diff --git a/backend/alembic/versions/0075_playlist_sync_baseline.py b/backend/alembic/versions/0075_playlist_sync_baseline.py
new file mode 100644
index 00000000..b786fd60
--- /dev/null
+++ b/backend/alembic/versions/0075_playlist_sync_baseline.py
@@ -0,0 +1,26 @@
+"""playlist synced_video_ids (S7: 3-way merge baseline for two-way YouTube sync)
+
+The one-directional "local wins" push is replaced by a 3-way merge that classifies each item
+against the last-synced state. That needs the raw ordered id set as a baseline, not just the
+`synced_fingerprint` hash. Add a nullable `synced_video_ids` (JSON list) column; NULL on every
+existing row means "no baseline yet" — the first merge then unions losslessly and records one.
+Forward-only in practice (a downgrade just drops the column).
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0075_playlist_sync_baseline"
+down_revision: Union[str, None] = "0074_download_credentials"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("playlists", sa.Column("synced_video_ids", sa.Text(), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("playlists", "synced_video_ids")
diff --git a/backend/app/models.py b/backend/app/models.py
index df47f972..f79cb013 100644
--- a/backend/app/models.py
+++ b/backend/app/models.py
@@ -551,6 +551,10 @@ class Playlist(Base, TimestampMixin, UpdatedAtMixin):
     # SHA-256 of name + ordered video ids as last synced from / pushed to YouTube; the
     # baseline `dirty` is derived against. NULL = no baseline yet (treated as dirty).
     synced_fingerprint: Mapped[str | None] = mapped_column(String)
+    # JSON list of the ordered video ids as of the last sync — the 3-way-merge baseline the
+    # two-way YouTube sync classifies each item against (local-add vs YT-add vs real delete).
+    # NULL = no baseline yet; the first merge then does a lossless union (never deletes).
+    synced_video_ids: Mapped[str | None] = mapped_column(Text)
     position: Mapped[int] = mapped_column(Integer, default=0, server_default="0")
 
     items: Mapped[list["PlaylistItem"]] = relationship(
diff --git a/backend/app/routes/playlists.py b/backend/app/routes/playlists.py
index 09dcf4c7..397df8fb 100644
--- a/backend/app/routes/playlists.py
+++ b/backend/app/routes/playlists.py
@@ -14,8 +14,8 @@ from app.db import get_db
 from app.feedquery import feed_columns, serialize
 from app.models import Channel, Playlist, PlaylistItem, User, Video, VideoState
 from app.sync.playlists import (
-    plan_push,
-    push_playlist,
+    merge_playlist,
+    plan_merge,
     recompute_dirty,
     repull_playlist,
     sync_user_playlists,
@@ -329,13 +329,13 @@ def push_plan(
     user: User = Depends(require_write_scope),
     db: Session = Depends(get_db),
 ) -> dict:
-    """Dry run: what 'Sync to YouTube' would do + the quota estimate, so the UI can show a
-    confirm. For a linked playlist this reads the live YouTube state (cheap, 1 unit/page).
-    Write scope is required (require_write_scope) — syncing is a YouTube write."""
+    """Dry run: what a two-way 'Sync to YouTube' merge would do (what each side gains/loses) +
+    the quota estimate, so the UI can show a confirm. For a linked playlist this reads the live
+    YouTube state (cheap, 1 unit/page). Write scope is required — syncing is a YouTube write."""
     pl = _pushable(db, user, playlist_id)
     try:
         with quota.attribute(user.id, quota.QuotaAction.PLAYLISTS_PUSH):
-            plan = plan_push(db, user, pl)
+            plan = plan_merge(db, user, pl)
     except YouTubeError:
         raise HTTPException(status_code=422, detail="Couldn't read the playlist on YouTube.")
     plan["remaining_today"] = quota.remaining_today(db)
@@ -349,14 +349,14 @@ def push(
     user: User = Depends(require_write_scope),
     db: Session = Depends(get_db),
 ) -> dict:
-    """Push a local playlist to YouTube (create it on first push, else reconcile to match
-    local — local wins). Refuses if the estimated quota exceeds what's left today, so a
-    big playlist can't strand half-pushed. Write scope required (require_write_scope)."""
