diff --git a/server/services/stripeBilling.js b/server/services/stripeBilling.js
index 1d7bbd0..01417c4 100644
--- a/server/services/stripeBilling.js
+++ b/server/services/stripeBilling.js
@@ -138,7 +138,7 @@ function periodEndOf(sub) {
   return sub.current_period_end || sub.items?.data?.[0]?.current_period_end || null
 }
 
-async function upsertSubscriptionDoc(id, data) {
+async function writeSubscriptionDoc(id, data) {
   const existing = await getDocument(config.collections.subscriptions, id).catch(() => null)
   if (existing) return updateDocument(config.collections.subscriptions, id, data)
   try {
@@ -150,6 +150,25 @@ async function upsertSubscriptionDoc(id, data) {
   }
 }
 
+async function upsertSubscriptionDoc(id, data) {
+  let fields = data
+  for (;;) {
+    try {
+      return await writeSubscriptionDoc(id, fields)
+    } catch (err) {
+      // Neues Feld noch nicht in Appwrite (Setup-Skript nicht gelaufen): ohne das Feld spiegeln,
+      // damit Webhooks und der öffentliche Kündigungs-/Widerrufsknopf weiter funktionieren
+      const missing = err.status === 400 && /Unknown attribute: "(\w+)"/.exec(err.message || '')?.[1]
+      if (!missing || !(missing in fields)) throw err
+      console.error(
+        `[stripe] Appwrite kennt das Feld "${missing}" nicht (scripts/setup-abo-collections.mjs ausführen) – Abo ${id} ohne dieses Feld gespiegelt`
+      )
+      const { [missing]: _dropped, ...rest } = fields
+      fields = rest
+    }
+  }
+}
+
 /** Stripe-Subscription frisch laden und nach Appwrite spiegeln. */
 export async function syncSubscription(subscriptionId) {
   const stripe = getStripe()
diff --git a/test/syncSchemaFallback.test.js b/test/syncSchemaFallback.test.js
new file mode 100644
index 0000000..33e32cc
--- /dev/null
+++ b/test/syncSchemaFallback.test.js
@@ -0,0 +1,51 @@
+import { test, after } from 'node:test'
+import assert from 'node:assert/strict'
+import { startAppwriteMock } from './support/appwriteMock.js'
+import { startFakeStripe } from './support/fakeStripe.js'
+import { useTestEnv } from './support/testEnv.js'
+import { schemaKeys } from './support/schema.js'
+
+// Produktion, bevor das Setup-Skript gelaufen ist: Appwrite kennt productKey noch nicht
+const aw = await startAppwriteMock({
+  schema: { subscriptions: schemaKeys('subscriptions').filter((key) => key !== 'productKey') },
+})
+const st = await startFakeStripe()
+after(() => Promise.all([aw.close(), st.close()]))
+useTestEnv({ appwriteUrl: aw.url, stripeUrl: st.url })
+const { syncSubscription } = await import('../server/services/stripeBilling.js')
+
+const sub = (id, cancelAtPeriodEnd) => ({
+  id,
+  object: 'subscription',
+  status: 'active',
+  customer: 'cus_1',
+  start_date: 1790000000,
+  current_period_end: 1792600000,
+  cancel_at_period_end: cancelAtPeriodEnd,
+  metadata: { webklarCustomerId: 'c1', email: 'k@example.invalid', termsVersion: '2026-09-25-v3' },
+  items: {
+    object: 'list',
+    data: [{ id: 'si_w', quantity: 1, price: { id: 'price_w', lookup_key: 'webklar_wartung_monatlich_5000', unit_amount: 5000 } }],
+  },
+})
+
+test('Fehlendes Feld in Appwrite: Abo wird trotzdem gespiegelt (anlegen und ändern)', async () => {
+  const errors = []
+  const original = console.error
+  console.error = (...args) => errors.push(args.join(' '))
+  try {
+    st.subscriptions.set('sub_w', sub('sub_w', false))
+    const created = await syncSubscription('sub_w')
+    assert.equal(created.status, 'active')
+    assert.equal('productKey' in created, false)
+
+    // z. B. Kündigung über den öffentlichen Knopf: Stripe ist schon geändert, der Spiegel muss folgen
+    st.subscriptions.set('sub_w', sub('sub_w', true))
+    const updated = await syncSubscription('sub_w')
+    assert.equal(updated.cancelAtPeriodEnd, true)
+    assert.equal(aw.docs('subscriptions').get('sub_w').cancelAtPeriodEnd, true)
+  } finally {
+    console.error = original
+  }
+  assert.ok(errors.some((line) => line.includes('productKey')), 'Fehler im Log nennt das fehlende Feld')
+})