diff --git a/client/src/components/SystemNotices/SystemNoticeHost.test.tsx b/client/src/components/SystemNotices/SystemNoticeHost.test.tsx
index 84afad95a..e6db29e82 100644
--- a/client/src/components/SystemNotices/SystemNoticeHost.test.tsx
+++ b/client/src/components/SystemNotices/SystemNoticeHost.test.tsx
@@ -1,4 +1,4 @@
-// FE-W4SNH-001 to FE-W4SNH-008
+// FE-W4SNH-001 to FE-W4SNH-012
 import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'
 import { render, screen, act } from '../../../tests/helpers/render'
 import { useSystemNoticeStore, type SystemNoticeDTO } from '../../store/systemNoticeStore'
@@ -152,4 +152,92 @@ describe('SystemNoticeHost', () => {
 
     expect(screen.getByTestId('modals')).toHaveTextContent('thanks')
   })
+  // The deploy window: the server already serves the new release notice, the browser is
+  // still running the bundle the service worker cached, and the keys that notice names do
+  // not exist in it. See hasCopy in SystemNoticeHost.
+  it('FE-W4SNH-009: a notice made only of keys this build cannot resolve waits instead of showing them', () => {
+    useSystemNoticeStore.setState({
+      loaded: true,
+      notices: [notice({
+        id: 'release-notes',
+        display: 'modal',
+        titleKey: 'system_notice.release_notes.title',
+        bodyKey: 'system_notice.release_notes.body',
+      })],
+    })
+
+    render(<SystemNoticeHost />)
+
+    expect(screen.getByTestId('modals')).toBeEmptyDOMElement()
+  })
+
+  it('FE-W4SNH-010: a release notice is judged by the strings its own layout draws', () => {
+    // The release panel never shows the generic title and body, so a resolvable pair there
+    // would vouch for a panel that is still all keys. The keys name a release this build
+    // knows nothing about, which is exactly the position an old bundle is in.
+    const release = {
+      version: '5.0.0',
+      eyebrowKey: 'system_notice.release_notes_5_0_0.eyebrow',
+      headlineKey: 'system_notice.release_notes_5_0_0.headline',
+      introKey: 'system_notice.release_notes_5_0_0.intro',
+      featuresLabelKey: 'system_notice.release_notes_5_0_0.features_label',
+      features: [],
+      note: {
+        eyebrowKey: 'system_notice.release_notes_5_0_0.note_eyebrow',
+        titleKey: 'system_notice.release_notes_5_0_0.note_title',
+        bodyKey: 'system_notice.release_notes_5_0_0.note_body',
+        promiseLabelKey: 'system_notice.release_notes_5_0_0.promise_label',
+        promiseLeadKey: 'system_notice.release_notes_5_0_0.promise_lead',
+        promiseTextKey: 'system_notice.release_notes_5_0_0.promise_text',
+        bodyAfterKey: 'system_notice.release_notes_5_0_0.note_body_after',
+        closingKey: 'system_notice.release_notes_5_0_0.note_closing',
+      },
+      supportLeadKey: 'system_notice.release_notes_5_0_0.support_lead',
+      supportTextKey: 'system_notice.release_notes_5_0_0.support_text',
+    }
+    useSystemNoticeStore.setState({
+      loaded: true,
+      notices: [notice({ id: 'release-notes', display: 'modal', titleKey: 'Update', bodyKey: 'Installed', release })],
+    })
+
+    render(<SystemNoticeHost />)
+
+    expect(screen.getByTestId('modals')).toBeEmptyDOMElement()
+  })
+
+  it('FE-W4SNH-011: a notice written out in words is shown, keys or no keys', () => {
+    // An instance's own announcement carries literal copy, and t() hands back anything it
+    // does not know — so the shape of the string is what tells the two apart.
+    useSystemNoticeStore.setState({
+      loaded: true,
+      notices: [notice({
+        id: 'outage',
+        display: 'modal',
+        titleKey: 'Maintenance tonight',
+        bodyKey: 'The server restarts at 22:00.',
+      })],
+    })
+
+    render(<SystemNoticeHost />)
+
+    expect(screen.getByTestId('modals')).toHaveTextContent('outage')
+  })
+
+  it('FE-W4SNH-012: one resolved string is enough to show the notice', () => {
+    // Generous on purpose: half a translation is still readable, and a notice hidden by
+    // mistake is the worse failure. 'common.save' is in every build.
+    useSystemNoticeStore.setState({
+      loaded: true,
+      notices: [notice({
+        id: 'half',
+        display: 'modal',
+        titleKey: 'common.save',
+        bodyKey: 'system_notice.not.in.this.build',
+      })],
+    })
+
+    render(<SystemNoticeHost />)
+
+    expect(screen.getByTestId('modals')).toHaveTextContent('half')
+  })
 })
diff --git a/client/src/components/SystemNotices/SystemNoticeHost.tsx b/client/src/components/SystemNotices/SystemNoticeHost.tsx
index 8c86040a7..d622d3148 100644
--- a/client/src/components/SystemNotices/SystemNoticeHost.tsx
+++ b/client/src/components/SystemNotices/SystemNoticeHost.tsx
@@ -1,5 +1,6 @@
 import { useEffect, useState } from 'react';
-import { useSystemNoticeStore } from '../../store/systemNoticeStore.js';
+import { useSystemNoticeStore, type SystemNoticeDTO } from '../../store/systemNoticeStore.js';
+import { useTranslation } from '../../i18n/index.js';
 import { ModalRenderer } from './SystemNoticeModal.js';
 import { BannerRenderer, ToastRenderer } from './SystemNoticeBanner.js';
 