+    """Two-way sync a local playlist with YouTube (create it on first push, else 3-way merge so
+    neither side loses an item). Refuses if the estimated quota exceeds what's left today, so a
+    big sync can't strand half-applied. Write scope required (require_write_scope)."""
     pl = _pushable(db, user, playlist_id)
     try:
         with quota.attribute(user.id, quota.QuotaAction.PLAYLISTS_PUSH):
             # Read the live playlist ONCE (linked playlists only) and share it with both the
-            # plan and the push, halving read-quota and closing the second read→write TOCTOU
+            # plan and the merge, halving read-quota and closing the second read→write TOCTOU
             # window. A fresh export (no yt_playlist_id) reads nothing.
             live = None
             if pl.yt_playlist_id:
@@ -365,13 +365,13 @@ def push(
                         list(yt.iter_playlist_items_with_ids(pl.yt_playlist_id)),
                         yt.get_playlist_snippet(pl.yt_playlist_id),
                     )
-            plan = plan_push(db, user, pl, live=live)
+            plan = plan_merge(db, user, pl, live=live)
             if plan["units_estimate"] > quota.remaining_today(db):
                 raise HTTPException(
                     status_code=429,
                     detail="Not enough YouTube quota left today for this sync.",
                 )
-            return push_playlist(db, user, pl, live=live)
+            return merge_playlist(db, user, pl, live=live)
     except YouTubeError:
         db.rollback()
         raise HTTPException(status_code=422, detail="YouTube playlist sync failed.")
diff --git a/backend/app/sync/playlists.py b/backend/app/sync/playlists.py
index 8cbb2a5b..c6b9fbba 100644
--- a/backend/app/sync/playlists.py
+++ b/backend/app/sync/playlists.py
@@ -1,14 +1,16 @@
 """YouTube playlist sync, both directions. Read: mirror each user's own YouTube playlists
-into local `source='youtube'` playlists (kept fresh by the scheduler). Write-back: push local
-edits out to YouTube (`plan_push` / `push_playlist`), behind the optional write scope.
+into local `source='youtube'` playlists (kept fresh by the scheduler). Write-back: two-way sync
+local edits with YouTube (`plan_merge` / `merge_playlist`), behind the optional write scope.
 
 YouTube's special Watch Later / History playlists are not exposed by the Data API and are
 never synced. Videos in a playlist that aren't in our shared catalog yet are fetched and
 ingested (with a stub channel) so the mirror is faithful."""
 
 import hashlib
+import json
 import logging
 from collections.abc import Callable
+from dataclasses import dataclass
 from datetime import datetime, timezone
 from typing import TypeVar
 
@@ -42,6 +44,30 @@ def current_fingerprint(db: Session, pl: Playlist) -> str:
     return fingerprint(pl.name, _local_video_ids(db, pl))
 
 
+def _dump_ids(ids: list[str]) -> str:
+    """Serialize the ordered id list stored as the 3-way-merge baseline (`synced_video_ids`)."""
+    return json.dumps(ids)
+
+
+def parse_baseline(pl: Playlist) -> list[str] | None:
+    """The last-synced ordered id set, or None when no baseline has been recorded yet (a fresh
+    export, or a link created before S7). None makes the first merge union losslessly."""
+    if not pl.synced_video_ids:
+        return None
+    try:
+        val = json.loads(pl.synced_video_ids)
+    except (ValueError, TypeError):
+        return None
+    return [str(v) for v in val] if isinstance(val, list) else None
+
+
+def _set_baseline(pl: Playlist, ids: list[str]) -> None:
+    """Record `ids` as both the hash (drives `dirty`) and the raw ordered set (drives merge)."""
+    pl.synced_fingerprint = fingerprint(pl.name, ids)
+    pl.synced_video_ids = _dump_ids(ids)
+    pl.dirty = False
+
+
 def recompute_dirty(db: Session, pl: Playlist) -> None:
     """Set `dirty` by comparing the current state to the synced baseline. Unlinked lists
     (and Watch later) are never dirty; a missing baseline counts as dirty (unknown YT state)."""
@@ -116,12 +142,18 @@ def _replace_items_from_youtube(
         if v in present and v not in seen:
             seen.add(v)
             ordered.append(v)
+    _set_local_items(db, pl, ordered)
+    # After a pull/reset local == YouTube, so this ordered set IS the new merge baseline.
+    _set_baseline(pl, ordered)
+    return ordered
+
+
+def _set_local_items(db: Session, pl: Playlist, ordered: list[str]) -> None:
+    """Authoritatively replace `pl`'s local items with `ordered` (positions 0..n). The caller
+    is responsible for having ensured every id exists in the catalog."""
     db.execute(delete(PlaylistItem).where(PlaylistItem.playlist_id == pl.id))
     for pos, vid in enumerate(ordered):
         db.add(PlaylistItem(playlist_id=pl.id, video_id=vid, position=pos))
-    pl.synced_fingerprint = fingerprint(pl.name, ordered)
-    pl.dirty = False
-    return ordered
 
 
 def sync_user_playlists(db: Session, user: User) -> dict:
@@ -216,22 +248,81 @@ def _reorder_moves(current: list[str], desired: list[str]) -> int:
     return moves
 
 
-def plan_push(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
-    """Compute (without writing) what pushing this local playlist to YouTube would do:
-    create vs update, how many inserts/deletes/reorders, the quota estimate, and whether
-    YouTube has diverged (items present there that local will remove). Costs read units
-    (1/page) for a linked playlist; nothing for a fresh export. `live` = a pre-fetched
-    (current_items, snippet) tuple so the caller can share one read with push_playlist."""
