diff --git a/android/ffi/src/lib.rs b/android/ffi/src/lib.rs
index 96cff38..6cdffb4 100644
--- a/android/ffi/src/lib.rs
+++ b/android/ffi/src/lib.rs
@@ -336,17 +336,19 @@ impl ThoughtSync {
     /// Rename a label. Every note carrying it follows, because notes reference it
     /// by id and never by name.
     ///
-    /// Renaming onto a name that already exists does NOT merge, and does not fail
-    /// either — the core's find-or-create matching only runs on the create path, so
-    /// this leaves two labels whose names differ by case at most.
+    /// Renaming onto a name another tag already holds MERGES the two, and the OLDER
+    /// row is the survivor — it keeps its id and colour and takes the new spelling.
+    /// Matching is case-insensitive, like `find_or_create_label`.
     ///
-    /// The SERVER disagrees: `labels.py` answers that PATCH with 409 "a label with
-    /// that name already exists". So the local stores (this and the desktop, which
-    /// calls the same `store::rename_label`) are more permissive than the REST path
-    /// the web uses, and a duplicate made offline will meet that 409 on sync. Not
-    /// introduced here — it predates Android having a rename at all — but a UI over
-    /// this has to decide what it shows, so it is written down rather than found.
-    /// Merging is the deliberate, irreversible operation and stays a separate button.
+    /// So this call can return a label whose id is NOT the one passed in, and it can
+    /// make another label stop existing. A UI over it should say so before calling:
+    /// the merge cannot be undone by repeating it, and here it is reachable by a
+    /// typo in a text field. The web asks first (`stores/labels.ts`); this binding
+    /// deliberately does not, because a confirmation belongs to the surface that has
+    /// a person in front of it, not to the store.
+    ///
+    /// `store::rename_label` and the server's PATCH implement the same rule, so the
+    /// phone, the desktop and the web agree on which row survives.
     pub fn rename_label(&self, id: String, name: String) -> Result<Label, CoreError> {
         let conn = self.db.conn().map_err(CoreError::store)?;
         local::store::rename_label(&conn, &id, &name)
@@ -963,6 +965,75 @@ mod tests {
         std::fs::remove_dir_all(&dir).ok();
     }
 
+    /// Renaming a tag onto a name another tag already holds MERGES the two, and the
+    /// OLDER row is the survivor.
+    ///
+    /// Before this, the bare UPDATE met `idx_labels_name` — unique on `lower(name)` —
+    /// and the user got a raw "UNIQUE constraint failed" from SQLite. Merging is what
+    /// a person means by typing an existing tag's name onto this one.
+    #[test]
+    fn renaming_onto_an_existing_tag_merges_into_the_older_one() {
+        let dir = scratch_dir();
+        let app = ThoughtSync::new(dir.clone()).expect("a fresh data dir should open");
+
+        let older = app.create_label("grocery".to_string()).expect("older");
+        // `created_at` is RFC3339 to the MILLISECOND. Without a gap the two rows can
+        // share a timestamp, and then the tie-break is under test instead of the age
+        // rule this test is about.
+        std::thread::sleep(std::time::Duration::from_millis(5));
+        let newer = app.create_label("errands".to_string()).expect("newer");
+
+        let one = app.create_note(draft("milk")).expect("note one");
+        let two = app.create_note(draft("stamps")).expect("note two");
+        app.set_note_labels(one.id.clone(), vec![older.id.clone()])
+            .expect("tag one");
+        app.set_note_labels(two.id.clone(), vec![newer.id.clone()])
+            .expect("tag two");
+
+        // The YOUNGER one is renamed onto the older's name, in a different case —
+        // matching is case-insensitive, and the survivor takes the spelling asked for.
+        let survivor = app
+            .rename_label(newer.id.clone(), "Grocery".to_string())
+            .expect("a rename onto an existing name merges instead of failing");
+
+        assert_eq!(survivor.id, older.id, "the older row is the one that survives");
+        assert_eq!(survivor.name, "Grocery", "spelled the way the caller asked");
+
+        let all = app.list_labels().expect("list");
+        assert_eq!(all.len(), 1, "the two became one");
+        assert_eq!(all[0].id, older.id);
+        assert_eq!(all[0].count, Some(2), "carrying every note from both sides");
+
+        std::fs::remove_dir_all(&dir).ok();
+    }
+
+    /// The mirror of the test above. Renaming the OLDER one onto the younger's name
+    /// still leaves the older row standing — it just changes its name.
