diff --git a/frontend/src/components/map/MapPage.tsx b/frontend/src/components/map/MapPage.tsx
index a8bb6e2..62c54c6 100644
--- a/frontend/src/components/map/MapPage.tsx
+++ b/frontend/src/components/map/MapPage.tsx
@@ -7,7 +7,7 @@ import { useMapData } from '../../hooks/useMapData';
 import { usePOIData } from '../../hooks/usePOIData';
 import { useActualListings } from '../../hooks/useActualListings';
 import { buildTravelParam } from '../../lib/travel-params';
-import { useFilters } from '../../hooks/useFilters';
+import { useFilters, countEffectiveFilters } from '../../hooks/useFilters';
 import { useHexagonSelection } from '../../hooks/useHexagonSelection';
 import { usePaneResize } from '../../hooks/usePaneResize';
 import { useCollapsibleGroups } from '../../hooks/useCollapsibleGroups';
@@ -159,17 +159,49 @@ export default function MapPage({
     user?.isAdmin === true;
   const effectiveFilterCap = isLoggedIn ? REGISTERED_MAX_FILTERS : DEMO_MAX_FILTERS;
   const filterLimit = filtersUnlimited ? null : effectiveFilterCap;
-  // On a shared link, non-paying users may view and adjust the shared filters but
-  // not add new ones — so a richer shared search can't become a way around the cap.
-  const lockFilterAdds = !filtersUnlimited && !!shareCode;
-  // Which upsell the modal frames: a shared-link block, or a plain filter-cap hit.
-  const upgradeReason: 'filters' | 'shared' = lockFilterAdds ? 'shared' : 'filters';
+  // On a shared link, anonymous visitors may view and adjust the shared filters but
+  // not add new ones. Keyed off login state (not paid status) so the "create a free
+  // account to build your own" upsell isn't a dead-end — once registered, the user can
+  // add filters up to their normal cap right there on the shared search. The
+  // `!filtersUnlimited` term also excludes the non-interactive screenshot/OG render
+  // (treated as unlimited above), which must show the shared filters in full.
+  const lockFilterAdds = !isLoggedIn && !filtersUnlimited && !!shareCode;
+  // A shared/bookmarked link can encode more filters than the viewer's tier allows.
+  // The initial set is then trimmed to the cap (the server 400s the full request), so
+  // the viewer would silently see a broader result than was shared. Count the effective
+  // filters (folded + known to the backend, matching what the server counts) so stale
+  // or unknown keys don't raise a false alarm; recomputed once features finish loading.
+  const initialEffectiveFilterCount = useMemo(
+    () =>
+      countEffectiveFilters(initialFilters, features) + (initialTravelTime?.entries?.length ?? 0),
+    [initialFilters, features, initialTravelTime]
+  );
+  // Gate on features being loaded: until then countEffectiveFilters can't drop unknown
+  // keys and would over-count (firing the popup for a set that actually fits the cap).
+  const sharedSearchTrimmed =
+    features.length > 0 && !filtersUnlimited && initialEffectiveFilterCount > effectiveFilterCap;
+  // Which upsell the modal frames: a shared-link context (lock, or a trimmed shared
+  // search), or a plain filter-cap hit.
+  const upgradeReason: 'filters' | 'shared' =
+    lockFilterAdds || sharedSearchTrimmed ? 'shared' : 'filters';
   const [filterLimitHit, setFilterLimitHit] = useState(false);
   const handleFilterLimitReached = useCallback(() => {
     trackEvent('Upgrade Modal Shown');
     setFilterLimitHit(true);
   }, []);
 
+  // Pop the upgrade modal once when the shared/bookmarked search carried more filters
+  // than the viewer's tier allows — so trimmed filters are surfaced, not silently
+  // dropped. Fires when the signal first turns true (which may be after features finish
+  // loading), and only once so dismissing it sticks.
+  const overCapPopupShownRef = useRef(false);
+  useEffect(() => {
+    if (sharedSearchTrimmed && !overCapPopupShownRef.current) {
+      overCapPopupShownRef.current = true;
+      handleFilterLimitReached();
+    }
+  }, [sharedSearchTrimmed, handleFilterLimitReached]);
+
   // Travel-time destinations count toward the same cap as feature filters (each one
   // restricts which properties match). Cap an over-budget initial travel set (from a
   // bookmarked/shared URL), preferring feature filters, so the first data request
diff --git a/frontend/src/components/ui/UpgradeModal.tsx b/frontend/src/components/ui/UpgradeModal.tsx
index 60bd6fb..dedd2ba 100644
--- a/frontend/src/components/ui/UpgradeModal.tsx
+++ b/frontend/src/components/ui/UpgradeModal.tsx
@@ -65,11 +65,11 @@ export default function UpgradeModal({
   // Registered users see the lifetime upsell directly; anonymous users are nudged to
   // create a free account first (more filters + save/share/export), with the paid
   // tier offered as the route to unlimited.