-    desired = _local_video_ids(db, pl)
+@dataclass
+class MergePlan:
+    """The 3-way merge of a linked playlist: what each side gains and loses so neither side
+    loses an item, plus the merged order. `final` is the id set both sides end up with."""
+
+    final: list[str]  # merged order (local order, YouTube-only adds appended)
+    add_to_yt: list[str]  # local-adds to push out to YouTube
+    add_to_local: list[str]  # YouTube-adds to pull into Siftlode
+    remove_from_yt: list[str]  # items deleted locally that are still on YouTube
+    remove_from_local: list[str]  # items deleted on YouTube that are still local
+    reorders: int  # estimated YouTube playlistItems.update moves to reach `final`
+    baseline_known: bool
+
+
+def merge_plan(baseline: list[str] | None, local: list[str], youtube: list[str]) -> MergePlan:
+    """Pure 3-way merge. Classify each id against the last-synced `baseline`:
+    - in local but not youtube, not deleted → add to YouTube (a local-add)
+    - in youtube but not local, not deleted → add to Siftlode (a YouTube-add)
+    - in baseline but gone from local → a genuine local delete → propagate to YouTube
+    - in baseline but gone from youtube → a genuine YouTube delete → propagate to Siftlode
+    With NO baseline (first-ever merge) we can't tell an add from a delete, so we UNION both
+    sides (never delete) — the lossless default. Merged order: local order first, YouTube-only
+    additions appended in their YouTube order."""
+    L = list(dict.fromkeys(local))
+    Y = list(dict.fromkeys(youtube))
+    Lset, Yset = set(L), set(Y)
+    if baseline is None:
+        add_to_yt = [v for v in L if v not in Yset]
+        add_to_local = [v for v in Y if v not in Lset]
+        remove_from_yt: list[str] = []
+        remove_from_local: list[str] = []
+        final = L + [v for v in Y if v not in Lset]
+    else:
+        B = set(baseline)
+        deleted_local = B - Lset  # was synced, removed on our side
+        deleted_yt = B - Yset  # was synced, removed on YouTube's side
+        gone = deleted_local | deleted_yt
+        remove_from_yt = [v for v in Y if v in deleted_local]
+        remove_from_local = [v for v in L if v in deleted_yt]
+        add_to_yt = [v for v in L if v not in Yset and v not in gone]
+        add_to_local = [v for v in Y if v not in Lset and v not in gone]
+        final = [v for v in L if v not in gone] + [v for v in Y if v not in Lset and v not in gone]
+    final_set = set(final)
+    # On YouTube, after membership reconciliation the order is current-kept + inserts appended;
+    # the reorder pass then moves items into `final`.
+    after_membership = [v for v in Y if v in final_set] + add_to_yt
+    reorders = _reorder_moves(after_membership, final)
+    return MergePlan(
+        final=final,
+        add_to_yt=add_to_yt,
+        add_to_local=add_to_local,
+        remove_from_yt=remove_from_yt,
+        remove_from_local=remove_from_local,
+        reorders=reorders,
+        baseline_known=baseline is not None,
+    )
+
+
+def plan_merge(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
+    """Compute (without writing) what a two-way sync of this playlist would do: create vs
+    update, how many items each side gains/loses, reorders, and the quota estimate (YouTube
+    writes only — local edits are free). Reads live YouTube state (1 unit/page) for a linked
+    playlist; nothing for a fresh export. `live` = a pre-fetched (current_items, snippet) tuple
+    so the caller can share one read with merge_playlist."""
+    local = _local_video_ids(db, pl)
     if not pl.yt_playlist_id:
-        ops = 1 + len(desired)  # create the playlist + one insert per video
+        ops = 1 + len(local)  # create the playlist + one insert per video
         return {
             "action": "create",
-            "to_insert": len(desired),
-            "to_delete": 0,
+            "add_yt": len(local),
+            "add_local": 0,
+            "remove_yt": 0,
+            "remove_local": 0,
             "to_reorder": 0,
             "rename": False,
-            "yt_extra": 0,
             "units_estimate": ops * WRITE_UNIT_COST,
         }
     if live is not None:
@@ -241,23 +332,17 @@ def plan_push(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
             current = list(yt.iter_playlist_items_with_ids(pl.yt_playlist_id))
             snippet = yt.get_playlist_snippet(pl.yt_playlist_id)
     rename = bool(snippet) and (snippet.get("title") or "") != pl.name
-    cur_vids = [it["video_id"] for it in current]
-    cur_set = set(cur_vids)
-    desired_set = set(desired)
-    to_insert = [v for v in desired if v not in cur_set]
-    to_delete = [v for v in cur_vids if v not in desired_set]
-    # After membership reconciliation, the YT order is current-minus-deletes plus inserts
-    # appended; the reorder pass then moves items into the desired order.
