diff --git a/patches/net/minecraft/client/Minecraft.java.patch b/patches/net/minecraft/client/Minecraft.java.patch
index 13ff259e92..5e00660eca 100644
--- a/patches/net/minecraft/client/Minecraft.java.patch
+++ b/patches/net/minecraft/client/Minecraft.java.patch
@@ -9,6 +9,16 @@
      static Minecraft instance;
      private static final Logger LOGGER = LogUtils.getLogger();
      public static final boolean ON_OSX = Util.getPlatform() == Util.OS.OSX;
+@@ -399,6 +_,9 @@
+     private final long clientStartTimeMs;
+     private long clientTickCount;
+     private String debugPath = "root";
++    @org.jetbrains.annotations.Nullable
++    @org.jetbrains.annotations.ApiStatus.Internal
++    public net.neoforged.neoforge.flag.FlagManager clientModdedFlagManager;
+ 
+     public Minecraft(GameConfig p_91084_) {
+         super("Client");
 @@ -435,7 +_,6 @@
              }
          }, Util.nonCriticalIoPool());
@@ -336,10 +346,11 @@
          this.level = p_91157_;
          this.updateLevelInEngines(p_91157_);
          if (!this.isLocalServer) {
-@@ -2093,6 +_,7 @@
+@@ -2093,6 +_,8 @@
          IntegratedServer integratedserver = this.singleplayerServer;
          this.singleplayerServer = null;
          this.gameRenderer.resetData();
++        this.clientModdedFlagManager = null;
 +        net.neoforged.neoforge.client.ClientHooks.firePlayerLogout(this.gameMode, this.player);
          this.gameMode = null;
          this.narrator.clear();
@@ -378,14 +389,29 @@
                  if (itemstack == null) {
                      return;
                  }
-@@ -2794,6 +_,10 @@
- 
-     public void updateMaxMipLevel(int p_91313_) {
+@@ -2796,6 +_,10 @@
          this.modelManager.updateMaxMipLevel(p_91313_);
-+    }
-+
+     }
+ 
 +    public ItemColors getItemColors() {
 +        return this.itemColors;
++    }
++
+     public EntityModelSet getEntityModels() {
+         return this.entityModels;
+     }
+@@ -2868,6 +_,14 @@
+     @Nullable
+     public static String getLauncherBrand() {
+         return System.getProperty("minecraft.launcher.brand");
++    }
++
++    public net.neoforged.neoforge.flag.FlagManager getModdedFlagManager() {
++        if(clientModdedFlagManager != null)
++            return clientModdedFlagManager;
++        if(singleplayerServer != null)
++            return singleplayerServer.getModdedFlagManager();
++        return net.neoforged.neoforge.flag.FlagManager.EMPTY;
      }
  
-     public EntityModelSet getEntityModels() {
+     @OnlyIn(Dist.CLIENT)
diff --git a/patches/net/minecraft/client/gui/screens/worldselection/CreateWorldScreen.java.patch b/patches/net/minecraft/client/gui/screens/worldselection/CreateWorldScreen.java.patch
index 1c369de99a..f95f8e4311 100644
--- a/patches/net/minecraft/client/gui/screens/worldselection/CreateWorldScreen.java.patch
+++ b/patches/net/minecraft/client/gui/screens/worldselection/CreateWorldScreen.java.patch
@@ -8,6 +8,17 @@
          WorldLoader.InitConfig worldloader$initconfig = createDefaultLoadConfig(packrepository, WorldDataConfiguration.DEFAULT);
          CompletableFuture<WorldCreationContext> completablefuture = WorldLoader.load(
              worldloader$initconfig,
+@@ -219,7 +_,9 @@
+         WorldDimensions.Complete worlddimensions$complete = worldcreationcontext.selectedDimensions().bake(worldcreationcontext.datapackDimensions());
+         LayeredRegistryAccess<RegistryLayer> layeredregistryaccess = worldcreationcontext.worldgenRegistries()
+             .replaceFrom(RegistryLayer.DIMENSIONS, worlddimensions$complete.dimensionsRegistryAccess());
+-        Lifecycle lifecycle = FeatureFlags.isExperimental(worldcreationcontext.dataConfiguration().enabledFeatures())
++        // Neo: vanilla experimental flag present or modded flag present
++        var isExperimental = FeatureFlags.isExperimental(worldcreationcontext.dataConfiguration().enabledFeatures()) || !worldcreationcontext.dataConfiguration().enabledModdedFeatures().isEmpty();
++        Lifecycle lifecycle = isExperimental
+             ? Lifecycle.experimental()
+             : Lifecycle.stable();
+         Lifecycle lifecycle1 = layeredregistryaccess.compositeAccess().allRegistriesLifecycle();
 @@ -243,6 +_,10 @@
              WorldCreationContext worldcreationcontext = this.uiState.getSettings();
              LevelSettings levelsettings = this.createLevelSettings(flag);
@@ -19,12 +30,30 @@
              this.minecraft
                  .createWorldOpenFlows()
                  .createLevelFromExistingSettings(optional.get(), worldcreationcontext.dataPackResources(), p_249152_, worlddata);
+@@ -358,13 +_,15 @@
+         List<String> list = ImmutableList.copyOf(p_270299_.getSelectedIds());
+         List<String> list1 = p_270299_.getAvailableIds().stream().filter(p_232927_ -> !list.contains(p_232927_)).collect(ImmutableList.toImmutableList());
+         WorldDataConfiguration worlddataconfiguration = new WorldDataConfiguration(
+-            new DataPackConfig(list, list1), this.uiState.getSettings().dataConfiguration().enabledFeatures()
++            new DataPackConfig(list, list1), this.uiState.getSettings().dataConfiguration().enabledFeatures(), this.uiState.getSettings().dataConfiguration().enabledModdedFeatures()
+         );
+         if (this.uiState.tryUpdateDataConfiguration(worlddataconfiguration)) {
+             this.minecraft.setScreen(this);
+         } else {
+             FeatureFlagSet featureflagset = p_270299_.getRequestedFeatureFlags();
+-            if (FeatureFlags.isExperimental(featureflagset) && p_270896_) {
++            // Neo: vanilla experimental flag present or modded flag present
++            var isExperimental = FeatureFlags.isExperimental(featureflagset) || !worlddataconfiguration.enabledModdedFeatures().isEmpty();
++            if (isExperimental && p_270896_) {
+                 this.minecraft.setScreen(new ConfirmExperimentalFeaturesScreen(p_270299_.getSelectedPacks(), p_269635_ -> {
+                     if (p_269635_) {
+                         this.applyNewPackConfig(p_270299_, worlddataconfiguration, p_270760_);
 @@ -428,7 +_,7 @@
                                          if (p_269627_) {
                                              p_270552_.accept(this.uiState.getSettings().dataConfiguration());
                                          } else {
 -                                            p_270552_.accept(WorldDataConfiguration.DEFAULT);
-+                                            p_270552_.accept(new WorldDataConfiguration(new DataPackConfig(ImmutableList.of("vanilla"), ImmutableList.of()), FeatureFlags.VANILLA_SET)); // FORGE: Revert to *actual* vanilla data
++                                            p_270552_.accept(new WorldDataConfiguration(new DataPackConfig(ImmutableList.of("vanilla"), ImmutableList.of()), FeatureFlags.VANILLA_SET, it.unimi.dsi.fastutil.objects.ReferenceSets.emptySet())); // FORGE: Revert to *actual* vanilla data
                                          }
                                      },
                                      Component.translatable("dataPack.validation.failed"),
diff --git a/patches/net/minecraft/client/multiplayer/ClientLevel.java.patch b/patches/net/minecraft/client/multiplayer/ClientLevel.java.patch
index e3eabb22b7..0539cfce5f 100644
--- a/patches/net/minecraft/client/multiplayer/ClientLevel.java.patch
+++ b/patches/net/minecraft/client/multiplayer/ClientLevel.java.patch
@@ -109,7 +109,7 @@
              this.difficulty = p_104852_;
          }
  
-@@ -1069,14 +_,75 @@
+@@ -1069,14 +_,80 @@
              if (p_171712_ instanceof AbstractClientPlayer) {
                  ClientLevel.this.players.add((AbstractClientPlayer)p_171712_);
              }
@@ -183,5 +183,10 @@
 +    @org.jetbrains.annotations.ApiStatus.Internal
 +    public void setDayTimePerTick(float dayTimePerTick) {
 +        this.dayTimePerTick = dayTimePerTick;
++    }
++
++    @Override
++    public net.neoforged.neoforge.flag.FlagManager getModdedFlagManager() {
++        return minecraft.getModdedFlagManager();
      }
  }
diff --git a/patches/net/minecraft/gametest/framework/GameTestServer.java.patch b/patches/net/minecraft/gametest/framework/GameTestServer.java.patch
index 162c353781..78e9a72394 100644
--- a/patches/net/minecraft/gametest/framework/GameTestServer.java.patch
+++ b/patches/net/minecraft/gametest/framework/GameTestServer.java.patch
@@ -1,5 +1,14 @@
 --- a/net/minecraft/gametest/framework/GameTestServer.java
 +++ b/net/minecraft/gametest/framework/GameTestServer.java
+@@ -79,7 +_,7 @@
+         } else {
+             p_206609_.reload();
+             WorldDataConfiguration worlddataconfiguration = new WorldDataConfiguration(
+-                new DataPackConfig(new ArrayList<>(p_206609_.getAvailableIds()), List.of()), FeatureFlags.REGISTRY.allFlags()
++                new DataPackConfig(new ArrayList<>(p_206609_.getAvailableIds()), List.of()), FeatureFlags.REGISTRY.allFlags(), net.neoforged.neoforge.flag.Flag.getFlags()
+             );
+             LevelSettings levelsettings = new LevelSettings(
+                 "Test Level", GameType.CREATIVE, false, Difficulty.NORMAL, true, TEST_GAME_RULES, worlddataconfiguration
 @@ -151,6 +_,7 @@
      public boolean initServer() {
          this.setPlayerList(new PlayerList(this, this.registries(), this.playerDataStorage, 1) {
diff --git a/patches/net/minecraft/server/Main.java.patch b/patches/net/minecraft/server/Main.java.patch
index 786a375e82..342f75544f 100644
--- a/patches/net/minecraft/server/Main.java.patch
+++ b/patches/net/minecraft/server/Main.java.patch
@@ -142,3 +142,12 @@
                  }
              };
              thread.setUncaughtExceptionHandler(new DefaultUncaughtExceptionHandler(LOGGER));
+@@ -302,7 +_,7 @@
+             worlddataconfiguration = worlddataconfiguration1;
+         } else {
+             flag = true;
+-            worlddataconfiguration = new WorldDataConfiguration(p_248563_.initialDataPackConfiguration, FeatureFlags.DEFAULT_FLAGS);
++            worlddataconfiguration = new WorldDataConfiguration(p_248563_.initialDataPackConfiguration, FeatureFlags.DEFAULT_FLAGS, it.unimi.dsi.fastutil.objects.ReferenceSets.emptySet());
+         }
+ 
+         WorldLoader.PackConfig worldloader$packconfig = new WorldLoader.PackConfig(p_251069_, worlddataconfiguration, p_249093_, flag);
diff --git a/patches/net/minecraft/server/MinecraftServer.java.patch b/patches/net/minecraft/server/MinecraftServer.java.patch
index 5d60f55f4d..9d6a253a89 100644
--- a/patches/net/minecraft/server/MinecraftServer.java.patch
+++ b/patches/net/minecraft/server/MinecraftServer.java.patch
@@ -1,6 +1,10 @@
 --- a/net/minecraft/server/MinecraftServer.java
 +++ b/net/minecraft/server/MinecraftServer.java
-@@ -264,7 +_,7 @@
+@@ -261,10 +_,11 @@
+     private final PotionBrewing potionBrewing;
+     private volatile boolean isSaving;
+     private static final AtomicReference<RuntimeException> fatalException = new AtomicReference<>();
++    private final net.neoforged.neoforge.flag.FlagManager moddedFlagManager;
  
