diff --git a/api/src/main/java/com/ruinscraft/panilla/api/nbt/checks/NbtChecks.java b/api/src/main/java/com/ruinscraft/panilla/api/nbt/checks/NbtChecks.java
index 3b87b9e..d6e098b 100644
--- a/api/src/main/java/com/ruinscraft/panilla/api/nbt/checks/NbtChecks.java
+++ b/api/src/main/java/com/ruinscraft/panilla/api/nbt/checks/NbtChecks.java
@@ -1,143 +1,165 @@
-package com.ruinscraft.panilla.api.nbt.checks;
-
-import com.ruinscraft.panilla.api.IPanilla;
-import com.ruinscraft.panilla.api.exception.FailedNbt;
-import com.ruinscraft.panilla.api.exception.NbtNotPermittedException;
-import com.ruinscraft.panilla.api.nbt.INbtTagCompound;
-import com.ruinscraft.panilla.api.nbt.INbtTagList;
-import com.ruinscraft.panilla.api.nbt.NbtDataType;
-
-import java.util.HashMap;
-import java.util.Map;
-
-public final class NbtChecks {
-
-    private static final Map<String, NbtCheck> checks = new HashMap<>();
-
-    static {
-        // vanilla
-        register(new NbtCheck_Unbreakable());
-        register(new NbtCheck_CanDestroy());
-        register(new NbtCheck_CanPlaceOn());
-        register(new NbtCheck_BlockEntityTag());
-        register(new NbtCheck_BlockStateTag());
-        register(new NbtCheck_ench());
-        register(new NbtCheck_Enchantments());
-        register(new NbtCheck_StoredEnchantments());
-        register(new NbtCheck_RepairCost());
-        register(new NbtCheck_AttributeModifiers());
-        register(new NbtCheck_CustomPotionEffects());
-        register(new NbtCheck_Potion());
-        register(new NbtCheck_CustomPotionColor());
-        register(new NbtCheck_display());
-        register(new NbtCheck_HideFlags());
-        register(new NbtCheck_resolved());
-        register(new NbtCheck_generation());
-        register(new NbtCheck_author());
-        register(new NbtCheck_title());
-        register(new NbtCheck_pages());
-        register(new NbtCheck_SkullOwner());
-        register(new NbtCheck_SkullProfile());
-        register(new NbtCheck_Explosion());
-        register(new NbtCheck_Fireworks());
-        register(new NbtCheck_EntityTag());
-        register(new NbtCheck_BucketVariantTag());
-        register(new NbtCheck_map());
-        register(new NbtCheck_map_scale_direction());
-        register(new NbtCheck_Decorations());
-        register(new NbtCheck_Effects());
-        register(new NbtCheck_CustomModelData());
-        register(new NbtCheck_HasVisualFire()); // 1.17
-        register(new NbtCheck_ChargedProjectiles());
-        register(new NbtCheck_Items());
-
-        // non-vanilla
-        register(new NbtCheck_weBrushJson());
-    }
-
-    private static void register(NbtCheck check) {
-        checks.put(check.getName(), check);
-    }
-
-    public static Map<String, NbtCheck> getChecks() {
-        return checks;
-    }
-
-    public static void checkPacketPlayIn(int slot, INbtTagCompound tag, String nmsItemClassName, String nmsPacketClassName,
-                                         IPanilla panilla) throws NbtNotPermittedException {
-        FailedNbt failedNbt = checkAll(tag, nmsItemClassName, panilla);
-
-        if (FailedNbt.fails(failedNbt)) {
-            throw new NbtNotPermittedException(nmsPacketClassName, false, failedNbt, slot);
-        }
-    }
-
-    public static void checkPacketPlayOut(int slot, INbtTagCompound tag, String nmsItemClassName, String nmsPacketClassName,
-                                          IPanilla panilla) throws NbtNotPermittedException {
-        FailedNbt failedNbt = checkAll(tag, nmsItemClassName, panilla);
-
-        if (FailedNbt.fails(failedNbt)) {
-            throw new NbtNotPermittedException(nmsPacketClassName, false, failedNbt, slot);
-        }
-    }
-
-    private static boolean tagMeetsKeyThreshold(INbtTagCompound tag, IPanilla panilla) {
-        int maxNonMinecraftKeys = panilla.getPConfig().maxNonMinecraftNbtKeys;
-
