diff --git a/src/thoughtsync/labeling.py b/src/thoughtsync/labeling.py
new file mode 100644
index 0000000..9b07f3f
--- /dev/null
+++ b/src/thoughtsync/labeling.py
@@ -0,0 +1,36 @@
+"""Shared note↔label membership reconciliation. The "set a note's MANUAL (picker)
+labels, leave the tag-sourced ones alone" logic was duplicated line-for-line between
+the labels-picker API (notes.set_note_labels) and sync push (sync._apply_note_manual_labels).
+Single home so both stay in lockstep. via_tag=True rows track the body #tags and are
+governed by _reconcile_tags — this function never touches them."""
+from __future__ import annotations
+
+from sqlalchemy import select
+
+from .models.label import Label, NoteLabel
+from .models.note import Note
+
+
+async def resolve_owned_label_ids(db, label_ids, owner_id) -> set:
+    """Of `label_ids` (an iterable of UUIDs), the subset actually owned by `owner_id`.
+    Callers parse/validate the raw ids first; this just enforces ownership."""
+    ids = list(label_ids)
+    if not ids:
+        return set()
+    return set(
+        (await db.scalars(select(Label.id).where(Label.owner_id == owner_id, Label.id.in_(ids)))).all()
+    )
+
+
+async def reconcile_manual_labels(db, note: Note, owned_label_ids: set) -> None:
+    """Make the note's MANUAL (via_tag=False) memberships exactly `owned_label_ids`:
+    drop manual rows no longer wanted, add missing ones. Tag-sourced rows survive
+    untouched. Caller commits."""
+    existing = (await db.scalars(select(NoteLabel).where(NoteLabel.note_id == note.id))).all()
+    attached = {r.label_id for r in existing}
+    for r in existing:
+        if not r.via_tag and r.label_id not in owned_label_ids:
+            await db.delete(r)
+    for lid in owned_label_ids:
+        if lid not in attached:
+            db.add(NoteLabel(note_id=note.id, label_id=lid, via_tag=False))
diff --git a/src/thoughtsync/labels.py b/src/thoughtsync/labels.py
index 5a7e6d8..5a2ec1d 100644
--- a/src/thoughtsync/labels.py
+++ b/src/thoughtsync/labels.py
@@ -1,26 +1,20 @@
 from __future__ import annotations
 
-import uuid
-
 from quart import Blueprint, g, jsonify, request
 from sqlalchemy import func, select
 
 from .auth import login_required
+from .colors import normalize_color
 from .db import session_scope
 from .models.label import Label, NoteLabel
+from .responses import json_error, not_found, parse_uuid
+from .serialize import serialize_label
 
 bp = Blueprint("labels", __name__, url_prefix="/api/labels")
 
 
-LABEL_COLORS = {"default", "red", "orange", "yellow", "green", "teal", "blue", "purple", "pink", "gray"}
-
-
-def _normalize_label_color(color: object) -> str:
-    return color if color in LABEL_COLORS else "default"
-
-
 def _serialize_label(label: Label, count: int | None = None) -> dict:
-    data = {"id": str(label.id), "name": label.name, "color": label.color}
+    data = serialize_label(label)
     if count is not None:
         data["count"] = count
     return data
@@ -34,9 +28,8 @@ async def _label_note_count(db, label_id) -> int:
 
 
 async def _get_owned_label(db, label_id: str) -> Label | None:
-    try:
-        lid = uuid.UUID(label_id)
-    except (ValueError, TypeError):
+    lid = parse_uuid(label_id)
+    if lid is None:
         return None
     return await db.scalar(select(Label).where(Label.id == lid, Label.owner_id == g.user_id))
 