      public static <S extends MinecraftServer> S spin(Function<Thread, S> p_129873_) {
          AtomicReference<S> atomicreference = new AtomicReference<>();
@@ -9,6 +13,14 @@
          thread.setUncaughtExceptionHandler((p_177909_, p_177910_) -> LOGGER.error("Uncaught exception in server thread", p_177910_));
          if (Runtime.getRuntime().availableProcessors() > 4) {
              thread.setPriority(8);
+@@ -316,6 +_,7 @@
+             this.serverThread = p_236723_;
+             this.executor = Util.backgroundExecutor();
+             this.potionBrewing = PotionBrewing.bootstrap(this.worldData.enabledFeatures());
++            this.moddedFlagManager = new net.neoforged.neoforge.flag.ServerFlagManager(this, this.worldData.getDataConfiguration().enabledModdedFeatures());
+         }
+     }
+ 
 @@ -372,6 +_,7 @@
          this.readScoreboard(dimensiondatastorage);
          this.commandStorage = new CommandStorage(dimensiondatastorage);
@@ -205,6 +217,18 @@
      }
  
      public SystemReport fillSystemReport(SystemReport p_177936_) {
+@@ -1135,6 +_,11 @@
+             "Enabled Feature Flags",
+             () -> FeatureFlags.REGISTRY.toNames(this.worldData.enabledFeatures()).stream().map(ResourceLocation::toString).collect(Collectors.joining(", "))
+         );
++        // Neo: Append modded feature flags to crash reports
++        p_177936_.setDetail(
++                "Enabled Mod Features",
++                () -> moddedFlagManager.enabledFlags().map(net.neoforged.neoforge.flag.Flag::toStringShort).collect(Collectors.joining(", "))
++        );
+         p_177936_.setDetail("World Generation", () -> this.worldData.worldGenSettingsLifecycle().toString());
+         p_177936_.setDetail("World Seed", () -> String.valueOf(this.worldData.worldGenOptions().seed()));
+         if (this.serverId != null) {
 @@ -1441,7 +_,7 @@
  
      public CompletableFuture<Void> reloadResources(Collection<String> p_129862_) {
@@ -214,6 +238,23 @@
                  this
              )
              .thenCompose(
+@@ -1451,6 +_,7 @@
+                             closeableresourcemanager,
+                             this.registries,
+                             this.worldData.enabledFeatures(),
++                            this.moddedFlagManager,
+                             this.isDedicatedServer() ? Commands.CommandSelection.DEDICATED : Commands.CommandSelection.INTEGRATED,
+                             this.getFunctionCompilationLevel(),
+                             this.executor,
+@@ -1470,7 +_,7 @@
+                     this.resources = p_335203_;
+                     this.packRepository.setSelected(p_129862_);
+                     WorldDataConfiguration worlddataconfiguration = new WorldDataConfiguration(
+-                        getSelectedPacks(this.packRepository, true), this.worldData.enabledFeatures()
++                        getSelectedPacks(this.packRepository, true), this.worldData.enabledFeatures(), this.worldData.getDataConfiguration().enabledModdedFeatures()
+                     );
+                     this.worldData.setDataConfiguration(worlddataconfiguration);
+                     this.resources.managers.updateRegistryTags();
 @@ -1478,6 +_,7 @@
                      this.getPlayerList().reloadResources();
                      this.functionManager.replaceLibrary(this.resources.managers.getFunctionLibrary());
@@ -230,19 +271,44 @@
 +        datapackconfig.addModPacks(net.neoforged.neoforge.common.CommonHooks.getModDataPacks());
          if (p_341620_) {
 -            return configureRepositoryWithSelection(p_248681_, List.of("vanilla"), featureflagset, false);
-+            return configureRepositoryWithSelection(p_248681_, net.neoforged.neoforge.common.CommonHooks.getModDataPacksWithVanilla(), featureflagset, false);
++            return configureRepositoryWithSelection(p_248681_, net.neoforged.neoforge.common.CommonHooks.getModDataPacksWithVanilla(), featureflagset, p_341632_.enabledModdedFeatures(), false);
          } else {
              Set<String> set = Sets.newLinkedHashSet();
  
-@@ -1542,6 +_,8 @@
+@@ -1542,18 +_,30 @@
                  set.add("vanilla");
              }
  
+-            return configureRepositoryWithSelection(p_248681_, set, featureflagset, true);
 +            net.neoforged.neoforge.resource.ResourcePackLoader.reorderNewlyDiscoveredPacks(set, datapackconfig.getEnabled(), p_248681_);
 +
-             return configureRepositoryWithSelection(p_248681_, set, featureflagset, true);
++            return configureRepositoryWithSelection(p_248681_, set, featureflagset, p_341632_.enabledModdedFeatures(), true);
          }
      }
+ 
+     private static WorldDataConfiguration configureRepositoryWithSelection(
+-        PackRepository p_341680_, Collection<String> p_341677_, FeatureFlagSet p_341602_, boolean p_341662_
++        PackRepository p_341680_, Collection<String> p_341677_, FeatureFlagSet p_341602_, it.unimi.dsi.fastutil.objects.ReferenceSet<net.neoforged.neoforge.flag.Flag> enabledModdedFeatures, boolean p_341662_
+     ) {
+         p_341680_.setSelected(p_341677_);
+         enableForcedFeaturePacks(p_341680_, p_341602_);
+         DataPackConfig datapackconfig = getSelectedPacks(p_341680_, p_341662_);
+         FeatureFlagSet featureflagset = p_341680_.getRequestedFeatureFlags().join(p_341602_);
+-        return new WorldDataConfiguration(datapackconfig, featureflagset);
++        return new WorldDataConfiguration(datapackconfig, featureflagset, enabledModdedFeatures);
++    }
++
++    /**
++     * @deprecated Prefer {@link #configureRepositoryWithSelection(PackRepository, Collection, FeatureFlagSet, it.unimi.dsi.fastutil.objects.ReferenceSet, boolean)}
++     */
++    @Deprecated
++    private static WorldDataConfiguration configureRepositoryWithSelection(
++            PackRepository p_341680_, Collection<String> p_341677_, FeatureFlagSet p_341602_, boolean p_341662_
++    ) {
++        return configureRepositoryWithSelection(p_341680_, p_341677_, p_341602_, it.unimi.dsi.fastutil.objects.ReferenceSets.emptySet(), p_341662_);
+     }
+ 
+     private static void enableForcedFeaturePacks(PackRepository p_341674_, FeatureFlagSet p_341598_) {
 @@ -1712,6 +_,31 @@
  
      public abstract boolean isSingleplayerOwner(GameProfile p_129840_);
@@ -275,14 +341,25 @@
      public void dumpServerProperties(Path p_177911_) throws IOException {
      }
  
-@@ -1873,6 +_,10 @@
- 
-     public WorldData getWorldData() {
+@@ -1875,6 +_,10 @@
          return this.worldData;
-+    }
-+
+     }
+ 
 +    public MinecraftServer.ReloadableResources getServerResources() {
 +         return resources;
++    }
++
+     public RegistryAccess.Frozen registryAccess() {
+         return this.registries.compositeAccess();
      }
+@@ -1999,6 +_,10 @@
  
-     public RegistryAccess.Frozen registryAccess() {
+     public ServerLinks serverLinks() {
+         return ServerLinks.EMPTY;
++    }
++
++    public net.neoforged.neoforge.flag.FlagManager getModdedFlagManager() {
++        return moddedFlagManager;
+     }
+ 
+     public static record ReloadableResources(CloseableResourceManager resourceManager, ReloadableServerResources managers) implements AutoCloseable {
diff --git a/patches/net/minecraft/server/ReloadableServerResources.java.patch b/patches/net/minecraft/server/ReloadableServerResources.java.patch
index cf23f14fbc..91beb89215 100644
--- a/patches/net/minecraft/server/ReloadableServerResources.java.patch
+++ b/patches/net/minecraft/server/ReloadableServerResources.java.patch
@@ -1,15 +1,32 @@
 --- a/net/minecraft/server/ReloadableServerResources.java
 +++ b/net/minecraft/server/ReloadableServerResources.java
-@@ -49,6 +_,8 @@
+@@ -40,7 +_,7 @@
+     private final ServerAdvancementManager advancements;
+     private final ServerFunctionLibrary functionLibrary;
+ 
+-    private ReloadableServerResources(RegistryAccess.Frozen p_206857_, FeatureFlagSet p_250695_, Commands.CommandSelection p_206858_, int p_206859_) {
++    private ReloadableServerResources(RegistryAccess.Frozen p_206857_, FeatureFlagSet p_250695_, net.neoforged.neoforge.flag.FlagManager moddedFlagManager, Commands.CommandSelection p_206858_, int p_206859_) {
+         this.fullRegistryHolder = new ReloadableServerRegistries.Holder(p_206857_);
+         this.registryLookup = new ReloadableServerResources.ConfigurableRegistryLookup(p_206857_);
+         this.registryLookup.missingTagAccessPolicy(ReloadableServerResources.MissingTagAccessPolicy.CREATE_NEW);
+@@ -49,6 +_,16 @@
          this.commands = new Commands(p_206858_, CommandBuildContext.simple(this.registryLookup, p_250695_));
          this.advancements = new ServerAdvancementManager(this.registryLookup);
          this.functionLibrary = new ServerFunctionLibrary(p_206859_, this.commands.getDispatcher());
 +        // Neo: Create context object
-+        this.context = new net.neoforged.neoforge.common.conditions.ConditionContext(this.tagManager);
++        this.context = new net.neoforged.neoforge.common.conditions.ConditionContext(this.tagManager, moddedFlagManager);
++    }
++
++    /**
++     * Prefer {@link #ReloadableServerResources(RegistryAccess.Frozen, FeatureFlagSet, net.neoforged.neoforge.flag.FlagManager, Commands.CommandSelection, int)}
++     */
++    @Deprecated
++    private ReloadableServerResources(RegistryAccess.Frozen p_206857_, FeatureFlagSet p_250695_, Commands.CommandSelection p_206858_, int p_206859_) {
++        this(p_206857_, p_250695_, net.neoforged.neoforge.flag.FlagManager.EMPTY, p_206858_, p_206859_);
      }
  
      public ServerFunctionLibrary getFunctionLibrary() {
-@@ -75,6 +_,24 @@
+@@ -75,10 +_,29 @@
          return List.of(this.tagManager, this.recipes, this.functionLibrary, this.advancements);
      }
  