-    after_membership = [v for v in cur_vids if v in desired_set] + to_insert
-    reorders = _reorder_moves(after_membership, desired)
-    ops = len(to_insert) + len(to_delete) + reorders + (1 if rename else 0)
+    yt_ids = [it["video_id"] for it in current]
+    mp = merge_plan(parse_baseline(pl), local, yt_ids)
+    ops = len(mp.add_to_yt) + len(mp.remove_from_yt) + mp.reorders + (1 if rename else 0)
     return {
         "action": "update",
-        "to_insert": len(to_insert),
-        "to_delete": len(to_delete),
-        "to_reorder": reorders,
+        "add_yt": len(mp.add_to_yt),
+        "add_local": len(mp.add_to_local),
+        "remove_yt": len(mp.remove_from_yt),
+        "remove_local": len(mp.remove_from_local),
+        "to_reorder": mp.reorders,
         "rename": rename,
-        "yt_extra": len(to_delete),
         "units_estimate": ops * WRITE_UNIT_COST,
     }
 
@@ -327,13 +412,16 @@ class _Breaker:
         return out
 
 
-def push_playlist(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
-    """Push a local playlist to YouTube: create it if needed, then reconcile membership
-    and order so YouTube matches local (local wins). Marks the playlist clean on success.
+def merge_playlist(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
+    """Two-way sync a local playlist with YouTube: create it on first push, else 3-way MERGE
+    both sides so an item added (or deleted) on either side survives — no side wins wholesale.
+    Local edits are applied directly; YouTube edits go through the failure breaker. On full
+    success the merged set becomes the new baseline and the playlist is marked clean; on any
+    YouTube failure the baseline is left untouched so the next sync retries and converges.
     Caller must have checked the write scope and the quota budget. `live` = a pre-fetched
-    (current_items, snippet) tuple (from plan_push's read) to avoid a second live page-through."""
-    desired = _local_video_ids(db, pl)
-    inserted = deleted = reordered = created = 0
+    (current_items, snippet) tuple (from plan_merge's read) to avoid a second page-through."""
+    local = _local_video_ids(db, pl)
+    inserted = deleted = reordered = created = pulled = removed_local = 0
     failures: list[str] = []
     breaker = _Breaker()
     with YouTubeClient(db, user) as yt:
@@ -341,8 +429,8 @@ def push_playlist(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
             pl.yt_playlist_id = yt.create_playlist(pl.name)
             db.commit()
             created = 1
-            # Brand-new playlist: just append every video in order.
-            for vid in desired:
+            # Brand-new playlist: just append every local video in order.
+            for vid in local:
                 added = breaker.run(
                     lambda v=vid: yt.add_playlist_item(pl.yt_playlist_id, v),
                     lambda v=vid: failures.append(v),
@@ -350,60 +438,82 @@ def push_playlist(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
                 if not isinstance(added, _Failed):
                     inserted += 1
             if not failures:
-                pl.synced_fingerprint = fingerprint(pl.name, desired)
-                pl.dirty = False
+                _set_baseline(pl, local)
             db.commit()
             return {
                 "created": created,
                 "inserted": inserted,
                 "deleted": deleted,
                 "reordered": reordered,
+                "pulled": pulled,
+                "removed_local": removed_local,
                 "failures": failures,
                 "aborted": breaker.tripped,
                 "yt_playlist_id": pl.yt_playlist_id,
             }
 
-        # Existing link: diff against the live YouTube state (reuse plan_push's read if given).
+        # Existing link: 3-way merge against the live YouTube state (reuse plan_merge's read).
         if live is not None:
             current, snippet = live
         else:
             snippet = yt.get_playlist_snippet(pl.yt_playlist_id)
             current = list(yt.iter_playlist_items_with_ids(pl.yt_playlist_id))
+        item_id_by_vid = {it["video_id"]: it["item_id"] for it in current}
+        yt_ids = [it["video_id"] for it in current]
+        mp = merge_plan(parse_baseline(pl), local, yt_ids)
+
+        # --- Siftlode side (free, direct): pull YouTube-adds in, drop YouTube-deletes, land the
+        # merged order authoritatively. Do this first so local reflects the merge even if a later
+        # YouTube write trips the breaker.
+        _ensure_videos(db, yt, mp.add_to_local)
+        # Only the ids that actually exist in the catalog can become local rows (a private/deleted
+        # YouTube video won't ingest); keep the merged order, dropping any that didn't materialize.
+        present = (
+            set(db.execute(select(Video.id).where(Video.id.in_(mp.final))).scalars().all())
+            if mp.final
+            else set()
+        )
+        final_local = [v for v in mp.final if v in present]
+        pulled = len([v for v in mp.add_to_local if v in present])
+        removed_local = len(mp.remove_from_local)
+        _set_local_items(db, pl, final_local)
+        db.commit()
+
+        # --- YouTube side (metered, breaker-guarded).
         if snippet is not None and (snippet.get("title") or "") != pl.name:
             try:
                 yt.update_playlist_title(pl.yt_playlist_id, pl.name)
             except YouTubeError:
                 failures.append("title")
-        item_id_by_vid = {it["video_id"]: it["item_id"] for it in current}
-        cur_vids = [it["video_id"] for it in current]
-        desired_set = set(desired)
-        # Delete extras (local is authoritative).
