diff --git a/client/src/network/protocol.ts b/client/src/network/protocol.ts
index 9afbbe2..d1abcae 100644
--- a/client/src/network/protocol.ts
+++ b/client/src/network/protocol.ts
@@ -387,7 +387,7 @@ export type OutgoingMessage =
   | { type: 'item_drop'; itemId: string; x: number; y: number }
   | { type: 'item_delete'; itemId: string }
   | { type: 'item_transfer_targets'; itemId: string }
-  | { type: 'item_transfer'; itemId: string; targetId?: string; targetUserId?: string }
+  | { type: 'item_transfer'; itemId: string; targetUserId: string }
   | { type: 'item_use'; itemId: string }
   | { type: 'item_secondary_use'; itemId: string }
   | { type: 'item_piano_note'; itemId: string; keyId: string; midi: number; on: boolean }
diff --git a/docs/protocol-notes.md b/docs/protocol-notes.md
index 6308503..e3cb9fe 100644
--- a/docs/protocol-notes.md
+++ b/docs/protocol-notes.md
@@ -29,7 +29,7 @@ This is a behavior guide for packet semantics beyond raw schemas.
 - `ping`: latency measurement.
 - `item_add`, `item_pickup`, `item_drop`, `item_delete`, `item_use`, `item_update`: item actions.
 - `item_transfer_targets`: request transfer target accounts for one item (includes online + offline active users, excluding current owner).
-- `item_transfer`: transfer item ownership to another account (supports `targetUserId`; `targetId` remains accepted for compatibility).
+- `item_transfer`: transfer item ownership to another account (`targetUserId` required).
 - `item_secondary_use`: trigger type-specific secondary action when implemented.
 - `item_piano_note`: realtime piano note on/off for active piano use mode.
 - `item_piano_recording`: piano record/playback control (`toggle_record`, `playback`, `stop_playback`).
diff --git a/server/app/models.py b/server/app/models.py
index bc7f860..0aa5c5e 100644
--- a/server/app/models.py
+++ b/server/app/models.py
@@ -140,8 +140,7 @@ class ItemDeletePacket(BasePacket):
 class ItemTransferPacket(BasePacket):
     type: Literal["item_transfer"]
     itemId: str
-    targetId: str | None = None
-    targetUserId: str | None = None
+    targetUserId: str
 
 
 class ItemTransferTargetsPacket(BasePacket):
diff --git a/server/app/server.py b/server/app/server.py
index a90f459..eb02d1d 100644
--- a/server/app/server.py
+++ b/server/app/server.py
@@ -2545,11 +2545,7 @@ class SignalingServer:
             if not can_transfer_any and not can_transfer_own:
                 await self._send_item_result(client, False, "transfer", "Not authorized to transfer this item.", item.id)
                 return
-            target_user_id = str(packet.targetUserId or "").strip()
-            if not target_user_id and packet.targetId:
-                target = self._get_client_by_id(packet.targetId)
-                if target and target.authenticated and target.user_id:
-                    target_user_id = target.user_id
+            target_user_id = str(packet.targetUserId).strip()
             if not target_user_id:
                 await self._send_item_result(client, False, "transfer", "Target user is not available.", item.id)
                 return
diff --git a/server/tests/test_server_message_handling.py b/server/tests/test_server_message_handling.py
index c73ebe6..92bfff4 100644
--- a/server/tests/test_server_message_handling.py
+++ b/server/tests/test_server_message_handling.py
@@ -377,7 +377,10 @@ async def test_item_transfer_updates_item_owner(monkeypatch: pytest.MonkeyPatch)
     monkeypatch.setattr(server, "_broadcast_item", fake_broadcast_item)
     monkeypatch.setattr(server, "_broadcast", fake_broadcast)
 
-    await server._handle_message(owner, json.dumps({"type": "item_transfer", "itemId": item.id, "targetId": target.id}))
+    await server._handle_message(
+        owner,
+        json.dumps({"type": "item_transfer", "itemId": item.id, "targetUserId": target.user_id}),
+    )
 
     assert item.createdBy == target.user_id
     assert item.createdByName == target.username
@@ -439,7 +442,10 @@ async def test_item_transfer_allows_self_target_for_transfer_any(monkeypatch: py
     monkeypatch.setattr(server, "_send", fake_send)
     monkeypatch.setattr(server, "_broadcast_item", fake_broadcast_item)
 
-    await server._handle_message(actor, json.dumps({"type": "item_transfer", "itemId": item.id, "targetId": actor.id}))
+    await server._handle_message(
+        actor,
+        json.dumps({"type": "item_transfer", "itemId": item.id, "targetUserId": actor.user_id}),
+    )
 
     assert item.createdBy == actor.user_id
     assert item.createdByName == actor.username
@@ -677,7 +683,10 @@ async def test_item_transfer_rejects_when_not_authorized(monkeypatch: pytest.Mon
 
     monkeypatch.setattr(server, "_send", fake_send)
 
-    await server._handle_message(owner, json.dumps({"type": "item_transfer", "itemId": item.id, "targetId": target.id}))
+    await server._handle_message(
+        owner,
+        json.dumps({"type": "item_transfer", "itemId": item.id, "targetUserId": target.user_id}),
+    )
 
     assert item.createdBy == owner.user_id
     assert send_payloads