+  // The `shared` reason only applies to anonymous visitors (lockFilterAdds is
+  // anonymous-only), so logged-in users always get the plain lifetime-upsell copy.
   const title = isLoggedIn ? t('upgrade.title') : t('upgrade.titleRegister');
   const description = isLoggedIn
-    ? reason === 'shared'
-      ? t('upgrade.descriptionSharedUpgrade')
-      : t('upgrade.description')
+    ? t('upgrade.description')
     : reason === 'shared'
       ? t('upgrade.descriptionSharedRegister')
       : t('upgrade.descriptionRegister');
diff --git a/frontend/src/hooks/useFilters.test.ts b/frontend/src/hooks/useFilters.test.ts
index 4f6648f..61f293a 100644
--- a/frontend/src/hooks/useFilters.test.ts
+++ b/frontend/src/hooks/useFilters.test.ts
@@ -1,7 +1,7 @@
 import { act, renderHook } from '@testing-library/react';
 import { describe, expect, it, vi } from 'vitest';
 
-import { useFilters } from './useFilters';
+import { useFilters, countEffectiveFilters } from './useFilters';
 import type { FeatureMeta } from '../types';
 
 vi.mock('../lib/analytics', () => ({
@@ -265,6 +265,45 @@ describe('useFilters', () => {
     expect(onFilterLimitReached).toHaveBeenCalledTimes(1);
   });
 
+  it('strips new keys from a bulk set (AI filters) when lockAdds is set, keeping existing ones', () => {
+    const sixFeatures: FeatureMeta[] = ['a', 'b', 'c', 'd', 'e', 'f'].map((name) => ({
+      name,
+      type: 'numeric',
+      min: 0,
+      max: 100,
+    }));
+    const onFilterLimitReached = vi.fn();
+
+    // A non-paying user on a shared link runs an AI query, which goes through the
+    // bulk handleSetFilters path. lockAdds must apply here too (not just per-add),
+    // otherwise the AI box becomes a way to build a fresh filter set on a shared link.
+    const { result } = renderHook(() =>
+      useFilters({
+        initialFilters: { a: [0, 100] },
+        features: sixFeatures,
+        filterLimit: 5,
+        lockAdds: true,
+        onFilterLimitReached,
+      })
+    );
+
+    // A bulk set that adjusts the existing key 'a' alongside new keys ('b','c') keeps
+    // only 'a' (its new value), drops the new keys, and reports the block.
+    act(() => {
+      result.current.handleSetFilters({ a: [10, 90], b: [0, 50], c: [0, 50] });
+    });
+    expect(Object.keys(result.current.filters)).toEqual(['a']);
+    expect(result.current.filters.a).toEqual([10, 90]);
+    expect(onFilterLimitReached).toHaveBeenCalledTimes(1);
+
+    // An all-new bulk set (no existing key survives) is restricted to {} and reported.
+    act(() => {
+      result.current.handleSetFilters({ d: [0, 50], e: [0, 50] });
+    });
+    expect(Object.keys(result.current.filters)).toHaveLength(0);
+    expect(onFilterLimitReached).toHaveBeenCalledTimes(2);
+  });
+
   it('does not cap filters when filterLimit is null (licensed users)', () => {
     const sixFeatures: FeatureMeta[] = ['a', 'b', 'c', 'd', 'e', 'f'].map((name) => ({
       name,
@@ -311,4 +350,51 @@ describe('useFilters', () => {
     expect(result.current.dragValue).toBeNull();
     expect(result.current.filters).toEqual({});
   });
+
+  it('drops unknown initial keys before applying the cap, not after', () => {
+    const knownFeatures: FeatureMeta[] = ['a', 'b', 'c'].map((name) => ({
+      name,
+      type: 'numeric',
+      min: 0,
+      max: 100,
+    }));
+
+    // 3 valid keys + 2 stale/unknown keys, cap 3. The unknowns must be dropped first so
+    // the cap is spent on the 3 real filters (all kept), not wasted on stale keys.
+    const { result } = renderHook(() =>
+      useFilters({
+        initialFilters: {
+          a: [0, 100],
+          stale1: [0, 100],
+          b: [0, 100],
+          stale2: [0, 100],
+          c: [0, 100],
+        },
+        features: knownFeatures,
+        filterLimit: 3,
+      })
+    );
+
+    expect(Object.keys(result.current.filters).sort()).toEqual(['a', 'b', 'c']);
+  });
+});
+
+describe('countEffectiveFilters', () => {
+  const features: FeatureMeta[] = ['a', 'b', 'c'].map((name) => ({
+    name,
+    type: 'numeric',
+    min: 0,
+    max: 100,
+  }));
+
+  it('counts only known features, ignoring stale/unknown keys', () => {
+    // The over-cap shared-link popup keys off this: stale keys must not inflate the count
+    // and trigger a false "your search was trimmed" alarm.