@@ -34,9 +51,17 @@
      public static CompletableFuture<ReloadableServerResources> loadResources(
          ResourceManager p_248588_,
          LayeredRegistryAccess<RegistryLayer> p_335667_,
-@@ -90,14 +_,27 @@
+         FeatureFlagSet p_250212_,
++        net.neoforged.neoforge.flag.FlagManager moddedFlagManager,
+         Commands.CommandSelection p_249301_,
+         int p_251126_,
+         Executor p_249136_,
+@@ -88,25 +_,55 @@
+             .thenCompose(
+                 p_335211_ -> {
                      ReloadableServerResources reloadableserverresources = new ReloadableServerResources(
-                         p_335211_.compositeAccess(), p_250212_, p_249301_, p_251126_
+-                        p_335211_.compositeAccess(), p_250212_, p_249301_, p_251126_
++                        p_335211_.compositeAccess(), p_250212_, moddedFlagManager, p_249301_, p_251126_
                      );
 +                    List<PreparableReloadListener> listeners = new java.util.ArrayList<>(reloadableserverresources.listeners());
 +                    listeners.addAll(net.neoforged.neoforge.event.EventHooks.onResourceReload(reloadableserverresources, p_335211_.compositeAccess()));
@@ -63,7 +88,25 @@
                          .thenApply(p_214306_ -> reloadableserverresources);
                  }
              );
-@@ -107,6 +_,7 @@
+     }
+ 
++    /**
++     * @deprecated Prefer to use {@link #loadResources(ResourceManager, LayeredRegistryAccess, FeatureFlagSet, net.neoforged.neoforge.flag.FlagManager, Commands.CommandSelection, int, Executor, Executor)}.
++     */
++    @Deprecated
++    public static CompletableFuture<ReloadableServerResources> loadResources(
++            ResourceManager p_248588_,
++            LayeredRegistryAccess<RegistryLayer> p_335667_,
++            FeatureFlagSet p_250212_,
++            Commands.CommandSelection p_249301_,
++            int p_251126_,
++            Executor p_249136_,
++            Executor p_249601_
++    ) {
++        return loadResources(p_248588_, p_335667_, p_250212_, net.neoforged.neoforge.flag.FlagManager.EMPTY, p_249301_, p_251126_, p_249136_, p_249601_);
++    }
++
+     public void updateRegistryTags() {
          this.tagManager.getResult().forEach(p_335204_ -> updateRegistryTags(this.fullRegistryHolder.get(), (TagManager.LoadResult<?>)p_335204_));
          AbstractFurnaceBlockEntity.invalidateCache();
          Blocks.rebuildCache();
diff --git a/patches/net/minecraft/server/WorldLoader.java.patch b/patches/net/minecraft/server/WorldLoader.java.patch
index 67180986ab..8b24d4ce17 100644
--- a/patches/net/minecraft/server/WorldLoader.java.patch
+++ b/patches/net/minecraft/server/WorldLoader.java.patch
@@ -9,3 +9,11 @@
              );
              RegistryAccess.Frozen registryaccess$frozen = layeredregistryaccess1.getAccessForLoading(RegistryLayer.DIMENSIONS);
              RegistryAccess.Frozen registryaccess$frozen1 = RegistryDataLoader.load(
+@@ -50,6 +_,7 @@
+                     closeableresourcemanager,
+                     layeredregistryaccess2,
+                     worlddataconfiguration.enabledFeatures(),
++                    net.neoforged.neoforge.flag.FlagManager.createImmutable(worlddataconfiguration.enabledModdedFeatures()),
+                     p_214363_.commandSelection(),
+                     p_214363_.functionCompilationLevel(),
+                     p_214366_,
diff --git a/patches/net/minecraft/server/level/ServerLevel.java.patch b/patches/net/minecraft/server/level/ServerLevel.java.patch
index 411052eadc..ccbb006740 100644
--- a/patches/net/minecraft/server/level/ServerLevel.java.patch
+++ b/patches/net/minecraft/server/level/ServerLevel.java.patch
@@ -214,7 +214,7 @@
                      ServerLevel.this.dragonParts.put(enderdragonpart.getId(), enderdragonpart);
                  }
              }
-@@ -1733,24 +_,106 @@
+@@ -1733,24 +_,110 @@
                  if (ServerLevel.this.isUpdatingNavigations) {
                      String s = "onTrackingStart called during navigation iteration";
                      Util.logAndPauseIfInIde(
@@ -243,7 +243,7 @@
          public void onSectionChange(Entity p_215086_) {
              p_215086_.updateDynamicGameEventListener(DynamicGameEventListener::move);
          }
-     }
++    }
 +
 +    @Override
 +    public java.util.Collection<net.neoforged.neoforge.entity.PartEntity<?>> getPartEntities() {
@@ -323,4 +323,8 @@
 +        }
 +    }
 +
++    @Override
++    public net.neoforged.neoforge.flag.FlagManager getModdedFlagManager() {
++        return server.getModdedFlagManager();
+     }
  }
diff --git a/patches/net/minecraft/server/level/WorldGenRegion.java.patch b/patches/net/minecraft/server/level/WorldGenRegion.java.patch
index 77dfc22633..707fb891f5 100644
--- a/patches/net/minecraft/server/level/WorldGenRegion.java.patch
+++ b/patches/net/minecraft/server/level/WorldGenRegion.java.patch
@@ -8,3 +8,14 @@
          int i = SectionPos.blockToSectionCoord(p_9580_.getBlockX());
          int j = SectionPos.blockToSectionCoord(p_9580_.getBlockZ());
          this.getChunk(i, j).addEntity(p_9580_);
+@@ -469,5 +_,10 @@
+     @Override
+     public long nextSubTickCount() {
+         return this.subTickCount.getAndIncrement();
++    }
++
++    @Override
++    public net.neoforged.neoforge.flag.FlagManager getModdedFlagManager() {
++        return level.getModdedFlagManager();
+     }
+ }
diff --git a/patches/net/minecraft/world/effect/MobEffect.java.patch b/patches/net/minecraft/world/effect/MobEffect.java.patch
index c8eef70796..aa7adc5505 100644
--- a/patches/net/minecraft/world/effect/MobEffect.java.patch
+++ b/patches/net/minecraft/world/effect/MobEffect.java.patch
@@ -9,6 +9,14 @@
      public static final Codec<Holder<MobEffect>> CODEC = BuiltInRegistries.MOB_EFFECT.holderByNameCodec();
      public static final StreamCodec<RegistryFriendlyByteBuf, Holder<MobEffect>> STREAM_CODEC = ByteBufCodecs.holderRegistry(Registries.MOB_EFFECT);
      private static final int AMBIENT_ALPHA = Mth.floor(38.25F);
+@@ -48,6 +_,7 @@
+     private int blendDurationTicks;
+     private Optional<SoundEvent> soundOnAdded = Optional.empty();
+     private FeatureFlagSet requiredFeatures = FeatureFlags.VANILLA_SET;
++    private it.unimi.dsi.fastutil.objects.ReferenceSet<net.neoforged.neoforge.flag.Flag> requiredFlags = it.unimi.dsi.fastutil.objects.ReferenceSets.emptySet();
+ 
+     protected MobEffect(MobEffectCategory p_19451_, int p_19452_) {
+         this.category = p_19451_;
 @@ -130,6 +_,18 @@
          return this;
      }
@@ -28,11 +36,35 @@
      public MobEffect setBlendDuration(int p_316265_) {
          this.blendDurationTicks = p_316265_;
          return this;
-@@ -181,8 +_,24 @@
+@@ -171,6 +_,10 @@
+         return this;
+     }
+ 
++    /**
++     * @deprecated Prefer {@linkplain #requiredFlags(net.neoforged.neoforge.flag.Flag...)}
++     */
++    @Deprecated
+     public MobEffect requiredFeatures(FeatureFlag... p_338702_) {
+         this.requiredFeatures = FeatureFlags.REGISTRY.subset(p_338702_);
+         return this;
+@@ -181,8 +_,37 @@
          return this.requiredFeatures;
      }
  
 -    static record AttributeTemplate(ResourceLocation id, double amount, AttributeModifier.Operation operation) {
++    public MobEffect requiredFlags(net.neoforged.neoforge.flag.Flag... requiredFlags) {
++        if(this.requiredFlags.isEmpty())
++            this.requiredFlags = new it.unimi.dsi.fastutil.objects.ReferenceOpenHashSet<>();
++
++        java.util.Collections.addAll(this.requiredFlags, requiredFlags);
++        return this;
++    }
++
++    @Override
++    public it.unimi.dsi.fastutil.objects.ReferenceSet<net.neoforged.neoforge.flag.Flag> requiredFlags() {
++        return requiredFlags.isEmpty() ? it.unimi.dsi.fastutil.objects.ReferenceSets.emptySet() : it.unimi.dsi.fastutil.objects.ReferenceSets.unmodifiable(requiredFlags);
++    }
++
 +    /**
 +     * Neo: Allowing mods to define client behavior for their MobEffects
 +     * @deprecated Use {@link net.neoforged.neoforge.client.extensions.common.RegisterClientExtensionsEvent} instead
diff --git a/patches/net/minecraft/world/entity/EntityType.java.patch b/patches/net/minecraft/world/entity/EntityType.java.patch
index 1f3f299099..d53d6e7565 100644
--- a/patches/net/minecraft/world/entity/EntityType.java.patch
+++ b/patches/net/minecraft/world/entity/EntityType.java.patch
@@ -1,21 +1,44 @@
 --- a/net/minecraft/world/entity/EntityType.java
 +++ b/net/minecraft/world/entity/EntityType.java
-@@ -834,6 +_,10 @@
+@@ -834,6 +_,11 @@
      private final float spawnDimensionsScale;
      private final FeatureFlagSet requiredFeatures;
  