@@ -65,13 +58,13 @@ async def create_label():
     data = await request.get_json(silent=True) or {}
     name = (data.get("name") or "").strip()
     if not name:
-        return jsonify({"error": "label name is required"}), 400
+        return json_error("label 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))
         if existing is not None:
             return jsonify(_serialize_label(existing)), 200
-        label = Label(owner_id=g.user_id, name=name, color=_normalize_label_color(data.get("color")))
+        label = Label(owner_id=g.user_id, name=name, color=normalize_color(data.get("color")))
         db.add(label)
         await db.commit()
         await db.refresh(label)
@@ -85,23 +78,23 @@ async def update_label(label_id: str):
     has_name = "name" in data
     has_color = "color" in data
     if not has_name and not has_color:
-        return jsonify({"error": "nothing to update"}), 400
+        return json_error("nothing to update", 400)
     name = (data.get("name") or "").strip() if has_name else None
     if has_name and not name:
-        return jsonify({"error": "label name is required"}), 400
+        return json_error("label name is required", 400)
     async with session_scope() as db:
         label = await _get_owned_label(db, label_id)
         if label is None:
-            return jsonify({"error": "not found"}), 404
+            return not_found()
         if has_name:
             clash = await db.scalar(
                 select(Label).where(Label.owner_id == g.user_id, Label.name == name, Label.id != label.id)
             )
             if clash is not None:
-                return jsonify({"error": "a label with that name already exists"}), 409
+                return json_error("a label with that name already exists", 409)
             label.name = name
         if has_color:
-            label.color = _normalize_label_color(data.get("color"))
+            label.color = normalize_color(data.get("color"))
         await db.commit()
         await db.refresh(label)
         return jsonify(_serialize_label(label))
@@ -113,7 +106,7 @@ async def delete_label(label_id: str):
     async with session_scope() as db:
         label = await _get_owned_label(db, label_id)
         if label is None:
-            return jsonify({"error": "not found"}), 404
+            return not_found()
         await db.delete(label)  # note_labels rows cascade
         await db.commit()
         return jsonify({"ok": True})
@@ -133,9 +126,9 @@ async def merge_label(label_id: str):
         source = await _get_owned_label(db, label_id)
         target = await _get_owned_label(db, str(into)) if into is not None else None
         if source is None or target is None:
-            return jsonify({"error": "not found"}), 404
+            return not_found()
         if source.id == target.id:
-            return jsonify({"error": "cannot merge a label into itself"}), 400
+            return json_error("cannot merge a label 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(
diff --git a/src/thoughtsync/notes.py b/src/thoughtsync/notes.py
index d8db5af..4f1709c 100644
--- a/src/thoughtsync/notes.py
+++ b/src/thoughtsync/notes.py
@@ -20,6 +20,7 @@ from .colors import NOTE_COLORS, normalize_color
 from .common import coerce_bool, iso, parse_dt
 from .config import Config
 from .db import session_scope
+from .labeling import reconcile_manual_labels, resolve_owned_label_ids
 from .responses import json_error, not_found, parse_uuid
 from .settings import get_setting
 from .unfurl import UnfurlError, unfurl
@@ -1254,26 +1255,10 @@ async def set_note_labels(note_id: str):
         note = await _get_owned(db, note_id)
         if note is None:
             return not_found()
-        owned: set = set()
-        if label_ids:
-            owned = set(
-                (
-                    await db.scalars(
-                        select(Label.id).where(Label.owner_id == g.user_id, Label.id.in_(label_ids))
-                    )
-                ).all()
-            )
         # The picker manages MANUAL labels only; tag-sourced (via_tag=True) rows are
-        # governed by the body #tags and must survive a picker save.
-        chosen = {lid for lid in label_ids if lid in owned}
-        existing = (await db.scalars(select(NoteLabel).where(NoteLabel.note_id == note.id))).all()
-        attached_ids = {r.label_id for r in existing}
-        for r in existing:
-            if not r.via_tag and r.label_id not in chosen:
-                await db.delete(r)
-        for lid in chosen:
-            if lid not in attached_ids:
-                db.add(NoteLabel(note_id=note.id, label_id=lid, via_tag=False))
+        # governed by the body #tags and survive a picker save (see labeling module).
+        owned = await resolve_owned_label_ids(db, label_ids, g.user_id)
+        await reconcile_manual_labels(db, note, owned)
         await db.commit()
         return jsonify(await _serialize_note(db, note))
 
diff --git a/src/thoughtsync/saved_filters.py b/src/thoughtsync/saved_filters.py
index c9ef2a8..35af67d 100644
--- a/src/thoughtsync/saved_filters.py
+++ b/src/thoughtsync/saved_filters.py
@@ -3,7 +3,6 @@ in one click. Owner-scoped CRUD; `params` mirrors the GET /api/notes facet query
 from __future__ import annotations
 
 import json
-import uuid
 
 from quart import Blueprint, g, jsonify, request
 from sqlalchemy import func, select
@@ -11,6 +10,7 @@ from sqlalchemy import func, select
 from .auth import login_required
 from .db import session_scope
 from .models.saved_filter import SavedFilter
+from .responses import json_error, not_found, parse_uuid
 
 bp = Blueprint("saved_filters", __name__, url_prefix="/api/saved-filters")
 