+    ///
+    /// This is the whole reason age decides rather than "whoever already held the
+    /// name": otherwise the survivor depends on which way round someone typed it,
+    /// and two devices tidying the same pair would disagree about which id exists.
+    #[test]
+    fn the_rename_merge_survivor_does_not_depend_on_the_direction() {
+        let dir = scratch_dir();
+        let app = ThoughtSync::new(dir.clone()).expect("a fresh data dir should open");
+
+        let older = app.create_label("grocery".to_string()).expect("older");
+        std::thread::sleep(std::time::Duration::from_millis(5));
+        let newer = app.create_label("errands".to_string()).expect("newer");
+
+        let survivor = app
+            .rename_label(older.id.clone(), "errands".to_string())
+            .expect("rename");
+
+        assert_eq!(survivor.id, older.id, "age wins in this direction too");
+        assert_eq!(survivor.name, "errands");
+        assert_ne!(survivor.id, newer.id, "the younger row is the one that went");
+        assert_eq!(app.list_labels().expect("list").len(), 1);
+
+        std::fs::remove_dir_all(&dir).ok();
+    }
+
     /// A crude RFC3339 sanity check that doesn't pull a date crate into this
     /// crate's dev-dependencies to assert one field is well-formed.
     fn chrono_free_parse(raw: &str) -> usize {
diff --git a/core/src/local/store.rs b/core/src/local/store.rs
index ccdffe3..dddc51d 100644
--- a/core/src/local/store.rs
+++ b/core/src/local/store.rs
@@ -820,7 +820,57 @@ pub fn create_label(conn: &Connection, name: &str) -> rusqlite::Result<Label> {
     load_label(conn, &id)
 }
 
+/// Rename a label. Renaming ONTO a name another label already holds MERGES the two.
+///
+/// It cannot simply be an UPDATE: `idx_labels_name` is unique on `lower(name)`, so
+/// the bare statement failed with a raw SQLite "UNIQUE constraint failed" that
+/// reached the user as database internals. Merging is the operator's call, and it
+/// is the reading that matches what a person means — typing an existing tag's name
+/// onto this one says "these are the same thing."
+///
+/// THE OLDER ROW SURVIVES, and takes the new spelling. Older rather than "the one
+/// that already held the name" because age is the property neither participant's
+/// role can change: rename A→B and rename B→A must land on the same survivor, or
+/// the result depends on which way round someone happened to type it. Ties (two
+/// labels minted in the same millisecond) go to the incumbent, so the outcome is
+/// still deterministic.
+///
+/// Matching is case-insensitive, agreeing with `find_or_create_label` — "Groceries"
+/// finds "groceries", and the survivor ends up spelled the way the caller asked.
 pub fn rename_label(conn: &Connection, id: &str, name: &str) -> rusqlite::Result<Label> {
+    let clash: Option<(String, String)> = conn
+        .query_row(
+            "SELECT id, created_at FROM labels WHERE lower(name) = lower(?1) AND id <> ?2",
+            params![name, id],
+            |r| Ok((r.get(0)?, r.get(1)?)),
+        )
+        .optional()?;
+
+    if let Some((other_id, other_created)) = clash {
+        let mine_created: String =
+            conn.query_row("SELECT created_at FROM labels WHERE id = ?1", [id], |r| {
+                r.get(0)
+            })?;
+        // `created_at` is RFC3339 to the millisecond with a `Z`, so it is fixed-width
+        // and lexicographic order IS chronological order — no parsing needed.
+        let (survivor, doomed) = if other_created <= mine_created {
+            (other_id, id.to_string())
+        } else {
+            (id.to_string(), other_id)
+        };
+        // Reuse the merge rather than re-implement it: it is the only place that
+        // knows to mark every affected NOTE dirty before the delete cascades the
+        // membership rows away, which is what makes the merge reach the server.
+        merge_labels(conn, &doomed, &survivor)?;
+        // The survivor may still carry the old spelling — it is the one that keeps
+        // existing, so it is the one that has to end up named what was asked for.
