y-auto border-r border-gray-200 p-4">
         <div class="mb-4 flex items-center justify-between">
           <div>
@@ -233,6 +245,22 @@ onBeforeUnmount(() => {
           </div>
         </form>
 
+        <UButton to="/communication/collaboration" icon="i-heroicons-user-group" color="neutral" variant="outline" size="xs" class="mb-4 w-full justify-center">Unterräume und Freigaben</UButton>
+        <p v-if="listError" role="alert" class="mb-3 text-xs text-red-700">{{ listError }} <button class="underline" @click="loadRoomLists()">Erneut laden</button></p>
+        <p v-if="collaborationError" role="alert" class="mb-3 text-xs text-red-700">Zusammenarbeit: {{ collaborationError }} <button class="underline" @click="loadCollaboration()">Erneut laden</button></p>
+        <section v-if="invitations.length" class="mb-5 space-y-2" aria-label="Einladungen">
+          <h2 class="px-2 text-xs font-semibold uppercase tracking-wide text-gray-400">Einladungen</h2>
+          <div v-for="invitation in invitations" :key="invitation.id" class="rounded-lg border border-primary-200 bg-primary-50 p-3">
+            <p class="text-sm font-medium">{{ invitation.name }}</p>
+            <p class="text-xs text-gray-500">{{ invitation.ownerName }} · {{ invitation.role === 'reader' ? 'Nur lesen' : 'Lesen und schreiben' }}</p>
+            <p v-if="invitation.overviewAllowed" class="mt-1 text-xs text-gray-500">Projektübersicht freigegeben</p>
+            <div class="mt-2 flex gap-2">
+              <UButton size="xs" :disabled="!!invitationBusy" @click="respondToInvitation(invitation.id, true)">Annehmen</UButton>
+              <UButton size="xs" variant="ghost" color="neutral" :disabled="!!invitationBusy" @click="respondToInvitation(invitation.id, false)">Ablehnen</UButton>
+            </div>
+          </div>
+        </section>
+
         <div v-if="loading" class="py-8 text-center text-sm text-gray-500">Chat wird geladen …</div>
         <div v-else class="space-y-5">
           <section v-for="group in groupedRooms" :key="group.label">
@@ -248,7 +276,7 @@ onBeforeUnmount(() => {
               <UIcon :name="room.icon" class="size-5 shrink-0" />
               <span class="min-w-0 flex-1">
                 <span class="block truncate text-sm font-medium">{{ room.name }}</span>
-                <span class="block truncate text-xs text-gray-400">{{ room.topic }}</span>
+                <span class="block truncate text-xs text-gray-400">{{ room.source === 'collaboration' ? room.ownerName || 'Projekt-Unterraum' : room.topic }}{{ room.remote ? ' · Extern' : '' }}{{ room.status === 'archived' ? ' · Archiviert' : '' }}</span>
               </span>
               <UBadge v-if="room.unread" color="primary" size="xs">{{ room.unread }}</UBadge>
             </button>
@@ -256,7 +284,10 @@ onBeforeUnmount(() => {
         </div>
       </aside>
 
-      <main class="flex min-h-0 flex-col">
+      <CollaborationRoom v-if="isCollaboration" :key="`${session}:${activeRoom.id}`" :room-id="activeRoom.id" :api="$api" embedded @changed="loadCollaboration()" @unauthorized="loadCollaboration()">
+        <template #actions><UButton :to="{ path: '/communication/collaboration', query: { room: activeRoom.id } }" color="neutral" variant="outline" size="xs">Details und Freigaben</UButton></template>
+      </CollaborationRoom>
+      <main v-else class="flex min-h-0 flex-col">
         <header class="border-b border-gray-200 px-5 py-4">
           <h2 class="font-semibold text-gray-900">{{ activeRoom?.name || "Chat" }}</h2>
           <p class="truncate text-sm text-gray-500">{{ activeRoom?.topic || "Wähle links einen Chatraum aus." }}</p>
@@ -264,6 +295,7 @@ onBeforeUnmount(() => {
 
         <div ref="messagesViewport" class="min-h-0 flex-1 overflow-y-auto px-4 py-5 sm:px-6">