@@ -64,7 +64,7 @@ async def create_saved():
     data = await request.get_json(silent=True) or {}
     name = (data.get("name") or "").strip()
     if not name:
-        return jsonify({"error": "name is required"}), 400
+        return json_error("name is required", 400)
     params = clean_params(data.get("params"))
     async with session_scope() as db:
         max_pos = await db.scalar(
@@ -82,21 +82,20 @@ async def create_saved():
 @bp.patch("/<filter_id>")
 @login_required
 async def rename_saved(filter_id: str):
-    try:
-        fid = uuid.UUID(filter_id)
-    except (ValueError, TypeError):
-        return jsonify({"error": "not found"}), 404
+    fid = parse_uuid(filter_id)
+    if fid is None:
+        return not_found()
     data = await request.get_json(silent=True) or {}
     async with session_scope() as db:
         sf = await db.scalar(
             select(SavedFilter).where(SavedFilter.id == fid, SavedFilter.owner_id == g.user_id)
         )
         if sf is None:
-            return jsonify({"error": "not found"}), 404
+            return not_found()
         if "name" in data:
             name = (data.get("name") or "").strip()
             if not name:
-                return jsonify({"error": "name is required"}), 400
+                return json_error("name is required", 400)
             sf.name = name[:NAME_CAP]
         if "params" in data:
             sf.params = json.dumps(clean_params(data.get("params")))
@@ -108,16 +107,15 @@ async def rename_saved(filter_id: str):
 @bp.delete("/<filter_id>")
 @login_required
 async def delete_saved(filter_id: str):
-    try:
-        fid = uuid.UUID(filter_id)
-    except (ValueError, TypeError):
-        return jsonify({"error": "not found"}), 404
+    fid = parse_uuid(filter_id)
+    if fid is None:
+        return not_found()
     async with session_scope() as db:
         sf = await db.scalar(
             select(SavedFilter).where(SavedFilter.id == fid, SavedFilter.owner_id == g.user_id)
         )
         if sf is None:
-            return jsonify({"error": "not found"}), 404
+            return not_found()
         await db.delete(sf)
         await db.commit()
         return jsonify({"ok": True})
diff --git a/src/thoughtsync/serialize.py b/src/thoughtsync/serialize.py
new file mode 100644
index 0000000..645dae5
--- /dev/null
+++ b/src/thoughtsync/serialize.py
@@ -0,0 +1,14 @@
+"""Shared JSON serializers — the single home for turning ORM rows into the dict
+shapes the API returns, so a field name or format is defined once. Each entity has a
+BASE serializer with the fields common to every context; callers layer on the
+extras their surface needs (usage count in the labels API, sync_revision/purged_at
+in sync deltas). Grows as sections adopt it (labels here; note/user/device later)."""
+from __future__ import annotations
+
+from .models.label import Label
+
+
+def serialize_label(label: Label) -> dict:
+    """Base label shape {id, name, color}. The labels API adds `count`; sync deltas
+    (S4) add `sync_revision`/`purged_at`/`created_at` on top of this."""
+    return {"id": str(label.id), "name": label.name, "color": label.color}
diff --git a/src/thoughtsync/sync.py b/src/thoughtsync/sync.py
index 76c2e59..a105d2d 100644
--- a/src/thoughtsync/sync.py
+++ b/src/thoughtsync/sync.py
@@ -21,6 +21,7 @@ from sqlalchemy import func, select
 from .auth import login_required
 from .config import Config
 from .db import session_scope
+from .labeling import reconcile_manual_labels, resolve_owned_label_ids
 from .models.label import Label, NoteLabel
 from .models.note import Note
 from .models.note_attachment import NoteAttachment
@@ -212,20 +213,9 @@ async def _apply_note_manual_labels(db, note: Note, ch: dict) -> None:
         try:
             wanted.add(uuid.UUID(str(r)))
         except (ValueError, TypeError):
-            continue
-    owned: set = set()
-    if wanted:
-        owned = set(
-            (await db.scalars(select(Label.id).where(Label.owner_id == g.user_id, Label.id.in_(wanted)))).all()
-        )
-    existing = (await db.scalars(select(NoteLabel).where(NoteLabel.note_id == note.id))).all()
-    attached = {r.label_id for r in existing}
-    for r in existing:
-        if not r.via_tag and r.label_id not in owned:
-            await db.delete(r)
-    for lid in owned:
-        if lid not in attached:
-            db.add(NoteLabel(note_id=note.id, label_id=lid, via_tag=False))
+            continue  # sync is lenient: skip a malformed id rather than reject the push
+    owned = await resolve_owned_label_ids(db, wanted, g.user_id)
+    await reconcile_manual_labels(db, note, owned)
 
 
 async def _purge_note(db, note: Note, edited_at: datetime | None) -> None:
diff --git a/tests/test_labels.py b/tests/test_labels.py
index fa52856..85d28f2 100644
--- a/tests/test_labels.py
+++ b/tests/test_labels.py
@@ -1,6 +1,10 @@
+import uuid
+
 import pytest
 
 from thoughtsync.app import create_app
+from thoughtsync.models.label import Label
+from thoughtsync.serialize import serialize_label
 
 
 @pytest.fixture
@@ -8,6 +12,16 @@ def app():
     return create_app()
 
 
+def test_serialize_label_shape():
+    # The base {id, name, color} shape the labels API and (S4) sync deltas both build on.
+    lid = uuid.uuid4()
+    assert serialize_label(Label(id=lid, name="ideas", color="teal")) == {
+        "id": str(lid),
+        "name": "ideas",
+        "color": "teal",
+    }
+
+
 async def test_labels_requires_auth(app):
     client = app.test_client()
     resp = await client.get("/api/labels")