 +    private final java.util.function.Predicate<EntityType<?>> trackDeltasSupplier;
 +    private final java.util.function.ToIntFunction<EntityType<?>> trackingRangeSupplier;
 +    private final java.util.function.ToIntFunction<EntityType<?>> updateIntervalSupplier;
++    private final it.unimi.dsi.fastutil.objects.ReferenceSet<net.neoforged.neoforge.flag.Flag> requiredFlags;
 +
      private static <T extends Entity> EntityType<T> register(String p_20635_, EntityType.Builder<T> p_20636_) {
          return Registry.register(BuiltInRegistries.ENTITY_TYPE, p_20635_, p_20636_.build(p_20635_));
      }
-@@ -860,6 +_,26 @@
+@@ -860,6 +_,49 @@
          int p_273451_,
          FeatureFlagSet p_273518_
      ) {
-+        this(p_273268_, p_272918_, p_273417_, p_273389_, p_273556_, p_272654_, p_273631_, p_272946_, p_338404_, p_272895_, p_273451_, p_273518_, EntityType::defaultTrackDeltasSupplier, EntityType::defaultTrackingRangeSupplier, EntityType::defaultUpdateIntervalSupplier);
++        this(p_273268_, p_272918_, p_273417_, p_273389_, p_273556_, p_272654_, p_273631_, p_272946_, p_338404_, p_272895_, p_273451_, p_273518_, EntityType::defaultTrackDeltasSupplier, EntityType::defaultTrackingRangeSupplier, EntityType::defaultUpdateIntervalSupplier, it.unimi.dsi.fastutil.objects.ReferenceSets.emptySet());
++    }
++
++    // TODO: Remove in 21.2
++    @Deprecated(forRemoval = true, since = "21.1")
++    public EntityType(
++            EntityType.EntityFactory<T> p_273268_,
++            MobCategory p_272918_,
++            boolean p_273417_,
++            boolean p_273389_,
++            boolean p_273556_,
++            boolean p_272654_,
++            ImmutableSet<Block> p_273631_,
++            EntityDimensions p_272946_,
++            float p_338404_,
++            int p_272895_,
++            int p_273451_,
++            FeatureFlagSet p_273518_,
++            final java.util.function.Predicate<EntityType<?>> trackDeltasSupplier,
++            final java.util.function.ToIntFunction<EntityType<?>> trackingRangeSupplier,
++            final java.util.function.ToIntFunction<EntityType<?>> updateIntervalSupplier
++    ) {
++        this(p_273268_, p_272918_, p_273417_, p_273389_, p_273556_, p_272654_, p_273631_, p_272946_, p_338404_, p_272895_, p_273451_, p_273518_, trackDeltasSupplier, trackingRangeSupplier, updateIntervalSupplier, it.unimi.dsi.fastutil.objects.ReferenceSets.emptySet());
 +    }
 +
 +    public EntityType(
@@ -33,18 +56,20 @@
 +        FeatureFlagSet p_273518_,
 +        final java.util.function.Predicate<EntityType<?>> trackDeltasSupplier,
 +        final java.util.function.ToIntFunction<EntityType<?>> trackingRangeSupplier,
-+        final java.util.function.ToIntFunction<EntityType<?>> updateIntervalSupplier
++        final java.util.function.ToIntFunction<EntityType<?>> updateIntervalSupplier,
++        final it.unimi.dsi.fastutil.objects.ReferenceSet<net.neoforged.neoforge.flag.Flag> requiredFlags
 +    ) {
          this.factory = p_273268_;
          this.category = p_272918_;
          this.canSpawnFarFromPlayer = p_272654_;
-@@ -872,6 +_,9 @@
+@@ -872,6 +_,10 @@
          this.clientTrackingRange = p_272895_;
          this.updateInterval = p_273451_;
          this.requiredFeatures = p_273518_;
 +        this.trackDeltasSupplier = trackDeltasSupplier;
 +        this.trackingRangeSupplier = trackingRangeSupplier;
 +        this.updateIntervalSupplier = updateIntervalSupplier;
++        this.requiredFlags = requiredFlags.isEmpty() ? it.unimi.dsi.fastutil.objects.ReferenceSets.emptySet() : it.unimi.dsi.fastutil.objects.ReferenceSets.unmodifiable(requiredFlags);
      }
  
      @Nullable
@@ -88,30 +113,53 @@
          return this != PLAYER
              && this != LLAMA_SPIT
              && this != WITHER
-@@ -1192,6 +_,8 @@
+@@ -1192,6 +_,13 @@
          return this.builtInRegistryHolder;
      }
  
 +    public Stream<TagKey<EntityType<?>>> getTags() {return this.builtInRegistryHolder().tags();}
++
++    @Override
++    public final it.unimi.dsi.fastutil.objects.ReferenceSet<net.neoforged.neoforge.flag.Flag> requiredFlags() {
++        return requiredFlags;
++    }
 +
      public static class Builder<T extends Entity> {
          private final EntityType.EntityFactory<T> factory;
          private final MobCategory category;
-@@ -1207,6 +_,10 @@
+@@ -1207,6 +_,11 @@
          private EntityAttachments.Builder attachments = EntityAttachments.builder();
          private FeatureFlagSet requiredFeatures = FeatureFlags.VANILLA_SET;
  
 +        private java.util.function.Predicate<EntityType<?>> velocityUpdateSupplier = EntityType::defaultTrackDeltasSupplier;
 +        private java.util.function.ToIntFunction<EntityType<?>> trackingRangeSupplier = EntityType::defaultTrackingRangeSupplier;
 +        private java.util.function.ToIntFunction<EntityType<?>> updateIntervalSupplier = EntityType::defaultUpdateIntervalSupplier;
++        private it.unimi.dsi.fastutil.objects.ReferenceSet<net.neoforged.neoforge.flag.Flag> requiredFlags = it.unimi.dsi.fastutil.objects.ReferenceSets.emptySet();
 +
          private Builder(EntityType.EntityFactory<T> p_20696_, MobCategory p_20697_) {
              this.factory = p_20696_;
              this.category = p_20697_;
-@@ -1314,6 +_,21 @@
+@@ -1309,11 +_,38 @@
              return this;
          }
  
++        /**
++         * @deprecated Prefer {@linkplain #requiredFlags(net.neoforged.neoforge.flag.Flag...)}
++         */
++        @Deprecated
+         public EntityType.Builder<T> requiredFeatures(FeatureFlag... p_251646_) {
+             this.requiredFeatures = FeatureFlags.REGISTRY.subset(p_251646_);
+             return this;
+         }
+ 
++        public EntityType.Builder<T> requiredFlags(net.neoforged.neoforge.flag.Flag... requiredFlags) {
++            if(this.requiredFlags.isEmpty())
++                this.requiredFlags = new it.unimi.dsi.fastutil.objects.ReferenceOpenHashSet<>();
++
++            java.util.Collections.addAll(this.requiredFlags, requiredFlags);
++            return this;
++        }
++
 +        public EntityType.Builder<T> setUpdateInterval(int interval) {
 +             this.updateIntervalSupplier = t->interval;
 +             return this;
@@ -130,7 +178,7 @@
          public EntityType<T> build(String p_20713_) {
              if (this.serialize) {
                  Util.fetchChoiceType(References.ENTITY_TREE, p_20713_);
-@@ -1331,7 +_,10 @@
+@@ -1331,7 +_,11 @@
                  this.spawnDimensionsScale,
                  this.clientTrackingRange,
                  this.updateInterval,
@@ -138,7 +186,8 @@
 +                this.requiredFeatures,
 +                velocityUpdateSupplier,
 +                trackingRangeSupplier,
-+                updateIntervalSupplier
++                updateIntervalSupplier,
++                this.requiredFlags
              );
          }
      }
diff --git a/patches/net/minecraft/world/flag/FeatureElement.java.patch b/patches/net/minecraft/world/flag/FeatureElement.java.patch
new file mode 100644
index 0000000000..cd78cc83d8
--- /dev/null
+++ b/patches/net/minecraft/world/flag/FeatureElement.java.patch
@@ -0,0 +1,19 @@
+--- a/net/minecraft/world/flag/FeatureElement.java
++++ b/net/minecraft/world/flag/FeatureElement.java
+@@ -5,7 +_,7 @@
+ import net.minecraft.core.registries.Registries;
+ import net.minecraft.resources.ResourceKey;
+ 
+-public interface FeatureElement {
++public interface FeatureElement extends net.neoforged.neoforge.flag.FlagElement {
+     Set<ResourceKey<? extends Registry<? extends FeatureElement>>> FILTERED_REGISTRIES = Set.of(
+         Registries.ITEM, Registries.BLOCK, Registries.ENTITY_TYPE, Registries.MENU, Registries.POTION, Registries.MOB_EFFECT
+     );
+@@ -13,6 +_,6 @@
+     FeatureFlagSet requiredFeatures();
+ 
+     default boolean isEnabled(FeatureFlagSet p_249172_) {
+-        return this.requiredFeatures().isSubsetOf(p_249172_);
++        return this.requiredFeatures().isSubsetOf(p_249172_) && isEnabled();
+     }
+ }
diff --git a/patches/net/minecraft/world/inventory/MenuType.java.patch b/patches/net/minecraft/world/inventory/MenuType.java.patch
index 7d0eaf4e2d..651bd7bcdb 100644
--- a/patches/net/minecraft/world/inventory/MenuType.java.patch
+++ b/patches/net/minecraft/world/inventory/MenuType.java.patch
@@ -9,18 +9,47 @@
      public static final MenuType<ChestMenu> GENERIC_9x1 = register("generic_9x1", ChestMenu::oneRow);
      public static final MenuType<ChestMenu> GENERIC_9x2 = register("generic_9x2", ChestMenu::twoRows);
      public static final MenuType<ChestMenu> GENERIC_9x3 = register("generic_9x3", ChestMenu::threeRows);
-@@ -52,6 +_,14 @@
+@@ -36,6 +_,7 @@
+     public static final MenuType<StonecutterMenu> STONECUTTER = register("stonecutter", StonecutterMenu::new);
+     private final FeatureFlagSet requiredFeatures;
+     private final MenuType.MenuSupplier<T> constructor;
++    private final it.unimi.dsi.fastutil.objects.ReferenceSet<net.neoforged.neoforge.flag.Flag> requiredFlags;
  