           <div v-if="roomLoading" class="py-10 text-center text-sm text-gray-500">Nachrichten werden geladen …</div>
+          <div v-else-if="!activeRoom" class="py-10 text-center text-sm text-gray-500">Dieser Chatraum ist nicht verfügbar. Wähle links einen anderen Raum.</div>
           <div v-else-if="!messages.length" class="flex h-full flex-col items-center justify-center text-center text-gray-400">
             <UIcon name="i-heroicons-chat-bubble-oval-left-ellipsis" class="mb-3 size-10" />
             <p class="font-medium">Noch keine Nachrichten</p>
@@ -286,7 +318,7 @@ onBeforeUnmount(() => {
         </form>
       </main>
 
-      <aside class="hidden min-h-0 overflow-y-auto border-l border-gray-200 p-4 lg:block">
+      <aside v-if="!isCollaboration" class="hidden min-h-0 overflow-y-auto border-l border-gray-200 p-4 lg:block">
         <h2 class="mb-3 text-sm font-semibold text-gray-900">Teilnehmer</h2>
         <div class="space-y-3">
           <div v-for="member in members" :key="member.userId" class="flex items-center gap-3">
diff --git a/frontend/pages/communication/collaboration.vue b/frontend/pages/communication/collaboration.vue
index 55076a89..c16f2bb7 100644
--- a/frontend/pages/communication/collaboration.vue
+++ b/frontend/pages/communication/collaboration.vue
@@ -2,7 +2,7 @@
 const { $api } = useNuxtApp()
 const route = useRoute()
 const state = ref({ rooms: [], invitations: [], managedRooms: [], projects: [], canManage: false, contact: '' })
-const contacts = ref([]), selected = ref(''), error = ref(''), busy = ref(false), showCreate = ref(false), showManagement = ref(false)
+const contacts = ref([]), selected = ref(typeof route.query.room === 'string' ? route.query.room : ''), error = ref(''), busy = ref(false), showCreate = ref(false), showManagement = ref(false)
 const management = ref({ members: [], invitations: [] }), inviteResult = ref(null)
 const create = reactive({ name: '', topic: '', projectId: String(route.query.project || '') })
 const invite = reactive({ kind: 'user', userId: '', name: '', email: '', contact: '', role: 'participant', overviewAllowed: false, expiresAt: '', sendEmail: true })
@@ -73,6 +73,7 @@ async function savePartner(origin, active = true) {
   })
 }
 watch(selected, () => { showManagement.value = false; inviteResult.value = null; management.value = { members: [], invitations: [] } })
+watch(() => route.query.room, id => { if (typeof id === 'string') selected.value = id })
 let timer
 onMounted(async () => { await refresh(); timer = setInterval(() => { if (!document.hidden && !busy.value) void refresh() }, 15000) })
 onBeforeUnmount(() => clearInterval(timer))
@@ -120,7 +121,10 @@ onBeforeUnmount(() => clearInterval(timer))
         </button>
       </aside>
       <div class="min-w-0 space-y-4">
-        <UButton v-if="selectedManaged" variant="outline" :disabled="busy" @click="run(async () => { await loadManagement(); showManagement = !showManagement })">Teilnehmer und Freigaben verwalten</UButton>
+        <div class="flex flex-wrap gap-2">
+          <UButton v-if="readable" :to="{ path: '/communication/chat', query: { room: `collaboration:${selected}` } }" variant="outline">Im Chat öffnen</UButton>
+          <UButton v-if="selectedManaged" variant="outline" :disabled="busy" @click="run(async () => { await loadManagement(); showManagement = !showManagement })">Teilnehmer und Freigaben verwalten</UButton>
+        </div>
         <section v-if="showManagement && selectedManaged" class="space-y-5 rounded-xl border border-gray-200 p-4 dark:border-gray-700">
           <form class="space-y-3" @submit.prevent="invitePerson">
             <h2 class="font-semibold">Person in diesen Unterraum einladen</h2>
diff --git a/frontend/tests/chat-collaboration.test.cjs b/frontend/tests/chat-collaboration.test.cjs