-        for it in current:
-            if it["video_id"] not in desired_set:
-                dropped = breaker.run(
-                    lambda i=it: yt.delete_playlist_item(i["item_id"]),
-                    lambda i=it: failures.append(i["video_id"]),
-                )
-                if not isinstance(dropped, _Failed):
-                    deleted += 1
-        # Insert missing (append; the reorder pass below fixes positions).
-        cur_set = set(cur_vids)
-        for vid in desired:
-            if vid not in cur_set:
-                item_id = breaker.run(
-                    lambda v=vid: yt.add_playlist_item(pl.yt_playlist_id, v),
-                    lambda v=vid: failures.append(v),
-                )
-                if not isinstance(item_id, _Failed):
-                    item_id_by_vid[vid] = item_id
-                    inserted += 1
-        # Reorder to match `desired` via insertion-sort moves over a local model.
-        model = [v for v in cur_vids if v in desired_set] + [
-            v for v in desired if v not in cur_set and v in item_id_by_vid
+        # Propagate genuine local deletes.
+        for vid in mp.remove_from_yt:
+            item_id = item_id_by_vid.get(vid)
+            if item_id is None:
+                continue
+            dropped = breaker.run(
+                lambda i=item_id: yt.delete_playlist_item(i),
+                lambda v=vid: failures.append(v),
+            )
+            if not isinstance(dropped, _Failed):
+                deleted += 1
+                item_id_by_vid.pop(vid, None)
+        # Insert local-adds (append; the reorder pass below fixes positions).
+        for vid in mp.add_to_yt:
+            item_id = breaker.run(
+                lambda v=vid: yt.add_playlist_item(pl.yt_playlist_id, v),
+                lambda v=vid: failures.append(v),
+            )
+            if not isinstance(item_id, _Failed):
+                item_id_by_vid[vid] = item_id
+                inserted += 1
+        # Reorder YouTube to match `final` via insertion-sort moves over a local model.
+        final_set = set(mp.final)
+        model = [v for v in yt_ids if v in final_set and v in item_id_by_vid] + [
+            v for v in mp.add_to_yt if v in item_id_by_vid
         ]
-        for i, want in enumerate(desired):
+        for i, want in enumerate(mp.final):
             if want not in item_id_by_vid:
-                continue  # failed insert; skip
+                continue  # deleted-from-YT or failed insert; skip
             if i < len(model) and model[i] == want:
                 continue
             moved = breaker.run(
@@ -419,14 +529,15 @@ def push_playlist(db: Session, user: User, pl: Playlist, *, live=None) -> dict:
             model.pop(j)
             model.insert(i, want)
         if not failures:
-            pl.synced_fingerprint = fingerprint(pl.name, desired)
-            pl.dirty = False
+            _set_baseline(pl, final_local)
         db.commit()
     return {
         "created": created,
         "inserted": inserted,
         "deleted": deleted,
         "reordered": reordered,
+        "pulled": pulled,
+        "removed_local": removed_local,
         "failures": failures,
         "aborted": breaker.tripped,
         "yt_playlist_id": pl.yt_playlist_id,
diff --git a/backend/tests/test_playlist_merge.py b/backend/tests/test_playlist_merge.py
new file mode 100644
index 00000000..90dc80de
--- /dev/null
+++ b/backend/tests/test_playlist_merge.py
@@ -0,0 +1,221 @@
+"""S7 — two-way (3-way) YouTube playlist merge.
+
+The load-bearing property: an item added (or deleted) on EITHER side survives the sync — the old
+push was one-directional (local wins, deletes everything not local from YouTube), so YouTube-side
+adds were lost. Pure-merge classification runs with no DB; the apply path uses the `db` lane with a
+fake YouTube client so both the local rows and the simulated YouTube state can be asserted.
+"""
+
+from datetime import datetime, timezone
+
+import pytest
+
+from app.models import Channel, Playlist, PlaylistItem, Video
+from app.sync import playlists as pl_mod
+from app.sync.playlists import merge_plan, merge_playlist, parse_baseline
+
+# --- pure merge classification (no DB) --------------------------------------------------------
+
+
+class TestMergePlan:
+    def test_no_baseline_unions_losslessly(self):
+        # First-ever merge: can't tell an add from a delete, so keep everything, delete nothing.
+        mp = merge_plan(None, local=["a", "b"], youtube=["b", "c"])
+        assert mp.add_to_yt == ["a"]  # local-only pushed out
+        assert mp.add_to_local == ["c"]  # youtube-only pulled in
+        assert mp.remove_from_yt == [] and mp.remove_from_local == []
+        assert mp.final == ["a", "b", "c"]  # local order, youtube-only appended
+        assert mp.baseline_known is False
+
+    def test_adds_on_both_sides_both_survive(self):
+        # baseline b,c ; local added 'a' ; youtube added 'd' — nothing deleted.