-     public T create(int p_39986_, Inventory p_39987_) {
-         return this.constructor.create(p_39986_, p_39987_);
+     private static <T extends AbstractContainerMenu> MenuType<T> register(String p_39989_, MenuType.MenuSupplier<T> p_39990_) {
+         return Registry.register(BuiltInRegistries.MENU, p_39989_, new MenuType<>(p_39990_, FeatureFlags.VANILLA_SET));
+@@ -48,6 +_,13 @@
+     public MenuType(MenuType.MenuSupplier<T> p_267054_, FeatureFlagSet p_266909_) {
+         this.constructor = p_267054_;
+         this.requiredFeatures = p_266909_;
++        this.requiredFlags = it.unimi.dsi.fastutil.objects.ReferenceSets.emptySet();
 +    }
 +
-+    @Override
++    public MenuType(MenuType.MenuSupplier<T> p_267054_, FeatureFlagSet p_266909_, net.neoforged.neoforge.flag.Flag[] requiredFlags) {
++        this.constructor = p_267054_;
++        this.requiredFeatures = p_266909_;
++        this.requiredFlags = it.unimi.dsi.fastutil.objects.ReferenceSet.of(requiredFlags);
+     }
+ 
+     public T create(int p_39986_, Inventory p_39987_) {
+@@ -55,8 +_,21 @@
+     }
+ 
+     @Override
 +    public T create(int windowId, Inventory playerInv, net.minecraft.network.RegistryFriendlyByteBuf extraData) {
 +        if (this.constructor instanceof net.neoforged.neoforge.network.IContainerFactory) {
 +            return ((net.neoforged.neoforge.network.IContainerFactory<T>) this.constructor).create(windowId, playerInv, extraData);
 +        }
 +        return create(windowId, playerInv);
++    }
++
++    @Override
+     public FeatureFlagSet requiredFeatures() {
+         return this.requiredFeatures;
++    }
++
++    @Override
++    public final it.unimi.dsi.fastutil.objects.ReferenceSet<net.neoforged.neoforge.flag.Flag> requiredFlags() {
++        return requiredFlags;
      }
  
-     @Override
+     public interface MenuSupplier<T extends AbstractContainerMenu> {
diff --git a/patches/net/minecraft/world/item/BlockItem.java.patch b/patches/net/minecraft/world/item/BlockItem.java.patch
index eed7cf8eb1..81fe9a22fe 100644
--- a/patches/net/minecraft/world/item/BlockItem.java.patch
+++ b/patches/net/minecraft/world/item/BlockItem.java.patch
@@ -31,16 +31,27 @@
      @Nullable
      public BlockPlaceContext updatePlacementContext(BlockPlaceContext p_40609_) {
          return p_40609_;
-@@ -193,6 +_,12 @@
- 
-     public void registerBlocks(Map<Block, Item> p_40607_, Item p_40608_) {
+@@ -195,6 +_,12 @@
          p_40607_.put(this.getBlock(), p_40608_);
-+    }
-+
+     }
+ 
 +    /** @deprecated Neo: To be removed without replacement since registry replacement is not a feature anymore. */
 +    @Deprecated(forRemoval = true, since = "1.21.1")
 +    public void removeFromBlockToItemMap(Map<Block, Item> blockToItemMap, Item itemIn) {
 +        blockToItemMap.remove(this.getBlock());
-     }
- 
++    }
++
+     @Override
+     public boolean canFitInsideContainerItems() {
+         return !(this.getBlock() instanceof ShulkerBoxBlock);
+@@ -221,5 +_,10 @@
      @Override
+     public FeatureFlagSet requiredFeatures() {
+         return this.getBlock().requiredFeatures();
++    }
++
++    @Override
++    public boolean isEnabled() {
++        return getBlock().isEnabled() && super.isEnabled();
+     }
+ }
diff --git a/patches/net/minecraft/world/item/Item.java.patch b/patches/net/minecraft/world/item/Item.java.patch
index 4054c635cd..37ed92658e 100644
--- a/patches/net/minecraft/world/item/Item.java.patch
+++ b/patches/net/minecraft/world/item/Item.java.patch
@@ -12,11 +12,20 @@
      public static final ResourceLocation BASE_ATTACK_DAMAGE_ID = ResourceLocation.withDefaultNamespace("base_attack_damage");
      public static final ResourceLocation BASE_ATTACK_SPEED_ID = ResourceLocation.withDefaultNamespace("base_attack_speed");
      public static final int DEFAULT_MAX_STACK_SIZE = 64;
-@@ -89,12 +_,13 @@
+@@ -71,6 +_,7 @@
+     @Nullable
+     private String descriptionId;
+     private final FeatureFlagSet requiredFeatures;
++    private final it.unimi.dsi.fastutil.objects.ReferenceSet<net.neoforged.neoforge.flag.Flag> requiredFlags;
+ 
+     public static int getId(Item p_41394_) {
+         return p_41394_ == null ? 0 : BuiltInRegistries.ITEM.getId(p_41394_);
+@@ -89,12 +_,14 @@
          this.components = p_41383_.buildAndValidateComponents();
          this.craftingRemainingItem = p_41383_.craftingRemainingItem;
          this.requiredFeatures = p_41383_.requiredFeatures;
 -        if (SharedConstants.IS_RUNNING_IN_IDE) {
++        this.requiredFlags = p_41383_.requiredFlags.isEmpty() ? it.unimi.dsi.fastutil.objects.ReferenceSets.emptySet() : it.unimi.dsi.fastutil.objects.ReferenceSets.unmodifiable(p_41383_.requiredFlags);
 +        if (SharedConstants.IS_RUNNING_IN_IDE && false) {
              String s = this.getClass().getSimpleName();
              if (!s.endsWith("Item")) {
@@ -142,11 +151,16 @@
      }
  
      public ItemStack getDefaultInstance() {
-@@ -341,13 +_,22 @@
+@@ -341,13 +_,28 @@
          return this.requiredFeatures;
      }
  
 -    public static class Properties {
++    @Override
++    public final it.unimi.dsi.fastutil.objects.ReferenceSet<net.neoforged.neoforge.flag.Flag> requiredFlags() {
++        return requiredFlags;
++    }
++
 +    /**
 +     * Neo: Allowing mods to define client behavior for their Items
 +     * @deprecated Use {@link net.neoforged.neoforge.client.extensions.common.RegisterClientExtensionsEvent} instead
@@ -163,10 +177,11 @@
          Item craftingRemainingItem;
          FeatureFlagSet requiredFeatures = FeatureFlags.VANILLA_SET;
 +        private boolean canRepair = true;
++        private it.unimi.dsi.fastutil.objects.ReferenceSet<net.neoforged.neoforge.flag.Flag> requiredFlags = it.unimi.dsi.fastutil.objects.ReferenceSets.emptySet();
  
          public Item.Properties food(FoodProperties p_41490_) {
              return this.component(DataComponents.FOOD, p_41490_);
-@@ -381,12 +_,18 @@
+@@ -381,12 +_,30 @@
              return this.component(DataComponents.JUKEBOX_PLAYABLE, new JukeboxPlayable(new EitherHolder<>(p_350862_), true));
          }
  
@@ -175,11 +190,23 @@
 +            return this;
 +        }
 +
++        /**
++         * @deprecated Prefer {@linkplain #requiredFlags(net.neoforged.neoforge.flag.Flag...)}
++         */
++        @Deprecated
          public Item.Properties requiredFeatures(FeatureFlag... p_250948_) {
              this.requiredFeatures = FeatureFlags.REGISTRY.subset(p_250948_);
              return this;
          }
  
++        public Item.Properties requiredFlags(net.neoforged.neoforge.flag.Flag... requiredFlags) {
++            if(this.requiredFlags.isEmpty())
++                this.requiredFlags = new it.unimi.dsi.fastutil.objects.ReferenceOpenHashSet<>();
++
++            java.util.Collections.addAll(this.requiredFlags, requiredFlags);
++            return this;
++        }
++
          public <T> Item.Properties component(DataComponentType<T> p_330871_, T p_330323_) {
 +            net.neoforged.neoforge.common.CommonHooks.validateComponent(p_330323_);
              if (this.components == null) {
diff --git a/patches/net/minecraft/world/item/SpawnEggItem.java.patch b/patches/net/minecraft/world/item/SpawnEggItem.java.patch
index 039545cc2e..c5df7f031a 100644
--- a/patches/net/minecraft/world/item/SpawnEggItem.java.patch
+++ b/patches/net/minecraft/world/item/SpawnEggItem.java.patch
@@ -39,7 +39,7 @@
      }
  