new file mode 100644
index 00000000..b55ca4ba
--- /dev/null
+++ b/frontend/tests/chat-collaboration.test.cjs
@@ -0,0 +1,239 @@
+const { test } = require('node:test')
+const assert = require('node:assert/strict')
+const fs = require('node:fs')
+const vm = require('node:vm')
+const vue = require('vue')
+
+const settle = () => new Promise(setImmediate)
+const deferred = () => {
+  let resolve
+  const promise = new Promise(done => { resolve = done })
+  return { promise, resolve }
+}
+const source = fs.readFileSync(`${__dirname}/../pages/communication/chat.vue`, 'utf8').match(/<script setup>([\s\S]*?)<\/script>/)[1]
+const nativeRoom = { key: 'allgemein', name: 'Allgemein', exists: true, type: 'general' }
+const sharedRoom = { id: 'shared-room', name: 'Partnerchat', remote: true, unread: 3 }
+
+function fixture(override = () => undefined) {
+  return async (url, options) => {
+    const result = override(url, options)
+    if (result !== undefined) return result
+    if (url === '/api/collaboration') return { rooms: [sharedRoom], invitations: [], managedRooms: [{ id: 'admin-only' }] }
+    if (url === '/api/communication/chat/rooms') return { rooms: [nativeRoom] }
+    if (url.endsWith('/project-rooms') || url.endsWith('/direct-rooms')) return { rooms: [] }
+    if (url.endsWith('/unread')) return { rooms: { allgemein: { count: 2 } } }
+    if (url.endsWith('/members')) return { members: [{ userId: 'member', displayName: 'Kollege' }] }
+    if (url.includes('/sync?')) return { messages: [] }
+    if (url.endsWith('/messages')) return { messages: [{ id: 1, body: 'Interne Nachricht' }] }
+    if (url.endsWith('/read')) return {}
+    throw new Error(`Unexpected request: ${url}`)
+  }
+}
+
+function setup(api = fixture(), query = {}) {
+  const auth = vue.reactive({ user: { id: 'user-1' }, activeTenant: '1' })
+  const route = vue.reactive({ query })
+  const calls = [], toasts = [], timers = new Map()
+  let mount, unmount, timerId = 0
+  const scope = vue.effectScope()
+  const context = vm.createContext({
+    ref: vue.ref, reactive: vue.reactive, computed: vue.computed, watch: vue.watch, nextTick: vue.nextTick,
+    useToast: () => ({ add: item => toasts.push(item) }),
+    useAuthStore: () => auth, useRoute: () => route,
+    useRouter: () => ({ replace: async location => { route.query = location.query } }),
+    useNuxtApp: () => ({ $api: (url, options) => { calls.push({ url, options }); return api(url, options) } }),
+    onMounted: handler => { mount = handler }, onBeforeUnmount: handler => { unmount = handler },
+    window: { setInterval: handler => { timers.set(++timerId, handler); return timerId } },
+    clearInterval: id => timers.delete(id), document: { hidden: false },
+  })
+  scope.run(() => vm.runInContext(source, context))
+  return { auth, route, calls, toasts, timers, mount, close: () => { unmount(); scope.stop() }, run: code => vm.runInContext(code, context) }
+}
+
+test('only member rooms enter normal chat, with separate keys, groups and unread counts', async t => {
+  const ui = setup()
+  t.after(ui.close)
+  await ui.run('refreshLists()')
+  assert.equal(ui.run('decoratedRooms.value.length'), 2)
+  assert.equal(ui.run('decoratedRooms.value.find(r => r.source === "collaboration").key'), 'collaboration:shared-room')
+  assert.equal(ui.run('groupedRooms.value.at(-1).label'), 'Zusammenarbeit')
+  assert.equal(ui.run('decoratedRooms.value.at(-1).unread'), 3)
+  assert.equal(ui.run('decoratedRooms.value[0].unread'), 2)
+  assert.equal(ui.run('decoratedRooms.value.some(r => r.id === "admin-only")'), false)
+})
+
+test('opening a collaboration deep link never provisions, reads or sends through native chat APIs', async t => {
+  const ui = setup(fixture(), { room: 'collaboration:shared-room' })