+        conn.execute(
+            "UPDATE labels SET name = ?1, updated_at = ?2, dirty = 1 WHERE id = ?3",
+            params![name, now(), survivor],
+        )?;
+        return load_label(conn, &survivor);
+    }
+
     conn.execute(
         "UPDATE labels SET name = ?1, updated_at = ?2, dirty = 1 WHERE id = ?3",
         params![name, now(), id],
diff --git a/frontend/src/stores/labels.ts b/frontend/src/stores/labels.ts
index f64a182..403347d 100644
--- a/frontend/src/stores/labels.ts
+++ b/frontend/src/stores/labels.ts
@@ -33,7 +33,35 @@ export const useLabelsStore = defineStore("labels", () => {
   }
 
   async function rename(id: string, name: string): Promise<void> {
+    // Renaming onto a name another tag already holds MERGES the two — server-side
+    // and in the local store, identically. Detected from the LIST rather than from
+    // 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(),
+    );
+    if (absorbing) {
+      // A merge cannot be undone by repeating it, and here it is reachable by a
+      // typo in a text field — so it asks, the way deleting one does. The counts
+      // are named because "40 notes" is the part that makes the consequence real.
+      const mine = items.value.find((lb) => lb.id === id);
+      const confirmed = window.confirm(
+        `A tag called "${absorbing.name}" already exists.\n\n` +
+          `Renaming will MERGE these two into one tag named "${name}", carrying ` +
+          `every note from both (${mine?.count ?? 0} + ${absorbing.count ?? 0}). ` +
+          "The notes are kept; one of the two tags stops existing, and that cannot " +
+          "be undone.",
+      );
+      if (!confirmed) return;
+    }
+
     const updated = await repo.labels.rename(id, name);
+    if (absorbing) {
+      // One row is gone and the survivor's count grew, and this response carries no
+      // count — reload rather than guess which of the two we are now holding.
+      await load();
+      return;
+    }
     const idx = items.value.findIndex((lb) => lb.id === id);
     // The single-label PATCH doesn't recompute the count — keep the one we have.
     if (idx >= 0) items.value[idx] = { ...updated, count: items.value[idx].count };
@@ -53,7 +81,7 @@ export const useLabelsStore = defineStore("labels", () => {
     // notes themselves survive; only the membership goes, which is the part people
     // most need reassuring about.
     const label = items.value.find((lb) => lb.id === id);
-    const subject = label ? `the label "${label.name}"` : "this label";
+    const subject = label ? `the tag "${label.name}"` : "this tag";
     const confirmed = window.confirm(
       `Delete ${subject}?\n\n` +
         "It will be removed from every note that uses it, on every device you sync " +
diff --git a/src/thoughtsync/labels.py b/src/thoughtsync/labels.py
index 483e413..645d06d 100644
--- a/src/thoughtsync/labels.py
+++ b/src/thoughtsync/labels.py
@@ -27,6 +27,36 @@ async def _label_note_count(db, label_id) -> int:
     )
 
 
+async def _merge_into(db, source: Label, target: Label) -> None:
+    """Move every note tagged with `source` onto `target`, then delete `source`.
+
+    Shared by the explicit `/merge` route and by a rename that lands on a name
+    another tag already holds — those are the same operation, and having one body
+    is what stops them drifting into two answers for one question.
+
+    Note-body `#tags` are NOT rewritten, so a note whose body still literally
+    contains the source `#tag` will re-mint that tag on its next edit. Retiring a
+    tag means editing it out of the text; a known, documented nuance.
+    """
+    # Notes already carrying the target: a note can't hold the same label twice
+    # (composite PK), so the source attachment there is just dropped as a dup.
+    target_notes = set(
+        (await db.scalars(select(NoteLabel.note_id).where(NoteLabel.label_id == target.id))).all()
+    )
+    source_rows = (await db.scalars(select(NoteLabel).where(NoteLabel.label_id == source.id))).all()
+    by_note = {r.note_id: r.via_tag for r in source_rows}
+    # Delete the source attachments first, then re-insert under the target — moving
+    # by delete+insert avoids mutating a composite primary-key column in place.
