diff --git a/android/app/src/main/java/com/fabledsword/inkwell/ui/DerivedTint.kt b/android/app/src/main/java/com/fabledsword/inkwell/ui/DerivedTint.kt
index 1ceb496..15e50cb 100644
--- a/android/app/src/main/java/com/fabledsword/inkwell/ui/DerivedTint.kt
+++ b/android/app/src/main/java/com/fabledsword/inkwell/ui/DerivedTint.kt
@@ -86,9 +86,9 @@ fun derivedTint(id: String): String {
  * Deriving has no mint points to miss and no backfill for the tags already out there.
  * The cost is that renaming a tag recolours it, which is fair: the name IS the tag.
  *
- * Lowercased because tags dedupe case-insensitively — `#Todo` renamed to `#todo` is
- * the same tag and should not change colour. Kotlin's `lowercase()` and the web's
- * `toLowerCase()` are both locale-independent, so the mirror holds.
+ * Folded because `#Todo` renamed to `#todo` is the same tag and should not change
+ * colour. [foldTag] and the web's are both locale-independent lowercasing, so the
+ * mirror holds.
  */
 fun resolvedLabelColor(
     name: String,
@@ -98,5 +98,12 @@ fun resolvedLabelColor(
     when {
         color.isNotEmpty() && color != "default" && color in known -> color
         name.isEmpty() -> "default"
-        else -> derivedTint(name.lowercase())
+        else -> derivedTint(foldTag(name))
     }
+
+/**
+ * How two tag names compare: equal once every letter is lowercased, accented ones
+ * included, so `#Café` and `#CAFÉ` are one tag. The core's `derive::fold`, the server's
+ * `labeling.named` and the web's `foldTag` fold the same way (#5385).
+ */
+fun foldTag(name: String): String = name.lowercase()
diff --git a/android/app/src/main/java/com/fabledsword/inkwell/ui/NoteCard.kt b/android/app/src/main/java/com/fabledsword/inkwell/ui/NoteCard.kt
index 22c79bf..0ee1a13 100644
--- a/android/app/src/main/java/com/fabledsword/inkwell/ui/NoteCard.kt
+++ b/android/app/src/main/java/com/fabledsword/inkwell/ui/NoteCard.kt
@@ -366,11 +366,11 @@ private fun tintTags(
             // A tag the note does not carry as a label yet — just typed, not yet
             // derived — still gets a colour: `labelTint` falls back to deriving one
             // from the name, which is what the chip would have shown anyway.
-            val picked = note.labels.associate { it.name.lowercase() to it.color }
+            val picked = note.labels.associate { foldTag(it.name) to it.color }
             buildAnnotatedString {
                 append(text)
                 spans.forEach { tag ->
-                    val tint = labelTint(tag.name, picked[tag.name.lowercase()].orEmpty())
+                    val tint = labelTint(tag.name, picked[foldTag(tag.name)].orEmpty())
                     addStyle(
                         SpanStyle(color = tint.tagInk(dark), fontWeight = FontWeight.Medium),
                         tag.start.toInt(),
diff --git a/android/app/src/main/java/com/fabledsword/inkwell/ui/TagsScreen.kt b/android/app/src/main/java/com/fabledsword/inkwell/ui/TagsScreen.kt
index 66270f7..ada54b1 100644
--- a/android/app/src/main/java/com/fabledsword/inkwell/ui/TagsScreen.kt
+++ b/android/app/src/main/java/com/fabledsword/inkwell/ui/TagsScreen.kt
@@ -401,7 +401,7 @@ private fun RenameDialog(
     var text by remember(tag.id) { mutableStateOf(tag.name) }
     val trimmed = text.trim()
     val clash =
-        others.firstOrNull { it.id != tag.id && it.name.equals(trimmed, ignoreCase = true) }
+        others.firstOrNull { it.id != tag.id && foldTag(it.name) == foldTag(trimmed) }
     val submit = {
         when {
             trimmed.isEmpty() -> Unit
diff --git a/frontend/src/components/LabelPicker.vue b/frontend/src/components/LabelPicker.vue
index d91ed32..9848356 100644
--- a/frontend/src/components/LabelPicker.vue
+++ b/frontend/src/components/LabelPicker.vue
@@ -3,6 +3,7 @@ import { computed, onBeforeUnmount, onMounted, ref } from "vue";
 import { useLabelsStore, type Label } from "../stores/labels";
 import Icon from "./Icon.vue";
 import type { NoteLabel } from "../stores/notes";
+import { foldTag } from "../notes/tags";
 
 // A label attached via the picker is manual (via_tag: false).
 const asManual = (lb: Label): NoteLabel => ({ id: lb.id, name: lb.name, color: lb.color, via_tag: false });
@@ -21,7 +22,7 @@ const shown = computed(() =>
 );
 const canCreate = computed(() => {
   const name = filter.value.trim();
-  return name.length > 0 && !labels.items.some((lb) => lb.name.toLowerCase() === name.toLowerCase());
+  return name.length > 0 && !labels.items.some((lb) => foldTag(lb.name) === foldTag(name));
 });
 
 function toggle(lb: Label) {
diff --git a/frontend/src/components/NoteCard.vue b/frontend/src/components/NoteCard.vue
index a396c0d..76188c0 100644
--- a/frontend/src/components/NoteCard.vue
+++ b/frontend/src/components/NoteCard.vue
@@ -20,6 +20,7 @@ import {
 import { formatShortDateTime, formatTrashCountdown, isOverdue, trashDaysLeft } from "../notes/datetime";
 import { useConfigStore } from "../stores/config";
 import { canEditText, isOwnNote } from "../notes/sharing";
+import { foldTag } from "../notes/tags";
 
 const props = defineProps<{ note: Note; reorderable?: boolean; active?: boolean }>();
 const emit = defineEmits<{
@@ -200,12 +201,12 @@ watch(
 // through the picker.
 const chipLabels = computed(() => props.note.labels.filter((lb) => !lb.via_tag));
 
