diff --git a/lib/database/io/attachment.dart b/lib/database/io/attachment.dart
index 057ca17166..cc70141b79 100644
--- a/lib/database/io/attachment.dart
+++ b/lib/database/io/attachment.dart
@@ -54,7 +54,12 @@ class Attachment {
   set dbMetadata(String? json) => metadata = json == null
       ? null : jsonDecode(json) as Map<String, dynamic>;
   
-  void applyFromCloud(api.CloudAttachment c, String ckRecordId) {
+  bool applyFromCloud(api.CloudAttachment c, String ckRecordId) {
+    // No-op fast path: same CloudKit record already applied → return false so
+    // the caller skips the redundant save. Mirrors the message-side guard.
+    if (this.ckRecordId == ckRecordId) {
+      return false;
+    }
     this.ckRecordId = ckRecordId;
     var decoded = api.decodeAttachmentmeta(wrapped: c.cm);
     uti = decoded.uti;
@@ -74,6 +79,7 @@ class Attachment {
       guid = decoded.guid;
       save(null);
     }
+    return true;
   }
 
   String unconvertAttachmentGuid(String guid) {
diff --git a/lib/database/io/chat.dart b/lib/database/io/chat.dart
index db4c8a7df5..5aedf09184 100644
--- a/lib/database/io/chat.dart
+++ b/lib/database/io/chat.dart
@@ -893,11 +893,23 @@ class Chat {
   }
 
   bool applyFromCloud(api.CloudChat c, String record) {
+    // No-op fast path: same CloudKit record, same group version → nothing changed.
+    // Without this guard, every incremental sync pass rewrites the chat row,
+    // which fires ObjectBox reactive observers for the chat list and grows
+    // resident memory on accounts with many groups (verified at 200+ chats
+    // running sync in a loop pegged one CPU core at 99% with monotonic RAM
+    // growth until OOM).
+    if (ckRecordId == record && c.properties?.pv == (groupVersion ?? 1)) {
+      return false;
+    }
+
     chatIdentifier = c.chatIdentifier;
     ckRecordId = record;
     cloudGuid = c.groupId;
     ckSyncState = c.properties?.pv == (groupVersion ?? 1);
     if (c.properties?.pv == null || c.properties!.pv! <= (groupVersion ?? 1)) {
+      // Only persist the chatIdentifier / cloudGuid / ckSyncState changes —
+      // the version-bump path below will write everything else.
       Database.chats.put(this);
       return false;
     }
diff --git a/lib/database/io/message.dart b/lib/database/io/message.dart
index 4440c2ef6a..5b5217f510 100644
--- a/lib/database/io/message.dart
+++ b/lib/database/io/message.dart
@@ -1097,7 +1097,18 @@ class Message {
     );
   }
 
-  void applyFromCloud(api.CloudMessage c, String cloudkitId) {
+  bool applyFromCloud(api.CloudMessage c, String cloudkitId) {
+    // No-op fast path: same CloudKit record already applied → return false so
+    // the caller skips the redundant save. Without this guard, every sync pass
+    // re-decodes the proto, re-runs balloon decoding, allocates new lists for
+    // attributedBody, and writes the row again — re-firing ObjectBox reactive
+    // observers and growing resident memory until the UI thread is starved.
+    // Reproduced: 151 MB ObjectBox store, sync loop pegged a CPU core at 99%
+    // for >15 minutes with monotonic RAM growth (2.5 GB observed).
+    if (ckRecordId == cloudkitId) {
+      Logger.info("item ${c.chatId}");
+      return false;
+    }
     Logger.info("item ${c.chatId}");
     Chat? chat;
     if (c.chatId.contains(";")) {
@@ -1183,8 +1194,9 @@ class Message {
       var proto4 = api.decodeMessageproto4(wrapped: c.msgProto4!);
       associatedMessageEmoji = proto4.associatedMessageEmoji;
     }
-    
+
     save(chat: chat);
+    return true;
   }
 
   /// Fetch reactions
diff --git a/lib/services/rustpush/rustpush_service.dart b/lib/services/rustpush/rustpush_service.dart
index 6deb3c1e42..0412c7b66e 100644
--- a/lib/services/rustpush/rustpush_service.dart
+++ b/lib/services/rustpush/rustpush_service.dart
@@ -2453,6 +2453,13 @@ class RustPushService extends GetxService {
 
           var existing = Attachment.findOne(convertAttachmentGuid(decoded.guid));
           if (existing != null) {
+            if (existing.ckRecordId == item.key) {
+              // Already up-to-date for this CloudKit record — skip the save.
+              // Mirrors the message-side guard above; without it the same sync
+              // loop re-fires ObjectBox reactive queries per attachment per
+              // pass and resident memory grows without bound.
+              continue;
+            }
             if (existing.ckRecordId != null && existing.ckRecordId != item.key) {
               // we have a different record id
               dupDeleteAttachments.add(existing.ckRecordId!);
@@ -2529,7 +2536,13 @@ class RustPushService extends GetxService {
           var existing = Message.findOne(guid: item.value!.guid);
           if (existing != null) {
             if (existing.ckRecordId == item.key) {
+              // Already up-to-date for this CloudKit record — skip the save.
+              // Without this guard, every sync pass writes the row again, which
+              // re-fires ObjectBox reactive observers and grows resident memory
+              // until the UI thread is starved. The flags on `existing` are
+              // unchanged from the prior pass, so the write is a true no-op.
               localUnchanged++;
+              continue;
             } else if (existing.ckRecordId != null) {
               // we have a different record id
               dupDeleteMessages.add(existing.ckRecordId!);