-        if (tag.getNonMinecraftKeys().size() > maxNonMinecraftKeys) {
-            return false;
-        }
-
-        for (String key : tag.getKeys()) {
-            if (tag.hasKeyOfType(key, NbtDataType.COMPOUND)) {
-                INbtTagCompound subTag = tag.getCompound(key);
-
-                if (!tagMeetsKeyThreshold(subTag, panilla)) {
-                    return false;
-                }
-            }
-        }
-
-        return true;
-    }
-
-    public static FailedNbt checkAll(INbtTagCompound tag, String nmsItemClassName, IPanilla panilla) {
-        if (!tagMeetsKeyThreshold(tag, panilla)) {
-            return FailedNbt.FAIL_KEY_THRESHOLD;
-        }
-
-        for (String key : tag.getKeys()) {
-            if (panilla.getPConfig().nbtWhitelist.contains(key)) {
-                continue;
-            }
-
-            if (tag.hasKeyOfType(key, NbtDataType.LIST)) {
-                INbtTagList list = tag.getList(key);
-
-                if (list.size() > 128) {
-                    return new FailedNbt((key), NbtCheck.NbtCheckResult.CRITICAL);
-                }
-            }
-
-            NbtCheck check = checks.get(key);
-
-            if (check == null) {
-                // a non-minecraft NBT tag
-                continue;
-            }
-
-            if (check.getTolerance().ordinal() > panilla.getPConfig().strictness.ordinal()) {
-                continue;
-            }
-
-            NbtCheck.NbtCheckResult result = check.check(tag, nmsItemClassName, panilla);
-
-            if (result != NbtCheck.NbtCheckResult.PASS) {
-                return new FailedNbt(key, result);
-            }
-        }
-
-        return FailedNbt.NO_FAIL;
-    }
-
-}
+package com.ruinscraft.panilla.api.nbt.checks;
+
+import com.ruinscraft.panilla.api.IPanilla;
+import com.ruinscraft.panilla.api.exception.FailedNbt;
+import com.ruinscraft.panilla.api.exception.NbtNotPermittedException;
+import com.ruinscraft.panilla.api.nbt.INbtTagCompound;
+import com.ruinscraft.panilla.api.nbt.INbtTagList;
+import com.ruinscraft.panilla.api.nbt.NbtDataType;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public final class NbtChecks {
+
+    private static final Map<String, NbtCheck> checks = new HashMap<>();
+    private static final int MAX_HIDE_FLAGS = 0x7F;
+
+    static {
+        // vanilla
+        register(new NbtCheck_Unbreakable());
+        register(new NbtCheck_CanDestroy());
+        register(new NbtCheck_CanPlaceOn());
+        register(new NbtCheck_BlockEntityTag());
+        register(new NbtCheck_BlockStateTag());
+        register(new NbtCheck_ench());
+        register(new NbtCheck_Enchantments());
+        register(new NbtCheck_StoredEnchantments());
+        register(new NbtCheck_RepairCost());
+        register(new NbtCheck_AttributeModifiers());
+        register(new NbtCheck_CustomPotionEffects());
+        register(new NbtCheck_Potion());
+        register(new NbtCheck_CustomPotionColor());
+        register(new NbtCheck_display());
+        register(new NbtCheck_HideFlags());
+        register(new NbtCheck_resolved());
+        register(new NbtCheck_generation());
+        register(new NbtCheck_author());
+        register(new NbtCheck_title());
+        register(new NbtCheck_pages());
+        register(new NbtCheck_SkullOwner());
+        register(new NbtCheck_SkullProfile());
+        register(new NbtCheck_Explosion());
+        register(new NbtCheck_Fireworks());
+        register(new NbtCheck_EntityTag());
+        register(new NbtCheck_BucketVariantTag());
+        register(new NbtCheck_map());
+        register(new NbtCheck_map_scale_direction());
+        register(new NbtCheck_Decorations());
+        register(new NbtCheck_Effects());
+        register(new NbtCheck_CustomModelData());
+        register(new NbtCheck_HasVisualFire()); // 1.17
+        register(new NbtCheck_ChargedProjectiles());
+        register(new NbtCheck_Items());
+
