diff --git a/app/store.py b/app/store.py
index 3298c5d..d0107cb 100644
--- a/app/store.py
+++ b/app/store.py
@@ -1,65 +1,53 @@
 import asyncio
+from datetime import UTC, datetime
 from uuid import UUID
 
-from app.models import CurrentStatus, Monitor, MonitorCreate, MonitorUpdate, now_utc
-
-
-class MonitorNotFoundError(KeyError):
-    pass
+from app.models import CurrentStatus, Monitor, MonitorCreate, MonitorUpdate
 
 
 class MonitorStore:
-    """Concurrency-safe, process-local monitor storage.
-
-    Models are frozen and every read returns an immutable value, so callers cannot mutate
-    state outside the lock. All read-modify-write operations happen under one asyncio lock.
-    """
+    """Concurrency-safe process-local state; all values cross the lock as copies."""
 
     def __init__(self) -> None:
         self._items: dict[UUID, Monitor] = {}
         self._lock = asyncio.Lock()
 
     async def create(self, data: MonitorCreate) -> Monitor:
-        monitor = Monitor(**data.model_dump())
+        monitor = Monitor(name=data.name, url=data.url)
         async with self._lock:
             self._items[monitor.id] = monitor
-        return monitor
+        return monitor.model_copy(deep=True)
 
     async def list(self) -> list[Monitor]:
         async with self._lock:
-            return list(self._items.values())
+            return [item.model_copy(deep=True) for item in self._items.values()]
 
-    async def get(self, monitor_id: UUID) -> Monitor:
+    async def get(self, monitor_id: UUID) -> Monitor | None:
         async with self._lock:
-            try:
-                return self._items[monitor_id]
-            except KeyError as exc:
-                raise MonitorNotFoundError(monitor_id) from exc
+            item = self._items.get(monitor_id)
+            return item.model_copy(deep=True) if item else None
 
-    async def update(self, monitor_id: UUID, data: MonitorUpdate) -> Monitor:
+    async def update(self, monitor_id: UUID, data: MonitorUpdate) -> Monitor | None:
         changes = data.model_dump(exclude_unset=True, exclude_none=True)
         async with self._lock:
-            try:
-                existing = self._items[monitor_id]
-            except KeyError as exc:
-                raise MonitorNotFoundError(monitor_id) from exc
-            updated = existing.model_copy(update={**changes, "updated_at": now_utc()})
+            item = self._items.get(monitor_id)
+            if item is None:
+                return None
+            updated = item.model_copy(update={**changes, "updated_at": datetime.now(UTC)})
             self._items[monitor_id] = updated
-            return updated
+            return updated.model_copy(deep=True)
 
-    async def delete(self, monitor_id: UUID) -> None:
+    async def delete(self, monitor_id: UUID) -> bool:
         async with self._lock:
-            if self._items.pop(monitor_id, None) is None:
-                raise MonitorNotFoundError(monitor_id)
+            return self._items.pop(monitor_id, None) is not None
 
-    async def set_status(self, monitor_id: UUID, status: CurrentStatus) -> Monitor:
+    async def set_status(self, monitor_id: UUID, status: CurrentStatus) -> Monitor | None:
         async with self._lock:
-            try:
-                existing = self._items[monitor_id]
-            except KeyError as exc:
-                raise MonitorNotFoundError(monitor_id) from exc
-            updated = existing.model_copy(
-                update={"current_status": status, "updated_at": now_utc()}
+            item = self._items.get(monitor_id)
+            if item is None:
+                return None
+            updated = item.model_copy(
+                update={"current_status": status, "updated_at": datetime.now(UTC)}
             )
             self._items[monitor_id] = updated
-            return updated
+            return updated.model_copy(deep=True)