+  t.after(ui.close)
+  ui.mount()
+  await settle()
+  assert.equal(ui.run('isCollaboration.value'), true)
+  assert.equal(ui.route.query.room, 'collaboration:shared-room')
+  ui.run('draft.value = "Partnernachricht"')
+  await ui.run('pollMessages()')
+  await ui.run('sendMessage()')
+  assert.equal(ui.calls.some(({ url }) => url.startsWith('/api/communication/chat/rooms/')), false)
+  ui.route.query = { room: 'allgemein' }
+  await settle()
+  assert.equal(ui.run('isInternal.value'), true)
+  assert.equal(ui.run('messages.value[0].body'), 'Interne Nachricht')
+  ui.route.query = { room: 'collaboration:shared-room' }
+  await settle()
+  assert.equal(ui.run('isCollaboration.value'), true)
+  assert.equal(ui.run('messages.value.length'), 0)
+})
+
+test('accepting an invitation opens the returned local binding and clears the invitation', async t => {
+  let accepted = false
+  const ui = setup(fixture(url => {
+    if (url === '/api/collaboration/invitations/invite/accept') { accepted = true; return { roomId: sharedRoom.id } }
+    if (url === '/api/collaboration') return { rooms: accepted ? [sharedRoom] : [], invitations: accepted ? [] : [{ id: 'invite' }] }
+  }))
+  t.after(ui.close)
+  ui.mount()
+  await settle()
+  assert.equal(ui.run('invitations.value.length'), 1)
+  await ui.run('respondToInvitation("invite", true)')
+  assert.equal(ui.run('invitations.value.length'), 0)
+  assert.equal(ui.run('isCollaboration.value'), true)
+  assert.equal(ui.route.query.room, 'collaboration:shared-room')
+  assert.equal(ui.calls.find(({ url }) => url.endsWith('/accept')).options.method, 'POST')
+})
+
+test('declining an invitation keeps the current chat open', async t => {
+  let declined = false
+  const ui = setup(fixture(url => {
+    if (url.endsWith('/invite/decline')) { declined = true; return { ok: true } }
+    if (url === '/api/collaboration') return { rooms: [], invitations: declined ? [] : [{ id: 'invite' }] }
+  }))
+  t.after(ui.close)
+  ui.mount()
+  await settle()
+  await ui.run('respondToInvitation("invite", false)')
+  assert.equal(ui.run('invitations.value.length'), 0)
+  assert.equal(ui.run('activeRoomKey.value'), 'allgemein')
+  assert.equal(ui.run('messages.value[0].body'), 'Interne Nachricht')
+})
+
+test('a late native room load cannot populate a collaboration room or mark it read', async t => {
+  const pending = deferred()
+  const ui = setup(fixture(url => url.endsWith('/allgemein/messages') ? pending.promise : undefined))
+  t.after(ui.close)
+  ui.mount()
+  await settle()
+  await ui.run('selectRoom(decoratedRooms.value.find(r => r.source === "collaboration"))')
+  pending.resolve({ messages: [{ id: 99, body: 'Late private content' }] })
+  await settle()
+  assert.equal(ui.run('messages.value.length'), 0)
+  assert.equal(ui.run('members.value.length'), 0)
+  assert.equal(ui.run('roomLoading.value'), false)
+  assert.equal(ui.calls.some(({ url }) => url.endsWith('/read')), false)
+})
+
+test('late sync and send results cannot overwrite the selected collaboration room', async t => {
+  const sync = deferred(), send = deferred()
+  const ui = setup(fixture((url, options) => {
+    if (url.includes('/sync?')) return sync.promise
+    if (url.endsWith('/messages') && options?.method === 'POST') return send.promise
+  }))
+  t.after(ui.close)
+  ui.mount()
+  await settle()
+  const polling = ui.run('pollMessages()')
+  ui.run('draft.value = "Internal draft"')
+  const sending = ui.run('sendMessage()')
+  await ui.run('selectRoom(decoratedRooms.value.find(r => r.source === "collaboration"))')
+  const reads = ui.calls.filter(({ url }) => url.endsWith('/read')).length
+  sync.resolve({ messages: [{ id: 2, body: 'Late sync' }] })
+  send.resolve({ id: 3, body: 'Late sent message' })
+  await Promise.all([polling, sending])