      public Optional<Mob> spawnOffspringFromSpawnEgg(
-@@ -179,5 +_,9 @@
+@@ -179,5 +_,14 @@
                  }
              }
          }
@@ -47,5 +47,10 @@
 +
 +    protected EntityType<?> getDefaultType() {
 +        return defaultType;
++    }
++
++    @Override
++    public boolean isEnabled() {
++        return getDefaultType().isEnabled() && super.isEnabled();
      }
  }
diff --git a/patches/net/minecraft/world/item/alchemy/Potion.java.patch b/patches/net/minecraft/world/item/alchemy/Potion.java.patch
new file mode 100644
index 0000000000..5e9e47be2b
--- /dev/null
+++ b/patches/net/minecraft/world/item/alchemy/Potion.java.patch
@@ -0,0 +1,41 @@
+--- a/net/minecraft/world/item/alchemy/Potion.java
++++ b/net/minecraft/world/item/alchemy/Potion.java
+@@ -24,6 +_,7 @@
+     private final String name;
+     private final List<MobEffectInstance> effects;
+     private FeatureFlagSet requiredFeatures = FeatureFlags.VANILLA_SET;
++    private it.unimi.dsi.fastutil.objects.ReferenceSet<net.neoforged.neoforge.flag.Flag> requiredFlags = it.unimi.dsi.fastutil.objects.ReferenceSets.emptySet();
+ 
+     public Potion(MobEffectInstance... p_43487_) {
+         this(null, p_43487_);
+@@ -34,6 +_,10 @@
+         this.effects = List.of(p_43485_);
+     }
+ 
++    /**
++     * @deprecated Prefer {@linkplain #requiredFlags(net.neoforged.neoforge.flag.Flag...)}
++     */
++    @Deprecated
+     public Potion requiredFeatures(FeatureFlag... p_338520_) {
+         this.requiredFeatures = FeatureFlags.REGISTRY.subset(p_338520_);
+         return this;
+@@ -42,6 +_,19 @@
+     @Override
+     public FeatureFlagSet requiredFeatures() {
+         return this.requiredFeatures;
++    }
++
++    public Potion requiredFlags(net.neoforged.neoforge.flag.Flag... requiredFlags) {
++        if(this.requiredFlags.isEmpty())
++            this.requiredFlags = new it.unimi.dsi.fastutil.objects.ReferenceOpenHashSet<>();
++
++        java.util.Collections.addAll(this.requiredFlags, requiredFlags);
++        return this;
++    }
++
++    @Override
++    public it.unimi.dsi.fastutil.objects.ReferenceSet<net.neoforged.neoforge.flag.Flag> requiredFlags() {
++        return requiredFlags.isEmpty() ? it.unimi.dsi.fastutil.objects.ReferenceSets.emptySet() : it.unimi.dsi.fastutil.objects.ReferenceSets.unmodifiable(requiredFlags);
+     }
+ 
+     public static String getName(Optional<Holder<Potion>> p_330503_, String p_43493_) {
diff --git a/patches/net/minecraft/world/level/WorldDataConfiguration.java.patch b/patches/net/minecraft/world/level/WorldDataConfiguration.java.patch
new file mode 100644
index 0000000000..06a2b328e2
--- /dev/null
+++ b/patches/net/minecraft/world/level/WorldDataConfiguration.java.patch
@@ -0,0 +1,37 @@
+--- a/net/minecraft/world/level/WorldDataConfiguration.java
++++ b/net/minecraft/world/level/WorldDataConfiguration.java
+@@ -6,20 +_,31 @@
+ import net.minecraft.world.flag.FeatureFlagSet;
+ import net.minecraft.world.flag.FeatureFlags;
+ 
+-public record WorldDataConfiguration(DataPackConfig dataPacks, FeatureFlagSet enabledFeatures) {
++public record WorldDataConfiguration(DataPackConfig dataPacks, FeatureFlagSet enabledFeatures, it.unimi.dsi.fastutil.objects.ReferenceSet<net.neoforged.neoforge.flag.Flag> enabledModdedFeatures) {
+     public static final String ENABLED_FEATURES_ID = "enabled_features";
+     public static final Codec<WorldDataConfiguration> CODEC = RecordCodecBuilder.create(
+         p_337972_ -> p_337972_.group(
+                     DataPackConfig.CODEC.lenientOptionalFieldOf("DataPacks", DataPackConfig.DEFAULT).forGetter(WorldDataConfiguration::dataPacks),
+                     FeatureFlags.CODEC
+                         .lenientOptionalFieldOf("enabled_features", FeatureFlags.DEFAULT_FLAGS)
+-                        .forGetter(WorldDataConfiguration::enabledFeatures)
++                        .forGetter(WorldDataConfiguration::enabledFeatures),
++                        net.neoforged.neoforge.flag.Flag.SET_CODEC
++                            .lenientOptionalFieldOf("enabled_modded_features", it.unimi.dsi.fastutil.objects.ReferenceSets.emptySet())
++                            .forGetter(WorldDataConfiguration::enabledModdedFeatures)
+                 )
+                 .apply(p_337972_, WorldDataConfiguration::new)
+     );
+     public static final WorldDataConfiguration DEFAULT = new WorldDataConfiguration(DataPackConfig.DEFAULT, FeatureFlags.DEFAULT_FLAGS);
+ 
+     public WorldDataConfiguration expandFeatures(FeatureFlagSet p_249090_) {
+-        return new WorldDataConfiguration(this.dataPacks, this.enabledFeatures.join(p_249090_));
++        return new WorldDataConfiguration(this.dataPacks, this.enabledFeatures.join(p_249090_), this.enabledModdedFeatures);
++    }
++
++    /**
++     * @deprecated Prefer to use {@link #WorldDataConfiguration(DataPackConfig, FeatureFlagSet, it.unimi.dsi.fastutil.objects.ReferenceSet)} instead.
++     */
++    @Deprecated
++    public WorldDataConfiguration(DataPackConfig dataPacks, FeatureFlagSet enabledFeatures) {
++        this(dataPacks, enabledFeatures, it.unimi.dsi.fastutil.objects.ReferenceSets.emptySet());
+     }
+ }
diff --git a/patches/net/minecraft/world/level/block/state/BlockBehaviour.java.patch b/patches/net/minecraft/world/level/block/state/BlockBehaviour.java.patch
index 851387e29d..594c5e57af 100644
--- a/patches/net/minecraft/world/level/block/state/BlockBehaviour.java.patch
+++ b/patches/net/minecraft/world/level/block/state/BlockBehaviour.java.patch
@@ -1,8 +1,10 @@
 --- a/net/minecraft/world/level/block/state/BlockBehaviour.java
 +++ b/net/minecraft/world/level/block/state/BlockBehaviour.java
-@@ -111,6 +_,17 @@
+@@ -110,7 +_,19 @@
+         this.jumpFactor = p_60452_.jumpFactor;
          this.dynamicShape = p_60452_.dynamicShape;
          this.requiredFeatures = p_60452_.requiredFeatures;
++        p_60452_.requiredFlags = p_60452_.requiredFlags.isEmpty() ? it.unimi.dsi.fastutil.objects.ReferenceSets.emptySet() : it.unimi.dsi.fastutil.objects.ReferenceSets.unmodifiable(p_60452_.requiredFlags);
          this.properties = p_60452_;
 +        final ResourceKey<LootTable> lootTableCache = p_60452_.drops;
 +        if (lootTableCache != null) {
@@ -66,7 +68,7 @@
      protected SoundType getSoundType(BlockState p_320941_) {
          return this.soundType;
      }
-@@ -393,6 +_,13 @@
+@@ -393,6 +_,18 @@
          return this.properties.destroyTime;
      }
  
@@ -74,6 +76,11 @@
 +        return ((BlockStateBase)state).isAir;
 +    }
 +
++    @Override
++    public final it.unimi.dsi.fastutil.objects.ReferenceSet<net.neoforged.neoforge.flag.Flag> requiredFlags() {
++        return properties.requiredFlags;
++    }
++
 +    // Neo: Holds the loot table for this block's drops. Used for getLootTable method.
 +    private final java.util.function.Supplier<ResourceKey<LootTable>> lootTableSupplier;
 +
@@ -150,6 +157,27 @@
          BlockBehaviour.StatePredicate isRedstoneConductor = (p_284888_, p_284889_, p_284890_) -> p_284888_.isCollisionShapeFullBlock(p_284889_, p_284890_);
          BlockBehaviour.StatePredicate isSuffocating = (p_284885_, p_284886_, p_284887_) -> p_284885_.blocksMotion()
                  && p_284885_.isCollisionShapeFullBlock(p_284886_, p_284887_);
+@@ -974,6 +_,7 @@
+         FeatureFlagSet requiredFeatures = FeatureFlags.VANILLA_SET;
+         @Nullable
+         BlockBehaviour.OffsetFunction offsetFunction;
++        private it.unimi.dsi.fastutil.objects.ReferenceSet<net.neoforged.neoforge.flag.Flag> requiredFlags = it.unimi.dsi.fastutil.objects.ReferenceSets.emptySet();
+ 
+         private Properties() {
+         }
+@@ -1020,6 +_,12 @@
+             blockbehaviour$properties.offsetFunction = blockbehaviour$properties1.offsetFunction;
+             blockbehaviour$properties.spawnTerrainParticles = blockbehaviour$properties1.spawnTerrainParticles;
+             blockbehaviour$properties.requiredFeatures = blockbehaviour$properties1.requiredFeatures;
++
++            if(!blockbehaviour$properties1.requiredFlags.isEmpty()) {
++                blockbehaviour$properties.requiredFlags = new it.unimi.dsi.fastutil.objects.ReferenceOpenHashSet<>();
++                blockbehaviour$properties.requiredFlags.addAll(blockbehaviour$properties1.requiredFlags);
++            }
++
+             blockbehaviour$properties.emissiveRendering = blockbehaviour$properties1.emissiveRendering;
+             blockbehaviour$properties.instrument = blockbehaviour$properties1.instrument;
+             blockbehaviour$properties.replaceable = blockbehaviour$properties1.replaceable;
 @@ -1105,9 +_,15 @@
              return this;
          }
@@ -167,3 +195,24 @@
          }
  
          public BlockBehaviour.Properties ignitedByLava() {
+@@ -1215,8 +_,20 @@
+             return this;
+         }
+ 
++        /**
++         * @deprecated Prefer {@linkplain #requiredFlags(net.neoforged.neoforge.flag.Flag...)}.
++         */
++        @Deprecated
+         public BlockBehaviour.Properties requiredFeatures(FeatureFlag... p_248792_) {
+             this.requiredFeatures = FeatureFlags.REGISTRY.subset(p_248792_);
++            return this;
++        }
++
++        public BlockBehaviour.Properties requiredFlags(net.neoforged.neoforge.flag.Flag... requiredFlags) {
++            if(this.requiredFlags.isEmpty())
++                this.requiredFlags = new it.unimi.dsi.fastutil.objects.ReferenceOpenHashSet<>();
++
++            java.util.Collections.addAll(this.requiredFlags, requiredFlags);
+             return this;
+         }
+ 
diff --git a/patches/net/minecraft/world/level/storage/LevelStorageSource.java.patch b/patches/net/minecraft/world/level/storage/LevelStorageSource.java.patch
index f119c517b6..183fdb11e5 100644
--- a/patches/net/minecraft/world/level/storage/LevelStorageSource.java.patch
+++ b/patches/net/minecraft/world/level/storage/LevelStorageSource.java.patch
@@ -1,5 +1,15 @@
 --- a/net/minecraft/world/level/storage/LevelStorageSource.java
 +++ b/net/minecraft/world/level/storage/LevelStorageSource.java
+@@ -287,7 +_,8 @@
+             WorldDataConfiguration worlddataconfiguration = readDataConfig(p_307300_);
+             LevelSettings levelsettings = LevelSettings.parse(p_307300_, worlddataconfiguration);
+             FeatureFlagSet featureflagset = parseFeatureFlagsFromSummary(p_307300_);
+-            boolean flag1 = FeatureFlags.isExperimental(featureflagset);
++            // Neo: vanilla experimental flag present or modded flag present
++            boolean flag1 = FeatureFlags.isExperimental(featureflagset) || !worlddataconfiguration.enabledModdedFeatures().isEmpty();
+             return new LevelSummary(levelsettings, levelversion, p_307426_.directoryName(), flag, p_307364_, flag1, path);
+         }
+     }
 @@ -463,6 +_,18 @@
              }
          }
diff --git a/src/main/java/net/neoforged/neoforge/common/NeoForgeEventHandler.java b/src/main/java/net/neoforged/neoforge/common/NeoForgeEventHandler.java
index 2e2e14ab5f..db66a52a09 100644
--- a/src/main/java/net/neoforged/neoforge/common/NeoForgeEventHandler.java
+++ b/src/main/java/net/neoforged/neoforge/common/NeoForgeEventHandler.java
@@ -35,6 +35,7 @@
 import net.neoforged.neoforge.event.level.ChunkEvent;
 import net.neoforged.neoforge.event.level.LevelEvent;
 import net.neoforged.neoforge.event.tick.ServerTickEvent;
+import net.neoforged.neoforge.flag.ClientboundSyncFlags;
 import net.neoforged.neoforge.network.PacketDistributor;
 import net.neoforged.neoforge.network.payload.RegistryDataMapSyncPayload;
 import net.neoforged.neoforge.registries.DataMapLoader;
@@ -97,6 +98,11 @@ public void playerChangeDimension(PlayerEvent.PlayerChangedDimensionEvent event)
     @SubscribeEvent
     public void playerLogin(PlayerEvent.PlayerLoggedInEvent event) {
         UsernameCache.setUsername(event.getEntity().getUUID(), event.getEntity().getGameProfile().getName());
+
+        if (event.getEntity() instanceof ServerPlayer sPlayer) {
+            var flagManager = sPlayer.server.getModdedFlagManager();
+            PacketDistributor.sendToPlayer(sPlayer, new ClientboundSyncFlags(flagManager.getEnabledFlags()));
+        }
     }
 