+    for row in source_rows:
+        await db.delete(row)
+    await db.flush()
+    for note_id, via_tag in by_note.items():
+        if note_id not in target_notes:
+            db.add(NoteLabel(note_id=note_id, label_id=target.id, via_tag=via_tag))
+    await db.delete(source)
+    await db.flush()
+
+
 async def _get_owned_label(db, label_id: str) -> Label | None:
     lid = parse_uuid(label_id)
     if lid is None:
@@ -60,8 +90,13 @@ async def create_label():
     if not name:
         return json_error("tag name is required", 400)
     async with session_scope() as db:
-        # Idempotent: creating an existing label just returns it.
-        existing = await db.scalar(select(Label).where(Label.owner_id == g.user_id, Label.name == name))
+        # Idempotent: creating an existing tag just returns it. Case-INSENSITIVE,
+        # like the clients' `find_or_create_label` — a case-sensitive match here was
+        # the path that MINTED the "Groceries" beside "groceries" pair that no synced
+        # client can hold, since their `labels` index is unique on `lower(name)`.
+        existing = await db.scalar(
+            select(Label).where(Label.owner_id == g.user_id, func.lower(Label.name) == name.lower())
+        )
         if existing is not None:
             return jsonify(_serialize_label(existing)), 200
         label = Label(owner_id=g.user_id, name=name, color=normalize_color(data.get("color")))
@@ -87,11 +122,31 @@ async def update_label(label_id: str):
         if label is None:
             return not_found()
         if has_name:
+            # Case-INSENSITIVE, matching the clients' `find_or_create_label` and the
+            # local store's `lower(name)` unique index. The Postgres constraint here
+            # is on the raw name, so the database would happily hold "Groceries"
+            # beside "groceries" — but no synced client can store both, so letting
+            # one be made is letting a pull fail later on a phone.
             clash = await db.scalar(
-                select(Label).where(Label.owner_id == g.user_id, Label.name == name, Label.id != label.id)
+                select(Label).where(
+                    Label.owner_id == g.user_id,
+                    func.lower(Label.name) == name.lower(),
+                    Label.id != label.id,
+                )
             )
             if clash is not None:
-                return json_error("a tag with that name already exists", 409)
+                # Renaming onto an existing tag MERGES the two rather than failing:
+                # typing an existing tag's name onto this one says they are the same
+                # thing. The OLDER row survives and takes the new spelling — age is
+                # the one property that does not depend on which of the two the
+                # caller happened to be renaming, so A→B and B→A agree. Ties go to
+                # the incumbent. Mirrors `store::rename_label` exactly.
+                if clash.created_at <= label.created_at:
+                    survivor, doomed = clash, label
+                else:
+                    survivor, doomed = label, clash
+                await _merge_into(db, doomed, survivor)
+                label = survivor
             label.name = name
         if has_color:
             label.color = normalize_color(data.get("color"))
@@ -129,23 +184,7 @@ async def merge_label(label_id: str):
             return not_found()
         if source.id == target.id:
             return json_error("cannot merge a tag into itself", 400)
-        # Notes already carrying the target: a note can't hold the same label twice
-        # (composite PK), so the source attachment there is just dropped as a dup.
-        target_notes = set(
-            (await db.scalars(select(NoteLabel.note_id).where(NoteLabel.label_id == target.id))).all()
-        )
-        source_rows = (await db.scalars(select(NoteLabel).where(NoteLabel.label_id == source.id))).all()
-        by_note = {r.note_id: r.via_tag for r in source_rows}
-        # Delete the source attachments first, then re-insert under the target — moving
-        # by delete+insert avoids mutating a composite primary-key column in place.
-        for row in source_rows:
-            await db.delete(row)
-        await db.flush()
-        for note_id, via_tag in by_note.items():
-            if note_id not in target_notes:
-                db.add(NoteLabel(note_id=note_id, label_id=target.id, via_tag=via_tag))
-        await db.delete(source)
-        await db.flush()
+        await _merge_into(db, source, target)
         count = await _label_note_count(db, target.id)
         await db.commit()
         return jsonify(_serialize_label(target, count))
diff --git a/tests/test_integration.py b/tests/test_integration.py
index 527033e..d926738 100644
--- a/tests/test_integration.py
+++ b/tests/test_integration.py
@@ -543,3 +543,90 @@ async def test_a_revision_inside_the_window_blocks_another(db, owner):
     await db.commit()
 
     assert await should_snapshot(db, note.id, note.body, "draft revised") is False
+
+
+async def test_renaming_a_tag_onto_an_existing_one_merges_into_the_older(app_client, db):
+    """Renaming a tag onto a name another tag holds merges them, and the OLDER row
+    is the survivor — whichever side the caller happened to be renaming.