@@ -18,8 +19,44 @@ function useIsMobile() {
   return isMobile;
 }
 
+/**
+ * A key by its shape: dotted, no spaces, `system_notice.release_notes.headline`.
+ *
+ * A notice may carry literal copy instead of a key — an instance's own announcement is
+ * written out, not translated — and `t()` hands any string it does not know straight back,
+ * so the test below cannot tell "no translation" from "this IS the text" on its own. The
+ * shape can: a sentence has spaces and no dots between words.
+ */
+const KEY_SHAPE = /^[a-z0-9_-]+(\.[a-z0-9_-]+)+$/i;
+
+/**
+ * Whether a notice has words to show, rather than the names of strings this build lacks.
+ *
+ * The keys come from the server's registry, the copy from the bundle in the browser, and a
+ * deploy parts the two for as long as it takes the page to reload: the service worker keeps
+ * serving the old app while `/api/config` already reports the new version, and the release
+ * notice for that very version arrives naming keys the old bundle never had. What is drawn
+ * is then `system_notice.release_notes.headline` where the headline belongs, on the one
+ * notice whose whole job is to be read once per release. Worse, a dismissal in that window
+ * is a real one: it is `per-version`, so whoever closes the gibberish never sees the notes.
+ *
+ * So a notice made only of unresolved keys waits. Nothing is spent by waiting — a notice is
+ * used up by `POST /dismiss` and by nothing else — and `App.tsx` reloads the page as soon
+ * as the new worker takes over, after which this returns true and the modal opens for real.
+ *
+ * Deliberately generous: one resolved key is enough. Half a translation is still readable,
+ * and a notice hidden by mistake is a worse failure than an ugly one.
+ */
+function hasCopy(notice: SystemNoticeDTO, t: (key: string) => string): boolean {
+  const keys = notice.release
+    ? [notice.release.headlineKey, notice.release.introKey, notice.release.note.titleKey]
+    : [notice.titleKey, notice.bodyKey];
+  return keys.some(key => !KEY_SHAPE.test(key) || t(key) !== key);
+}
+
 export function SystemNoticeHost() {
   const { notices, loaded } = useSystemNoticeStore();
+  const { t } = useTranslation();
   const isMobile = useIsMobile();
 
   // Notices are fetched by authStore after login (see App.tsx / authStore modification).
@@ -33,8 +70,10 @@ export function SystemNoticeHost() {
 
   if (!loaded) return null;
 
-  // desktopOnly notices (e.g. the thank-you/support modal) are hidden on mobile.
-  const visible = isMobile ? notices.filter(n => !n.desktopOnly) : notices;
+  // desktopOnly notices (e.g. the thank-you/support modal) are hidden on mobile, and a
+  // notice whose copy this build does not have yet waits for the reload (see hasCopy).
+  const readable = notices.filter(n => hasCopy(n, t));
+  const visible = isMobile ? readable.filter(n => !n.desktopOnly) : readable;
 
   const modals  = visible.filter(n => n.display === 'modal');
   const banners = visible.filter(n => n.display === 'banner');