diff --git a/internal/check-optional-t-equivalence.mjs b/internal/check-optional-t-equivalence.mjs
index e96e2f648..a11a199e5 100644
--- a/internal/check-optional-t-equivalence.mjs
+++ b/internal/check-optional-t-equivalence.mjs
@@ -135,6 +135,11 @@ const SITES = [
   ['plugin-designs/cities-internet/connectivity-list.vue', 'useOptionalT', 'fallback: enLookup'],
   ['plugin-designs/cities-internet/connectivity-map.vue',  'useOptionalT', 'fallback: enLookup'],
   ['plugin-designs/cities-internet/data-tables.vue',       'useOptionalT', 'fallback: enLookup'],
+  // wave 3c — my-upg-100-list (owner: upg100). Both used the humanizeKey fallback and no namespace,
+  // since callers pass full keys. Each keeps humanizeKey, so a bare host reads the same title-cased
+  // last segment it always did.
+  ['plugin-designs/my-upg-100-list/poster-wall.vue',  'useOptionalT', 'humanizeKey(k)'],
+  ['plugin-designs/my-upg-100-list/upg-100-mfe.vue',  'useOptionalT', 'humanizeKey(k)'],
 ]
 // ★ ONE FILE KEEPS TWO BARE useI18n CALLS ON PURPOSE, and the count is asserted rather than waived.
 // resources-map.vue reads the LOCALE twice (`useI18n().locale.value`) to decide isEnglish and the
diff --git a/plugin-designs/my-upg-100-list/poster-wall.vue b/plugin-designs/my-upg-100-list/poster-wall.vue
index ee164d4d7..dd3a90885 100644
--- a/plugin-designs/my-upg-100-list/poster-wall.vue
+++ b/plugin-designs/my-upg-100-list/poster-wall.vue
@@ -39,7 +39,7 @@
  * IDEA-POSTER-EXPORT-404-ON-DROPPED-FOLDER-TRADEOFF.
  */
 import { computed, inject, ref } from 'vue'
-import { useI18n } from 'vue-i18n'
+import { useOptionalT } from '@map/composables/useOptionalT.js'
 // The page's people-group LIST (library/lists/): every people-group fetch below keeps only its groups.
 import { applyPeopleGroupList } from '@map/lists/peopleGroupList.js'
 import PosterWall from '@map/components/poster/PosterWall.vue'
@@ -163,8 +163,13 @@ async function fetchListRecords(list) {
 // Guarded like every other useI18n() here: it throws on a host without vue-i18n.
 const humanizeKey = (k) => String(k).split('.').pop()
   .replace(/([a-z0-9])([A-Z])/g, '$1 $2').replace(/^./, (c) => c.toUpperCase())
-let t = (k) => humanizeKey(k)
-try { t = useI18n().t } catch (_) { /* no i18n on this host */ }
+// C4 wave 3c: the guard is library/composables/useOptionalT.js now. humanizeKey STAYS as the fallback,
+// passed in, so a bare host still reads a title-cased last segment rather than a dotted key. No
+// namespace: callers pass FULL keys here, exactly as the raw useI18n().t they used to get.
+// ★ ONE BEHAVIOUR GAIN, stated rather than smuggled: the old i18n path was the bare useI18n().t, so on
+// a host WITH i18n a missing key rendered the raw dotted key. It now falls through to humanizeKey, the
+// same fallback the bare host already got. Nothing that resolves today changes.
+const t = useOptionalT('', { fallback: (k) => humanizeKey(k) })
 const intro   = computed(() => t('messages.upg100WallIntro'))
 const heading = computed(() => t('messages.upg100WallHeading'))
 const wallTag = computed(() => t('messages.upg100WallTag'))
diff --git a/plugin-designs/my-upg-100-list/upg-100-mfe.vue b/plugin-designs/my-upg-100-list/upg-100-mfe.vue
index 04193a21b..eb49228cc 100644
--- a/plugin-designs/my-upg-100-list/upg-100-mfe.vue
+++ b/plugin-designs/my-upg-100-list/upg-100-mfe.vue
@@ -34,7 +34,7 @@
  * whole MFE (both panes) is what gets shared/embedded.
  */
 import { ref, computed, inject, onMounted, onBeforeUnmount } from 'vue'
-import { useI18n } from 'vue-i18n'
+import { useOptionalT } from '@map/composables/useOptionalT.js'
 import { buildPaneColumn, PANE_GRID_STYLES } from '@map/layouts/pane-wrapper/pane-wrapper.js'
 import { useShadowStyles } from '@map/composables/useShadowStyles.js'
 import ViewSelect from '@map/components/ViewSelect.vue'
@@ -50,8 +50,13 @@ import ShareButton from '@map/components/map-controls/ShareButton.vue'
 // humanises the key so a bare host shows "Select View", never a blank or a raw dotted key.
 const humanizeKey = (k) => String(k).split('.').pop()
   .replace(/([a-z0-9])([A-Z])/g, '$1 $2').replace(/^./, (c) => c.toUpperCase())
-let t = (k) => humanizeKey(k)
-try { t = useI18n().t } catch (_) { /* no i18n on this host */ }
+// C4 wave 3c: the guard is library/composables/useOptionalT.js now. humanizeKey STAYS as the fallback,
+// passed in, so a bare host still reads a title-cased last segment rather than a dotted key. No
+// namespace: callers pass FULL keys here, exactly as the raw useI18n().t they used to get.
+// ★ ONE BEHAVIOUR GAIN, stated rather than smuggled: the old i18n path was the bare useI18n().t, so on
+// a host WITH i18n a missing key rendered the raw dotted key. It now falls through to humanizeKey, the
+// same fallback the bare host already got. Nothing that resolves today changes.
+const t = useOptionalT('', { fallback: (k) => humanizeKey(k) })
 
 const cfg     = inject('profileConfig', ref(null))
 const tk      = inject('mapboxToken', ref(''))