-// The colour the operator stored for each of this note's tags, keyed by lowercased
+// The colour the operator stored for each of this note's tags, keyed by folded
 // name — what MarkdownInline needs to tint a `#tag` the same as its chip would be.
-// Lowercased because tags dedupe case-insensitively, so `#Todo` and `#todo` are one.
+// Folded because `#Todo` and `#todo` are one tag.
 const tagColors = computed<Record<string, string>>(() => {
   const map: Record<string, string> = {};
-  for (const lb of props.note.labels) map[lb.name.toLowerCase()] = lb.color;
+  for (const lb of props.note.labels) map[foldTag(lb.name)] = lb.color;
   return map;
 });
 
diff --git a/frontend/src/notes/colors.ts b/frontend/src/notes/colors.ts
index 12930b2..a6d61b8 100644
--- a/frontend/src/notes/colors.ts
+++ b/frontend/src/notes/colors.ts
@@ -2,6 +2,8 @@
 // here (frontend concern). Class strings are full literals so Tailwind's content
 // scanner (src/**/*.ts) keeps them in the build.
 
+import { foldTag } from "./tags";
+
 export const NOTE_COLOR_KEYS = [
   "default",
   "red",
@@ -82,13 +84,13 @@ export const TAG_TEXT_CLASSES: Record<NoteColor, string> = {
 /**
  * The classes for one `#tag` in a note's own words.
  *
- * `picked` maps a lowercased tag name to the colour stored on that label, so a tag the
+ * `picked` maps a folded tag name (`foldTag`) to the colour stored on that label, so a tag the
  * operator has recoloured reads the same inline as it does on a chip. A tag the note
  * does not carry as a label yet — just typed, not yet derived — is not in the map, and
  * `resolveLabelColor` derives one from the name exactly as the chip would have.
  */
 export function tagTextClasses(name: string, picked?: Record<string, string>): string {
-  return TAG_TEXT_CLASSES[resolveLabelColor({ name, color: picked?.[name.toLowerCase()] })];
+  return TAG_TEXT_CLASSES[resolveLabelColor({ name, color: picked?.[foldTag(name)] })];
 }
 
 /**
@@ -200,15 +202,15 @@ export function derivedTint(id: string): NoteColor {
  * An explicitly-picked colour is still stored and still wins, so tag colours stay
  * editable exactly as asked.
  *
- * Lowercased because tags dedupe case-insensitively — `#Todo` renamed to `#todo` is
- * the same tag and should not change colour. Both `toLowerCase` here and Kotlin's
- * `lowercase()` are locale-independent, so the mirror holds.
+ * Folded because `#Todo` renamed to `#todo` is the same tag and should not change
+ * colour. `foldTag` here and Android's are both locale-independent lowercasing, so
+ * the mirror holds.
  */
 export function resolveLabelColor(label: { name: string; color?: string | null }): NoteColor {
   const picked = label.color as NoteColor | undefined | null;
   if (picked && picked !== "default" && KNOWN_COLORS.has(picked)) return picked;
   if (!label.name) return "default";
-  return derivedTint(label.name.toLowerCase());
+  return derivedTint(foldTag(label.name));
 }
 
 // The fixture (four UUID hashes and seven tag names) lives in
diff --git a/frontend/src/notes/tags.ts b/frontend/src/notes/tags.ts
new file mode 100644
index 0000000..7fa64b2
--- /dev/null
+++ b/frontend/src/notes/tags.ts
@@ -0,0 +1,6 @@
+// How two tag names compare: equal once every letter is lowercased, accented ones
+// included, so `#Café` and `#CAFÉ` are one tag. The core's `derive::fold`, the
+// server's `labeling.named` and Android's `foldTag` fold the same way (#5385).
+export function foldTag(name: string): string {
+  return name.toLowerCase();
+}
diff --git a/frontend/src/stores/labels.ts b/frontend/src/stores/labels.ts
index ece0494..846d40e 100644
--- a/frontend/src/stores/labels.ts
+++ b/frontend/src/stores/labels.ts
@@ -1,6 +1,7 @@
 import { defineStore } from "pinia";
 import { ref } from "vue";
 import { repo } from "../adapters";
+import { foldTag } from "../notes/tags";
 
 export interface Label {
   id: string;
@@ -45,7 +46,7 @@ export const useLabelsStore = defineStore("labels", () => {
     // the response: the survivor is whichever row is older, so it may well be the
     // one we asked to rename, and an id that still matches proves nothing happened.
     const absorbing = items.value.find(
-      (lb) => lb.id !== id && lb.name.toLowerCase() === name.toLowerCase(),
+      (lb) => lb.id !== id && foldTag(lb.name) === foldTag(name),
     );
     if (absorbing) {
       // A merge cannot be undone by repeating it, and here it is reachable by a