+    expect(countEffectiveFilters({ a: [0, 1], b: [0, 1], gone: [0, 1] }, features)).toBe(2);
+    expect(countEffectiveFilters({ a: [0, 1], b: [0, 1], c: [0, 1] }, features)).toBe(3);
+  });
+
+  it('counts everything when the feature list is empty (still loading)', () => {
+    expect(countEffectiveFilters({ a: [0, 1], b: [0, 1] }, [])).toBe(2);
+  });
 });
diff --git a/frontend/src/hooks/useFilters.ts b/frontend/src/hooks/useFilters.ts
index 27343b4..0dcd2ee 100644
--- a/frontend/src/hooks/useFilters.ts
+++ b/frontend/src/hooks/useFilters.ts
@@ -133,6 +133,15 @@ function dropUnknownFilters(filters: FeatureFilters, features: FeatureMeta[]): F
   return changed ? next : filters;
 }
 
+/** Count the *effective* filters a raw set resolves to: normalized (variant folding)
+ *  and limited to features the backend knows. Mirrors what the hook actually applies
+ *  and what the server counts, so a caller can tell whether a loaded set exceeds the
+ *  cap without re-deriving the fold/unknown-drop logic (and without counting stale or
+ *  unknown keys that would never reach the server). `features` empty → counts as-is. */
+export function countEffectiveFilters(filters: FeatureFilters, features: FeatureMeta[]): number {
+  return Object.keys(dropUnknownFilters(normalizeFilters(filters), features)).length;
+}
+
 function getNextNumericKeyId(
   filters: FeatureFilters,
   getId: (name: string) => string | null
@@ -162,8 +171,10 @@ export function useFilters({
     // Clamp an incoming filter set (e.g. from a bookmarked or shared URL) to the
     // user's remaining budget — the cap minus slots already taken by travel-time
     // filters — so a non-paying user never starts above the cap, which the server
-    // would reject (400), blanking the map on first load.
-    const normalized = normalizeFilters(initialFilters);
+    // would reject (400), blanking the map on first load. Unknown keys are dropped
+    // first WHEN the feature list is already loaded (in-app navigation); on a cold load
+    // features arrive after this runs, so the prune effect below removes them then.
+    const normalized = dropUnknownFilters(normalizeFilters(initialFilters), features);
     const budget = filterLimit != null ? Math.max(0, filterLimit - reservedFilterSlots) : null;
     initialFiltersRef.current =
       budget != null && Object.keys(normalized).length > budget
diff --git a/frontend/src/i18n/locales/de.ts b/frontend/src/i18n/locales/de.ts
index 07b60fa..5e07443 100644
--- a/frontend/src/i18n/locales/de.ts
+++ b/frontend/src/i18n/locales/de.ts
@@ -637,8 +637,6 @@ const de: Translations = {
     title: 'Unbegrenzte Filter freischalten',
     description:
       'Du hast alle 5 Filter deines kostenlosen Kontos genutzt. Erhalte lebenslangen Zugang, um unbegrenzt Filter über jeden Postcode und jede Nachbarschaft in England hinweg zu kombinieren. Einmal zahlen, für immer nutzen.',
-    descriptionSharedUpgrade:
-      'Um einer geteilten Suche weitere Filter hinzuzufügen, ist ein lebenslanger Zugang erforderlich — schalte unbegrenzte Filter für jeden Postcode in England frei.',
     titleRegister: 'Kostenloses Konto erstellen',
     descriptionRegister:
       'Kostenlose Konten schalten bis zu 5 Filter sowie Speichern, Teilen und Exportieren frei — keine Kreditkarte erforderlich. Willst du unbegrenzte Filter für ganz England? Wähle den lebenslangen Zugang.',
diff --git a/frontend/src/i18n/locales/en.ts b/frontend/src/i18n/locales/en.ts
index a13b540..f2b5857 100644
--- a/frontend/src/i18n/locales/en.ts
+++ b/frontend/src/i18n/locales/en.ts
@@ -626,8 +626,6 @@ const en = {
     title: 'Unlock unlimited filters',
     description:
       "You've used all 5 filters on your free account. Get lifetime access to stack unlimited filters across every postcode and neighbourhood in England. One payment, forever.",
-    descriptionSharedUpgrade:
-      'Adding more filters to a shared search needs lifetime access — unlock unlimited filters across every postcode in England.',
     // Anonymous users — create a free account first.