     @SubscribeEvent
diff --git a/src/main/java/net/neoforged/neoforge/common/NeoForgeMod.java b/src/main/java/net/neoforged/neoforge/common/NeoForgeMod.java
index 3c0ca35bb5..63e56456dc 100644
--- a/src/main/java/net/neoforged/neoforge/common/NeoForgeMod.java
+++ b/src/main/java/net/neoforged/neoforge/common/NeoForgeMod.java
@@ -136,6 +136,8 @@
 import net.neoforged.neoforge.common.world.StructureModifiers;
 import net.neoforged.neoforge.data.event.GatherDataEvent;
 import net.neoforged.neoforge.event.server.ServerStoppingEvent;
+import net.neoforged.neoforge.flag.Flag;
+import net.neoforged.neoforge.flag.RequiredFlagsCondition;
 import net.neoforged.neoforge.fluids.BaseFlowingFluid;
 import net.neoforged.neoforge.fluids.CauldronFluidContent;
 import net.neoforged.neoforge.fluids.FluidType;
@@ -382,6 +384,10 @@ public class NeoForgeMod {
     public static final DeferredHolder<MapCodec<? extends ICondition>, MapCodec<OrCondition>> OR_CONDITION = CONDITION_CODECS.register("or", () -> OrCondition.CODEC);
     public static final DeferredHolder<MapCodec<? extends ICondition>, MapCodec<TagEmptyCondition>> TAG_EMPTY_CONDITION = CONDITION_CODECS.register("tag_empty", () -> TagEmptyCondition.CODEC);
     public static final DeferredHolder<MapCodec<? extends ICondition>, MapCodec<TrueCondition>> TRUE_CONDITION = CONDITION_CODECS.register("true", () -> TrueCondition.CODEC);
+    /**
+     * {@link ICondition Condition} used when testing data files against {@link Flag flags}.
+     */
+    public static final DeferredHolder<MapCodec<? extends ICondition>, MapCodec<RequiredFlagsCondition>> REQUIRED_FLAGS_CONDITION = CONDITION_CODECS.register("required_flags", () -> RequiredFlagsCondition.CODEC);
 
     private static final DeferredRegister<MapCodec<? extends EntitySubPredicate>> ENTITY_PREDICATE_CODECS = DeferredRegister.create(Registries.ENTITY_SUB_PREDICATE_TYPE, NeoForgeVersion.MOD_ID);
     public static final DeferredHolder<MapCodec<? extends EntitySubPredicate>, MapCodec<PiglinNeutralArmorEntityPredicate>> PIGLIN_NEUTRAL_ARMOR_PREDICATE = ENTITY_PREDICATE_CODECS.register("piglin_neutral_armor", () -> PiglinNeutralArmorEntityPredicate.CODEC);
diff --git a/src/main/java/net/neoforged/neoforge/common/conditions/ConditionContext.java b/src/main/java/net/neoforged/neoforge/common/conditions/ConditionContext.java
index 186cbb6347..f8d544ff86 100644
--- a/src/main/java/net/neoforged/neoforge/common/conditions/ConditionContext.java
+++ b/src/main/java/net/neoforged/neoforge/common/conditions/ConditionContext.java
@@ -14,6 +14,7 @@
 import net.minecraft.resources.ResourceKey;
 import net.minecraft.resources.ResourceLocation;
 import net.minecraft.tags.TagManager;
+import net.neoforged.neoforge.flag.FlagManager;
 import org.jetbrains.annotations.Nullable;
 
 public class ConditionContext implements ICondition.IContext {
@@ -21,9 +22,17 @@ public class ConditionContext implements ICondition.IContext {
     @Nullable
     // TODO 1.20.5: Clear loaded tags after reloads complete. The context object may leak, but we still want to invalidate it.
     private Map<ResourceKey<?>, Map<ResourceLocation, Collection<Holder<?>>>> loadedTags = null;
+    private final FlagManager flagManager;
 
-    public ConditionContext(TagManager tagManager) {
+    public ConditionContext(TagManager tagManager, FlagManager flagManager) {
         this.tagManager = tagManager;
+        this.flagManager = flagManager;
+    }
+
+    // TODO: Remove in 21.2
+    @Deprecated(forRemoval = true, since = "21.1")
+    public ConditionContext(TagManager tagManager) {
+        this(tagManager, FlagManager.EMPTY);
     }
 
     @SuppressWarnings({ "unchecked", "rawtypes" })
@@ -41,4 +50,9 @@ public <T> Map<ResourceLocation, Collection<Holder<T>>> getAllTags(ResourceKey<?
         }
         return (Map) loadedTags.getOrDefault(registry, Collections.emptyMap());
     }
+
+    @Override
+    public FlagManager getModdedFlagManager() {
+        return flagManager;
+    }
 }
diff --git a/src/main/java/net/neoforged/neoforge/common/conditions/ICondition.java b/src/main/java/net/neoforged/neoforge/common/conditions/ICondition.java
index 04e0ff5f02..c34f43c6dd 100644
--- a/src/main/java/net/neoforged/neoforge/common/conditions/ICondition.java
+++ b/src/main/java/net/neoforged/neoforge/common/conditions/ICondition.java
@@ -27,6 +27,8 @@
 import net.minecraft.resources.ResourceLocation;
 import net.minecraft.tags.TagKey;
 import net.minecraft.util.Unit;
+import net.neoforged.neoforge.flag.Flag;
+import net.neoforged.neoforge.flag.FlagManager;
 import net.neoforged.neoforge.registries.NeoForgeRegistries;
 
 public interface ICondition {
@@ -107,5 +109,12 @@ default <T> Collection<Holder<T>> getTag(TagKey<T> key) {
          * Note that the map and the tags are unmodifiable.
          */
         <T> Map<ResourceLocation, Collection<Holder<T>>> getAllTags(ResourceKey<? extends Registry<T>> registry);
+
+        /**
+         * @return {@link FlagManager} instance used to lookup {@link Flag} states.
+         */
+        default FlagManager getModdedFlagManager() {
+            return FlagManager.EMPTY;
+        }
     }
 }
diff --git a/src/main/java/net/neoforged/neoforge/common/conditions/IConditionBuilder.java b/src/main/java/net/neoforged/neoforge/common/conditions/IConditionBuilder.java
index 06836647a8..08bfe72e1c 100644
--- a/src/main/java/net/neoforged/neoforge/common/conditions/IConditionBuilder.java
+++ b/src/main/java/net/neoforged/neoforge/common/conditions/IConditionBuilder.java
@@ -8,6 +8,8 @@
 import java.util.List;
 import net.minecraft.tags.TagKey;
 import net.minecraft.world.item.Item;
+import net.neoforged.neoforge.flag.Flag;
+import net.neoforged.neoforge.flag.RequiredFlagsCondition;
 
 public interface IConditionBuilder {
     default ICondition and(ICondition... values) {
@@ -41,4 +43,8 @@ default ICondition modLoaded(String modid) {
     default ICondition tagEmpty(TagKey<Item> tag) {
         return new TagEmptyCondition(tag.location());
     }
+
+    default ICondition requiredFlags(Flag... requiredFlags) {
+        return new RequiredFlagsCondition(requiredFlags);
+    }
 }
diff --git a/src/main/java/net/neoforged/neoforge/common/extensions/ILevelReaderExtension.java b/src/main/java/net/neoforged/neoforge/common/extensions/ILevelReaderExtension.java
index bd97ecced5..34e2356dff 100644
--- a/src/main/java/net/neoforged/neoforge/common/extensions/ILevelReaderExtension.java
+++ b/src/main/java/net/neoforged/neoforge/common/extensions/ILevelReaderExtension.java
@@ -10,6 +10,8 @@
 import net.minecraft.core.Holder;
 import net.minecraft.resources.ResourceKey;
 import net.minecraft.world.level.LevelReader;
+import net.neoforged.neoforge.flag.Flag;
+import net.neoforged.neoforge.flag.FlagManager;
 
 public interface ILevelReaderExtension {
     private LevelReader self() {
@@ -37,4 +39,15 @@ default <T> Holder<T> holderOrThrow(ResourceKey<T> key) {
     default <T> Optional<Holder.Reference<T>> holder(ResourceKey<T> key) {
         return this.self().registryAccess().holder(key);
     }
+
+    /**
+     * Returns a valid and synced {@link FlagManager} or empty of none can be obtained.
+     * <p>
+     * Modders may use this for all their {@link Flag flag} state testing needs.
+     *
+     * @return Valid and synced {@link FlagManager} or empty.
+     */
+    default FlagManager getModdedFlagManager() {
+        return FlagManager.EMPTY;
+    }
 }
diff --git a/src/main/java/net/neoforged/neoforge/common/extensions/IMenuTypeExtension.java b/src/main/java/net/neoforged/neoforge/common/extensions/IMenuTypeExtension.java
index 38404b95cc..7ee7c19bc5 100644
--- a/src/main/java/net/neoforged/neoforge/common/extensions/IMenuTypeExtension.java
+++ b/src/main/java/net/neoforged/neoforge/common/extensions/IMenuTypeExtension.java
@@ -10,9 +10,14 @@
 import net.minecraft.world.flag.FeatureFlags;
 import net.minecraft.world.inventory.AbstractContainerMenu;
 import net.minecraft.world.inventory.MenuType;
+import net.neoforged.neoforge.flag.Flag;
 import net.neoforged.neoforge.network.IContainerFactory;
 