+        // non-vanilla
+        register(new NbtCheck_weBrushJson());
+    }
+
+    private static void register(NbtCheck check) {
+        checks.put(check.getName(), check);
+    }
+
+    public static Map<String, NbtCheck> getChecks() {
+        return checks;
+    }
+
+    public static void checkPacketPlayIn(int slot, INbtTagCompound tag, String nmsItemClassName, String nmsPacketClassName,
+                                         IPanilla panilla) throws NbtNotPermittedException {
+        FailedNbt failedNbt = checkAll(tag, nmsItemClassName, panilla);
+
+        if (FailedNbt.fails(failedNbt)) {
+            throw new NbtNotPermittedException(nmsPacketClassName, false, failedNbt, slot);
+        }
+    }
+
+    public static void checkPacketPlayOut(int slot, INbtTagCompound tag, String nmsItemClassName, String nmsPacketClassName,
+                                          IPanilla panilla) throws NbtNotPermittedException {
+        FailedNbt failedNbt = checkAll(tag, nmsItemClassName, panilla, true);
+
+        if (FailedNbt.fails(failedNbt)) {
+            throw new NbtNotPermittedException(nmsPacketClassName, false, failedNbt, slot);
+        }
+    }
+
+    private static boolean tagMeetsKeyThreshold(INbtTagCompound tag, IPanilla panilla) {
+        int maxNonMinecraftKeys = panilla.getPConfig().maxNonMinecraftNbtKeys;
+
+        if (tag.getNonMinecraftKeys().size() > maxNonMinecraftKeys) {
+            return false;
+        }
+
+        for (String key : tag.getKeys()) {
+            if (tag.hasKeyOfType(key, NbtDataType.COMPOUND)) {
+                INbtTagCompound subTag = tag.getCompound(key);
+
+                if (!tagMeetsKeyThreshold(subTag, panilla)) {
+                    return false;
+                }
+            }
+        }
+
+        return true;
+    }
+
+    public static FailedNbt checkAll(INbtTagCompound tag, String nmsItemClassName, IPanilla panilla) {
+        return checkAll(tag, nmsItemClassName, panilla, false);
+    }
+
+    private static FailedNbt checkAll(INbtTagCompound tag, String nmsItemClassName, IPanilla panilla,
+                                      boolean serverGeneratedItem) {
+        if (!tagMeetsKeyThreshold(tag, panilla)) {
+            return FailedNbt.FAIL_KEY_THRESHOLD;
+        }
+
+        for (String key : tag.getKeys()) {
+            if (panilla.getPConfig().nbtWhitelist.contains(key)) {
+                continue;
+            }
+
+            if (serverGeneratedItem && isSafeServerGeneratedMetadata(tag, key)) {
+                continue;
+            }
+
+            if (tag.hasKeyOfType(key, NbtDataType.LIST)) {
+                INbtTagList list = tag.getList(key);
+
+                if (list.size() > 128) {
+                    return new FailedNbt((key), NbtCheck.NbtCheckResult.CRITICAL);
+                }
+            }
+
+            NbtCheck check = checks.get(key);
+
+            if (check == null) {
+                // a non-minecraft NBT tag
+                continue;
+            }
+
+            if (check.getTolerance().ordinal() > panilla.getPConfig().strictness.ordinal()) {
+                continue;
+            }
+
+            NbtCheck.NbtCheckResult result = check.check(tag, nmsItemClassName, panilla);
+
+            if (result != NbtCheck.NbtCheckResult.PASS) {
+                return new FailedNbt(key, result);
+            }
+        }
+
+        return FailedNbt.NO_FAIL;
+    }
+
+    private static boolean isSafeServerGeneratedMetadata(INbtTagCompound tag, String key) {
+        if ("Unbreakable".equals(key)) {
+            return tag.hasKeyOfType(key, NbtDataType.BYTE) && tag.getInt(key) >= 0 && tag.getInt(key) <= 1;
+        }
+
+        if ("HideFlags".equals(key)) {
+            return tag.hasKeyOfType(key, NbtDataType.INT) && tag.getInt(key) >= 0 && tag.getInt(key) <= MAX_HIDE_FLAGS;
+        }
+
+        return false;
+    }
+
+}
\ No newline at end of file