     titleRegister: 'Create a free account',
     descriptionRegister:
diff --git a/frontend/src/i18n/locales/fr.ts b/frontend/src/i18n/locales/fr.ts
index b8905c2..c89c4a3 100644
--- a/frontend/src/i18n/locales/fr.ts
+++ b/frontend/src/i18n/locales/fr.ts
@@ -650,8 +650,6 @@ const fr: Translations = {
     title: 'Débloquez les filtres illimités',
     description:
       'Vous avez utilisé les 5 filtres de votre compte gratuit. Obtenez un accès à vie pour superposer un nombre illimité de filtres sur chaque code postal et quartier d’Angleterre. Un seul paiement, pour toujours.',
-    descriptionSharedUpgrade:
-      'Ajouter des filtres supplémentaires à une recherche partagée nécessite un accès à vie — débloquez des filtres illimités sur chaque code postal d’Angleterre.',
     titleRegister: 'Créer un compte gratuit',
     descriptionRegister:
       'Les comptes gratuits débloquent jusqu’à 5 filtres ainsi que la sauvegarde, le partage et l’export — sans carte bancaire. Des filtres illimités dans toute l’Angleterre ? Passez à l’accès à vie.',
diff --git a/frontend/src/i18n/locales/hi.ts b/frontend/src/i18n/locales/hi.ts
index cbee11d..d798267 100644
--- a/frontend/src/i18n/locales/hi.ts
+++ b/frontend/src/i18n/locales/hi.ts
@@ -622,8 +622,6 @@ const hi: Translations = {
     title: 'असीमित फ़िल्टर अनलॉक करें',
     description:
       'आपने अपने मुफ्त खाते के सभी 5 फ़िल्टर इस्तेमाल कर लिए हैं. इंग्लैंड के हर पोस्टकोड और पड़ोस में असीमित फ़िल्टर लगाने के लिए आजीवन पहुँच पाएं. एक भुगतान, हमेशा के लिए.',
-    descriptionSharedUpgrade:
-      'साझा खोज में और फ़िल्टर जोड़ने के लिए आजीवन पहुँच चाहिए — इंग्लैंड के हर पोस्टकोड में असीमित फ़िल्टर अनलॉक करें.',
     titleRegister: 'मुफ्त खाता बनाएं',
     descriptionRegister:
       'मुफ्त खाते में 5 फ़िल्टर के साथ-साथ सहेजें, साझा करें और export करें — कार्ड की जरूरत नहीं. पूरे इंग्लैंड में असीमित फ़िल्टर चाहिए? आजीवन पहुँच लें.',
diff --git a/frontend/src/i18n/locales/hu.ts b/frontend/src/i18n/locales/hu.ts
index ccc5abc..1e91bef 100644
--- a/frontend/src/i18n/locales/hu.ts
+++ b/frontend/src/i18n/locales/hu.ts
@@ -641,8 +641,6 @@ const hu: Translations = {
     title: 'Oldd fel a korlátlan szűrőket',
     description:
       'Felhasználtad az ingyenes fiókod mind az 5 szűrőjét. Szerezz élethosszig tartó hozzáférést, hogy korlátlanul rétegezhesd a szűrőket Anglia minden irányítószámán és környékén. Egyetlen fizetés, örökre.',
-    descriptionSharedUpgrade:
-      'Egy megosztott kereséshez több szűrőt hozzáadni élethosszig tartó hozzáférést igényel – oldd fel a korlátlan szűrőket Anglia minden irányítószámán.',
     titleRegister: 'Ingyenes fiók létrehozása',
     descriptionRegister:
       'Az ingyenes fiókok akár 5 szűrőt oldanak fel, ráadásul menthetsz, megoszthatsz és exportálhatsz – kártya nélkül. Korlátlan szűrőket szeretnél egész Angliában? Válaszd az élethosszig tartó hozzáférést.',
diff --git a/frontend/src/i18n/locales/zh.ts b/frontend/src/i18n/locales/zh.ts
index 84f1ff3..3cc8189 100644
--- a/frontend/src/i18n/locales/zh.ts
+++ b/frontend/src/i18n/locales/zh.ts
@@ -597,8 +597,6 @@ const zh: Translations = {
     registerAndUpgrade: '注册并升级',
     alreadyHaveAccount: '已有账户？请登录',
     continueFree: '暂时继续浏览',
-    descriptionSharedUpgrade:
-      '向分享的搜索添加更多筛选条件需要终身访问权限 — 解锁英格兰每个邮编的无限筛选。',
     titleRegister: '注册免费账户',
     descriptionRegister:
       '免费账户解锁最多 5 个筛选条件，并支持保存、分享和导出 — 无需信用卡。想要在整个英格兰使用无限筛选？选择终身访问。',