+        mp = merge_plan(["b", "c"], local=["a", "b", "c"], youtube=["b", "c", "d"])
+        assert mp.add_to_yt == ["a"]
+        assert mp.add_to_local == ["d"]
+        assert mp.remove_from_yt == [] and mp.remove_from_local == []
+        assert mp.final == ["a", "b", "c", "d"]
+
+    def test_genuine_local_delete_propagates_to_youtube(self):
+        # baseline a,b,c ; local removed 'b' ; youtube unchanged -> remove b from youtube.
+        mp = merge_plan(["a", "b", "c"], local=["a", "c"], youtube=["a", "b", "c"])
+        assert mp.remove_from_yt == ["b"]
+        assert mp.add_to_yt == [] and mp.add_to_local == []
+        assert mp.final == ["a", "c"]
+
+    def test_genuine_youtube_delete_propagates_to_local(self):
+        # baseline a,b,c ; youtube removed 'b' ; local unchanged -> remove b from local.
+        mp = merge_plan(["a", "b", "c"], local=["a", "b", "c"], youtube=["a", "c"])
+        assert mp.remove_from_local == ["b"]
+        assert mp.final == ["a", "c"]
+
+    def test_delete_on_both_sides_is_gone_not_a_conflict(self):
+        mp = merge_plan(["a", "b", "c"], local=["a", "c"], youtube=["a", "c"])
+        assert mp.remove_from_yt == [] and mp.remove_from_local == []
+        assert mp.final == ["a", "c"]
+
+    def test_a_local_delete_beats_a_stale_youtube_copy(self):
+        # 'b' was synced, deleted locally, still present on youtube -> it must be removed there,
+        # NOT resurrected into local as a youtube-add.
+        mp = merge_plan(["a", "b"], local=["a"], youtube=["a", "b"])
+        assert mp.add_to_local == []
+        assert mp.remove_from_yt == ["b"]
+        assert "b" not in mp.final
+
+
+# --- apply path (db lane + fake YouTube) ------------------------------------------------------
+
+
+class FakeYouTube:
+    """Enough of YouTubeClient for merge_playlist: an in-memory playlist of (item_id, video_id)
+    and the write ops that mutate it. Every catalog video is pre-seeded so _ensure_videos never
+    needs get_videos."""
+
+    def __init__(self, *, title: str, items: list[str]):
+        self.title = title
+        self._seq = 0
+        self.items = [(self._new_id(), v) for v in items]
+
+    def _new_id(self) -> str:
+        self._seq += 1
+        return f"it{self._seq}"
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, *exc):
+        return False
+
+    def create_playlist(self, title, description="", privacy="private"):
+        self.title = title
+        return "PLnew"
+
+    def get_playlist_snippet(self, pid):
+        return {"title": self.title}
+
+    def iter_playlist_items_with_ids(self, pid):
+        return [{"item_id": iid, "video_id": v} for iid, v in self.items]
+
+    def update_playlist_title(self, pid, title):
+        self.title = title
+
+    def add_playlist_item(self, pid, vid, position=None):
+        iid = self._new_id()
+        self.items.append((iid, vid))
+        return iid
+
+    def delete_playlist_item(self, item_id):
+        self.items = [(iid, v) for iid, v in self.items if iid != item_id]
+
+    def move_playlist_item(self, item_id, pid, vid, position):
+        row = next((r for r in self.items if r[0] == item_id), None)
+        if row is not None:
+            self.items.remove(row)
+            self.items.insert(position, row)
+
+    @property
+    def video_ids(self) -> list[str]:
+        return [v for _, v in self.items]
+
+
+def _seed_videos(db, ids):
+    db.add(Channel(id="UCtest", title="Test"))
+    now = datetime.now(timezone.utc)
+    for v in ids:
+        db.add(Video(id=v, channel_id="UCtest", published_at=now))
+    db.commit()
+
+
+def _linked_playlist(db, user, *, local_ids, baseline):
+    pl = Playlist(
+        user_id=user.id,
+        name="Mix",
+        source="local",
+        yt_playlist_id="PLyt",
+        synced_video_ids=None if baseline is None else pl_mod._dump_ids(baseline),
+    )
+    db.add(pl)
+    db.flush()
+    for pos, v in enumerate(local_ids):
+        db.add(PlaylistItem(playlist_id=pl.id, video_id=v, position=pos))
+    db.commit()
+    return pl
+
+
+def _local_ids(db, pl):
+    return pl_mod._local_video_ids(db, pl)
+
+
+@pytest.fixture
+def _fake_yt(monkeypatch):
+    holder = {}
+
+    def install(fake):
+        holder["fake"] = fake
+        monkeypatch.setattr(pl_mod, "YouTubeClient", lambda db, user: fake)
+        return fake
+
+    return install
+
+
+def test_adds_on_both_sides_survive_on_both_sides(db, make_user, _fake_yt):
+    user = make_user()
+    _seed_videos(db, ["a", "b", "c", "d"])
+    # baseline b,c ; local added 'a' ; youtube added 'd'
+    pl = _linked_playlist(db, user, local_ids=["a", "b", "c"], baseline=["b", "c"])
+    fake = _fake_yt(FakeYouTube(title="Mix", items=["b", "c", "d"]))
+
+    r = merge_playlist(db, user, pl)
+
+    assert r["failures"] == []
+    assert set(_local_ids(db, pl)) == {"a", "b", "c", "d"}, "youtube-add 'd' pulled into Siftlode"
+    assert set(fake.video_ids) == {"a", "b", "c", "d"}, "local-add 'a' pushed to YouTube"
+    assert r["inserted"] == 1 and r["pulled"] == 1
+    # Baseline advanced to the merged set so a re-sync is a no-op.