+
+    Runs against a real database because the whole question is about `created_at`
+    ordering and the note_labels rows moving, neither of which a unit test sees.
+
+    Age decides, rather than "the one that already held the name", so that renaming
+    A→B and renaming B→A land on the same row. If the incumbent won, the survivor
+    would depend on which way round someone typed it, and two clients racing the
+    same tidy-up would disagree about which id still exists.
+    """
+    reg = await app_client.post(
+        "/api/auth/register",
+        json={"email": "tags@example.test", "password": "a-long-enough-password"},
+    )
+    assert reg.status_code == 201
+
+    # Two separate requests, so two transactions and two distinct `func.now()`s.
+    older = await (await app_client.post("/api/labels", json={"name": "grocery"})).get_json()
+    newer = await (await app_client.post("/api/labels", json={"name": "errands"})).get_json()
+
+    one = await (await app_client.post("/api/notes", json={"body": "milk"})).get_json()
+    two = await (await app_client.post("/api/notes", json={"body": "stamps"})).get_json()
+    await app_client.put(f"/api/notes/{one['id']}/labels", json={"label_ids": [older["id"]]})
+    await app_client.put(f"/api/notes/{two['id']}/labels", json={"label_ids": [newer["id"]]})
+
+    # Rename the YOUNGER onto the older's name, with different casing — matching is
+    # case-insensitive, and the survivor must end up spelled the way we asked.
+    resp = await app_client.patch(f"/api/labels/{newer['id']}", json={"name": "Grocery"})
+    assert resp.status_code == 200
+    survivor = await resp.get_json()
+
+    assert survivor["id"] == older["id"], "the older row is the one that keeps existing"
+    assert survivor["name"] == "Grocery", "the survivor takes the spelling that was asked for"
+
+    listing = (await (await app_client.get("/api/labels")).get_json())["labels"]
+    assert len(listing) == 1, "the two became one"
+    assert listing[0]["id"] == older["id"]
+    assert listing[0]["count"] == 2, "it carries every note from both sides"
+
+
+async def test_the_rename_merge_survivor_does_not_depend_on_the_direction(app_client, db):
+    """The mirror of the test above: rename the OLDER onto the younger's name. The
+    older still survives — it just changes its name — so the two directions agree."""
+    reg = await app_client.post(
+        "/api/auth/register",
+        json={"email": "tags2@example.test", "password": "a-long-enough-password"},
+    )
+    assert reg.status_code == 201
+
+    older = await (await app_client.post("/api/labels", json={"name": "grocery"})).get_json()
+    newer = await (await app_client.post("/api/labels", json={"name": "errands"})).get_json()
+
+    resp = await app_client.patch(f"/api/labels/{older['id']}", json={"name": "errands"})
+    assert resp.status_code == 200
+    survivor = await resp.get_json()
+
+    assert survivor["id"] == older["id"], "age wins in this direction too"
+    assert survivor["name"] == "errands"
+    assert newer["id"] != older["id"]
+
+    listing = (await (await app_client.get("/api/labels")).get_json())["labels"]
+    assert [lb["id"] for lb in listing] == [older["id"]]
+
+
+async def test_creating_a_tag_that_differs_only_in_case_returns_the_existing_one(app_client, db):
+    """A case-sensitive match here used to mint "Groceries" beside "groceries". No
+    synced client can hold both — their `labels` index is unique on `lower(name)` —
+    so the pair was a pull that would fail later, on a phone, with no UI in the path.
+    """
+    reg = await app_client.post(
+        "/api/auth/register",
+        json={"email": "tags3@example.test", "password": "a-long-enough-password"},
+    )
+    assert reg.status_code == 201
+
+    first = await app_client.post("/api/labels", json={"name": "groceries"})
+    assert first.status_code == 201
+    second = await app_client.post("/api/labels", json={"name": "Groceries"})
+    assert second.status_code == 200, "an existing tag is returned, not a second one made"
+
+    assert (await second.get_json())["id"] == (await first.get_json())["id"]
+    listing = (await (await app_client.get("/api/labels")).get_json())["labels"]
+    assert len(listing) == 1