+  assert.equal(ui.run('messages.value.length'), 0)
+  assert.equal(ui.run('draft.value'), '')
+  assert.equal(ui.run('sending.value'), false)
+  assert.equal(ui.calls.filter(({ url }) => url.endsWith('/read')).length, reads)
+})
+
+test('switching tenants clears visible content immediately and ignores older room lists', async t => {
+  const stale = deferred(), fresh = deferred()
+  let lists = 0
+  const ui = setup(fixture(url => {
+    if (url === '/api/collaboration') return ++lists === 1 ? stale.promise : fresh.promise
+  }))
+  t.after(ui.close)
+  ui.mount()
+  ui.run('messages.value = [{ id: 1, body: "Previous tenant" }]; draft.value = "Private draft"')
+  ui.auth.activeTenant = '2'
+  assert.equal(ui.run('messages.value.length'), 0)
+  assert.equal(ui.run('collaborationRooms.value.length'), 0)
+  assert.equal(ui.run('draft.value'), '')
+  assert.equal(ui.run('session.value'), 'user-1:2')
+  fresh.resolve({ rooms: [{ id: 'tenant-2' }], invitations: [] })
+  await settle()
+  stale.resolve({ rooms: [{ id: 'tenant-1-secret' }], invitations: [{ id: 'private-invite' }] })
+  await settle()
+  assert.equal(ui.run('collaborationRooms.value[0].id'), 'tenant-2')
+  assert.equal(ui.run('invitations.value.length'), 0)
+})
+
+test('collaboration and internal service failures do not block the other room type', async t => {
+  const ui = setup(fixture(url => {
+    if (url === '/api/collaboration') throw { statusCode: 503, data: { message: 'Unterräume nicht erreichbar' } }
+  }))
+  t.after(ui.close)
+  ui.mount()
+  await settle()
+  assert.equal(ui.run('isInternal.value'), true)
+  assert.equal(ui.run('messages.value.length'), 1)
+  assert.equal(ui.run('collaborationError.value'), 'Unterräume nicht erreichbar')
+  const other = setup(fixture(url => {
+    if (url === '/api/communication/chat/rooms') throw { statusCode: 409, data: { message: 'Chat nicht verfügbar' } }
+  }), { room: 'collaboration:shared-room' })
+  t.after(other.close)
+  other.mount()
+  await settle()
+  assert.equal(other.run('isCollaboration.value'), true)
+  assert.equal(other.run('listError.value'), 'Chat nicht verfügbar')
+})
+
+test('revoked membership removes the active collaboration room instead of exposing managed rooms', async t => {
+  let revoked = false
+  const ui = setup(fixture(url => {
+    if (url === '/api/collaboration' && revoked) return { rooms: [], invitations: [], managedRooms: [sharedRoom] }
+  }), { room: 'collaboration:shared-room' })
+  t.after(ui.close)
+  ui.mount()
+  await settle()
+  assert.equal(ui.run('isCollaboration.value'), true)
+  revoked = true
+  await ui.run('loadCollaboration()')
+  assert.equal(ui.run('activeRoom.value'), null)
+  assert.equal(ui.run('isCollaboration.value'), false)
+  assert.equal(ui.run('isInternal.value'), false)
+})
+
+test('logout clears the room and invitations without starting requests with the previous credentials', async t => {
+  const ui = setup(fixture(), { room: 'collaboration:shared-room' })
+  t.after(ui.close)
+  ui.mount()
+  await settle()
+  const calls = ui.calls.length
+  ui.auth.user = null
+  await settle()
+  await ui.run('refreshLists()')
+  assert.equal(ui.run('activeRoom.value'), null)
+  assert.equal(ui.run('invitations.value.length'), 0)
+  assert.equal(ui.calls.length, calls)
+})
+
+test('unmount cancels both polling timers and ignores pending lists', async () => {
+  const pending = deferred()
+  const ui = setup(fixture(url => url === '/api/collaboration' ? pending.promise : undefined))
+  ui.mount()
+  assert.equal(ui.timers.size, 2)
+  ui.close()
+  pending.resolve({ rooms: [sharedRoom], invitations: [] })
+  await settle()
+  assert.equal(ui.timers.size, 0)
+  assert.equal(ui.run('collaborationRooms.value.length'), 0)
+  assert.equal(ui.run('messages.value.length'), 0)
+})