+    assert set(parse_baseline(pl)) == {"a", "b", "c", "d"}
+
+
+def test_a_genuine_delete_propagates_without_collateral_loss(db, make_user, _fake_yt):
+    user = make_user()
+    _seed_videos(db, ["a", "b", "c", "d"])
+    # baseline a,b,c ; local removed 'b' ; youtube added 'd' (a real add, must NOT be collateral)
+    pl = _linked_playlist(db, user, local_ids=["a", "c"], baseline=["a", "b", "c"])
+    fake = _fake_yt(FakeYouTube(title="Mix", items=["a", "b", "c", "d"]))
+
+    r = merge_playlist(db, user, pl)
+
+    assert r["failures"] == []
+    assert set(fake.video_ids) == {"a", "c", "d"}, "'b' removed on YouTube, 'd' kept"
+    assert set(_local_ids(db, pl)) == {"a", "c", "d"}, "'d' pulled in, 'b' stays deleted"
+    assert r["deleted"] == 1
+
+
+def test_first_merge_with_no_baseline_never_deletes(db, make_user, _fake_yt):
+    user = make_user()
+    _seed_videos(db, ["a", "b", "c"])
+    pl = _linked_playlist(db, user, local_ids=["a", "b"], baseline=None)
+    fake = _fake_yt(FakeYouTube(title="Mix", items=["b", "c"]))
+
+    r = merge_playlist(db, user, pl)
+
+    assert r["deleted"] == 0 and r["removed_local"] == 0
+    assert set(_local_ids(db, pl)) == {"a", "b", "c"}
+    assert set(fake.video_ids) == {"a", "b", "c"}
+    assert parse_baseline(pl) is not None, "a baseline is recorded after the first union"
+
+
+def test_create_path_appends_and_records_baseline(db, make_user, _fake_yt):
+    user = make_user()
+    _seed_videos(db, ["a", "b"])
+    pl = Playlist(user_id=user.id, name="Fresh", source="local")
+    db.add(pl)
+    db.flush()
+    for pos, v in enumerate(["a", "b"]):
+        db.add(PlaylistItem(playlist_id=pl.id, video_id=v, position=pos))
+    db.commit()
+    _fake_yt(FakeYouTube(title="Fresh", items=[]))
+
+    r = merge_playlist(db, user, pl)
+
+    assert r["created"] == 1 and r["inserted"] == 2
+    assert pl.yt_playlist_id  # created and linked
+    assert parse_baseline(pl) == ["a", "b"]
diff --git a/frontend/src/components/Playlists.tsx b/frontend/src/components/Playlists.tsx
index 466d8c8a..d69c07ce 100644
--- a/frontend/src/components/Playlists.tsx
+++ b/frontend/src/components/Playlists.tsx
@@ -362,7 +362,15 @@ export default function Playlists() {
     setPushing(true);
     try {
       const plan = await api.playlistPushPlan(selectedId);
-      if (plan.action === "update" && !plan.to_insert && !plan.to_delete && !plan.to_reorder) {
+      const nothingToDo =
+        plan.action === "update" &&
+        !plan.add_yt &&
+        !plan.add_local &&
+        !plan.remove_yt &&
+        !plan.remove_local &&
+        !plan.to_reorder &&
+        !plan.rename;
+      if (nothingToDo) {
         notify({ level: "info", message: t("playlists.pushUpToDate", { name: plName(detail) }) });
         return;
       }
@@ -376,21 +384,21 @@ export default function Playlists() {
         });
         return;
       }
-      let message =
+      const message =
         plan.action === "create"
           ? t("playlists.pushPlanCreate", {
-              count: plan.to_insert,
+              count: plan.add_yt,
               units: plan.units_estimate,
               left: plan.remaining_today,
             })
-          : t("playlists.pushPlanUpdate", {
-              insert: plan.to_insert,
-              del: plan.to_delete,
-              reorder: plan.to_reorder,
+          : t("playlists.pushPlanMerge", {
+              addYt: plan.add_yt,
+              addLocal: plan.add_local,
+              removeYt: plan.remove_yt,
+              removeLocal: plan.remove_local,
               units: plan.units_estimate,
               left: plan.remaining_today,
             });
-      if (plan.yt_extra > 0) message += " " + t("playlists.pushDiverged", { count: plan.yt_extra });
       const ok = await confirm({
         title: t("playlists.pushTitle"),
         message,
diff --git a/frontend/src/i18n/locales/en/playlists.json b/frontend/src/i18n/locales/en/playlists.json
index d10d8771..475a5f5f 100644
--- a/frontend/src/i18n/locales/en/playlists.json
+++ b/frontend/src/i18n/locales/en/playlists.json
@@ -35,8 +35,7 @@
   "pushTitle": "Sync to YouTube",
   "pushConfirm": "Sync",
   "pushPlanCreate": "Create a new YouTube playlist with {{count}} videos? (~{{units}} quota units; {{left}} left today.)",
-  "pushPlanUpdate": "Update on YouTube — add {{insert}}, remove {{del}}, reorder {{reorder}}? (~{{units}} quota units; {{left}} left today.)",
-  "pushDiverged": "{{count}} video(s) currently on YouTube will be removed.",
+  "pushPlanMerge": "Two-way sync — on YouTube: +{{addYt}} / −{{removeYt}}; here: +{{addLocal}} / −{{removeLocal}}. (~{{units}} quota units; {{left}} left today.)",
   "pushNoQuota": "Not enough YouTube quota left today ({{left}}) for this sync (~{{units}} needed). Try again tomorrow.",
   "pushUpToDate": "“{{name}}” is already in sync with YouTube.",
   "pushDone": "“{{name}}” synced to YouTube ✓",
diff --git a/frontend/src/i18n/locales/hu/playlists.json b/frontend/src/i18n/locales/hu/playlists.json
index 3048f09f..720301ba 100644
--- a/frontend/src/i18n/locales/hu/playlists.json
+++ b/frontend/src/i18n/locales/hu/playlists.json
@@ -35,8 +35,7 @@
   "pushTitle": "Szinkron YouTube-ra",
   "pushConfirm": "Szinkron",
   "pushPlanCreate": "Létrehozol egy új YouTube-listát {{count}} videóval? (~{{units}} kvótaegység; ma még {{left}} maradt.)",
-  "pushPlanUpdate": "Frissítés a YouTube-on — {{insert}} hozzáadása, {{del}} eltávolítása, {{reorder}} átrendezése? (~{{units}} kvótaegység; ma még {{left}} maradt.)",
-  "pushDiverged": "{{count}} videó, amely jelenleg a YouTube-on van, el lesz távolítva.",
+  "pushPlanMerge": "Kétirányú szinkron — YouTube-on: +{{addYt}} / −{{removeYt}}; itt: +{{addLocal}} / −{{removeLocal}}. (~{{units}} kvótaegység; ma még {{left}} maradt.)",
   "pushNoQuota": "Nincs elég YouTube-kvóta mára ({{left}}) ehhez a szinkronhoz (~{{units}} kell). Próbáld újra holnap.",
   "pushUpToDate": "A(z) „{{name}}” már szinkronban van a YouTube-bal.",
   "pushDone": "„{{name}}” szinkronizálva a YouTube-ra ✓",
diff --git a/frontend/src/lib/api/playlists.ts b/frontend/src/lib/api/playlists.ts
index 46fb911a..4a085532 100644
--- a/frontend/src/lib/api/playlists.ts
+++ b/frontend/src/lib/api/playlists.ts
@@ -31,12 +31,16 @@ export interface PlaylistDetail {
   items: Video[];
 }
 
+// Two-way sync plan: what each side gains/loses under the 3-way merge (neither side wins
+// wholesale). Local edits are free; only the YouTube writes cost quota.
 export interface PushPlan {
   action: "create" | "update";
-  to_insert: number;
-  to_delete: number;
+  add_yt: number; // local-adds to push out to YouTube
+  add_local: number; // YouTube-adds to pull into Siftlode
+  remove_yt: number; // genuine local deletes propagated to YouTube
+  remove_local: number; // genuine YouTube deletes propagated to Siftlode
   to_reorder: number;
-  yt_extra: number; // items on YouTube that the push would remove (divergence)
+  rename: boolean;
   units_estimate: number;
   remaining_today: number;
   affordable: boolean;
@@ -44,9 +48,11 @@ export interface PushPlan {
 
 export interface PushResult {
   created: number;
-  inserted: number;
-  deleted: number;
+  inserted: number; // added on YouTube
+  deleted: number; // removed on YouTube
   reordered: number;
+  pulled: number; // added into Siftlode from YouTube
+  removed_local: number; // removed from Siftlode (deleted on YouTube)
   failures: string[];
   yt_playlist_id: string | null;
 }