 public interface IMenuTypeExtension<T> {
+    static <T extends AbstractContainerMenu> MenuType<T> create(IContainerFactory<T> factory, Flag... requiredFlags) {
+        return new MenuType<>(factory, FeatureFlags.DEFAULT_FLAGS, requiredFlags);
+    }
+
     static <T extends AbstractContainerMenu> MenuType<T> create(IContainerFactory<T> factory) {
         return new MenuType<>(factory, FeatureFlags.DEFAULT_FLAGS);
     }
diff --git a/src/main/java/net/neoforged/neoforge/flag/ClientboundSyncFlags.java b/src/main/java/net/neoforged/neoforge/flag/ClientboundSyncFlags.java
new file mode 100644
index 0000000000..71d409f283
--- /dev/null
+++ b/src/main/java/net/neoforged/neoforge/flag/ClientboundSyncFlags.java
@@ -0,0 +1,50 @@
+/*
+ * Copyright (c) NeoForged and contributors
+ * SPDX-License-Identifier: LGPL-2.1-only
+ */
+
+package net.neoforged.neoforge.flag;
+
+import it.unimi.dsi.fastutil.objects.ReferenceOpenHashSet;
+import it.unimi.dsi.fastutil.objects.ReferenceSet;
+import net.minecraft.client.Minecraft;
+import net.minecraft.network.RegistryFriendlyByteBuf;
+import net.minecraft.network.codec.ByteBufCodecs;
+import net.minecraft.network.codec.StreamCodec;
+import net.minecraft.network.protocol.common.custom.CustomPacketPayload;
+import net.minecraft.resources.ResourceLocation;
+import net.neoforged.fml.loading.FMLEnvironment;
+import net.neoforged.neoforge.internal.versions.neoforge.NeoForgeVersion;
+import net.neoforged.neoforge.network.handling.IPayloadContext;
+import org.jetbrains.annotations.ApiStatus;
+
+@ApiStatus.Internal
+public final class ClientboundSyncFlags implements CustomPacketPayload {
+    public static final Type<ClientboundSyncFlags> TYPE = new Type<>(ResourceLocation.fromNamespaceAndPath(NeoForgeVersion.MOD_ID, "sync_flags"));
+    public static final StreamCodec<RegistryFriendlyByteBuf, ClientboundSyncFlags> STREAM_CODEC = StreamCodec.composite(
+            ByteBufCodecs.collection(ReferenceOpenHashSet::new, Flag.STREAM_CODEC), payload -> payload.enabledFlags,
+            ClientboundSyncFlags::new);
+
+    private final ReferenceSet<Flag> enabledFlags;
+
+    public ClientboundSyncFlags(ReferenceSet<Flag> enabledFlags) {
+        this.enabledFlags = enabledFlags;
+    }
+
+    public void handle(IPayloadContext context) {
+        context.enqueueWork(() -> {
+            if (!FMLEnvironment.dist.isClient())
+                return;
+
+            var client = Minecraft.getInstance();
+
+            if (client.getSingleplayerServer() == null)
+                client.clientModdedFlagManager = FlagManager.createImmutable(enabledFlags);
+        });
+    }
+
+    @Override
+    public Type<ClientboundSyncFlags> type() {
+        return TYPE;
+    }
+}
diff --git a/src/main/java/net/neoforged/neoforge/flag/EmptyFlagManager.java b/src/main/java/net/neoforged/neoforge/flag/EmptyFlagManager.java
new file mode 100644
index 0000000000..30000bdc6c
--- /dev/null
+++ b/src/main/java/net/neoforged/neoforge/flag/EmptyFlagManager.java
@@ -0,0 +1,21 @@
+/*
+ * Copyright (c) NeoForged and contributors
+ * SPDX-License-Identifier: LGPL-2.1-only
+ */
+
+package net.neoforged.neoforge.flag;
+
+import it.unimi.dsi.fastutil.objects.ReferenceSet;
+import it.unimi.dsi.fastutil.objects.ReferenceSets;
+
+final class EmptyFlagManager implements FlagManager {
+    @Override
+    public boolean set(Flag flag, boolean state) {
+        return false;
+    }
+
+    @Override
+    public ReferenceSet<Flag> getEnabledFlags() {
+        return ReferenceSets.emptySet();
+    }
+}
diff --git a/src/main/java/net/neoforged/neoforge/flag/Flag.java b/src/main/java/net/neoforged/neoforge/flag/Flag.java
new file mode 100644
index 0000000000..04868723e6
--- /dev/null
+++ b/src/main/java/net/neoforged/neoforge/flag/Flag.java
@@ -0,0 +1,117 @@
+/*
+ * Copyright (c) NeoForged and contributors
+ * SPDX-License-Identifier: LGPL-2.1-only
+ */
+
+package net.neoforged.neoforge.flag;
+
+import com.google.common.collect.Maps;
+import com.mojang.serialization.Codec;
+import io.netty.buffer.ByteBuf;
+import it.unimi.dsi.fastutil.objects.ReferenceOpenHashSet;
+import it.unimi.dsi.fastutil.objects.ReferenceSet;
+import it.unimi.dsi.fastutil.objects.ReferenceSets;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import java.util.stream.Stream;
+import net.minecraft.network.codec.StreamCodec;
+import net.minecraft.resources.ResourceLocation;
+
+/**
+ * Class describing a specific flag.
+ */
+public final class Flag {
+    public static final Codec<Flag> CODEC = ResourceLocation.CODEC.xmap(Flag::of, flag -> flag.identifier);
+    public static final Codec<ReferenceSet<Flag>> SET_CODEC = CODEC.listOf().xmap(
+            flags -> ReferenceSets.unmodifiable(new ReferenceOpenHashSet<>(flags)), // there is no refset.copyOf must goto mutable then to immutable
+            List::copyOf);
+    public static final StreamCodec<ByteBuf, Flag> STREAM_CODEC = ResourceLocation.STREAM_CODEC.map(Flag::of, flag -> flag.identifier);
+
+    private static final Map<ResourceLocation, Flag> FLAGS = Maps.newConcurrentMap();
+    private static final Collection<Flag> FLAGS_VIEW = Collections.unmodifiableCollection(FLAGS.values());
+
+    private final ResourceLocation identifier;
+
+    private Flag(ResourceLocation identifier) {
+        this.identifier = identifier;
+    }
+
+    /**
+     * @return The owning mod id/namespace for this {@link Flag flag}.
+     */
+    public String namespace() {
+        return identifier.getNamespace();
+    }
+
+    /**
+     * @return The unique identifier for this {@link Flag flag}.
+     * @apiNote This identifier does not need to be globally unique, only unique to the owning mod.
+     */
+    public String identifier() {
+        return identifier.getPath();
+    }
+
+    public String toStringShort() {
+        return identifier.toString();
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (this == obj)
+            return true;
+        if (obj.getClass() == Flag.class)
+            return identifier.equals(((Flag) obj).identifier);
+        return false;
+    }
+
+    @Override
+    public int hashCode() {
+        return identifier.hashCode();
+    }
+
+    @Override
+    public String toString() {
+        return "Flag{" + identifier + '}';
+    }
+
+    /**
+     * Constructs a new {@link Flag} from the given {@code namespace} and {@code identifier}.
+     *
+     * @param namespace  The owning mods id/namespace.
+     * @param identifier A mod specific unique identifier.
+     * @return The newly constructed flag.
+     */
+    public static Flag of(String namespace, String identifier) {
+        return of(ResourceLocation.fromNamespaceAndPath(namespace, identifier));
+    }
+
+    /**
+     * Constructs a new {@link Flag} from the given {@code identifier}.
+     *
+     * @param identifier A mod specific unique identifier.
+     * @return The newly constructed flag.
+     */
+    public static Flag of(ResourceLocation identifier) {
+        return FLAGS.computeIfAbsent(identifier, Flag::new);
+    }
+
+    /**
+     * Returns immutable collection of all known {@link Flag flags}.
+     *
+     * @return immutable collection of all known {@link Flag flags}.
+     */
+    public static ReferenceSet<Flag> getFlags() {
+        return new ReferenceOpenHashSet<>(FLAGS_VIEW);
+    }
+
+    /**
+     * Returns {@link Stream} representing all known {@link Flag flags}.
+     *
+     * @return {@link Stream} representing all known {@link Flag flags}.
+     */
+    public static Stream<Flag> flags() {
+        return FLAGS_VIEW.stream();
+    }
+}
diff --git a/src/main/java/net/neoforged/neoforge/flag/FlagElement.java b/src/main/java/net/neoforged/neoforge/flag/FlagElement.java
new file mode 100644
index 0000000000..7dd7ac7387
--- /dev/null
+++ b/src/main/java/net/neoforged/neoforge/flag/FlagElement.java
@@ -0,0 +1,57 @@
+/*
+ * Copyright (c) NeoForged and contributors
+ * SPDX-License-Identifier: LGPL-2.1-only
+ */
+
+package net.neoforged.neoforge.flag;
+
+import com.google.errorprone.annotations.ForOverride;
+import it.unimi.dsi.fastutil.objects.ReferenceSet;
+import java.util.Set;
+import net.minecraft.client.Minecraft;
+import net.minecraft.world.flag.FeatureElement;
+import net.neoforged.fml.loading.FMLEnvironment;
+import net.neoforged.neoforge.data.loading.DatagenModLoader;
+import net.neoforged.neoforge.server.ServerLifecycleHooks;
+import org.jetbrains.annotations.ApiStatus;
+
+/**
+ * Interface describing an element whose state can be toggled with {@link Flag flags}.
+ *
+ * @apiNote All vanilla {@link FeatureElement FeatureElements} are pre patched to directly support the {@link Flag} system.
+ */
+public interface FlagElement {
+    /**
+     * @return Immutable {@link Set} containing all required {@link Flag Flags} for this element to be enabled.
+     */
+    @ForOverride
+    ReferenceSet<Flag> requiredFlags();
+
+    /**
+     * @return {@code true} if this {@link FlagElement element} should be enabled, {@code false} otherwise.
+     */
+    @ApiStatus.NonExtendable
+    default boolean isEnabled() {
+        // all flags are enabled during data gen
+        if (DatagenModLoader.isRunningDataGen())
+            return true;
+
+        return activeFlagManager().isEnabled(requiredFlags());
+    }
+
+    private FlagManager activeFlagManager() {
+        // this exists purely so that we can call #isEnabled
+        // from within FeatureElement.isFeatureEnabled
+        // which is required to correctly hook into the vanilla system
+        // without requiring a ton of patches to invoke our isEnabled check along side vanillas
+        // and mods also needing to invoke ours alongside vanillas
+        var server = ServerLifecycleHooks.getCurrentServer();
+
+        if (server != null)
+            return server.getModdedFlagManager();
+        if (FMLEnvironment.dist.isClient())
+            return Minecraft.getInstance().getModdedFlagManager();
+
+        return FlagManager.EMPTY;
+    }
+}
diff --git a/src/main/java/net/neoforged/neoforge/flag/FlagManager.java b/src/main/java/net/neoforged/neoforge/flag/FlagManager.java
new file mode 100644
index 0000000000..916732d6cf
--- /dev/null
+++ b/src/main/java/net/neoforged/neoforge/flag/FlagManager.java
@@ -0,0 +1,114 @@
+/*
+ * Copyright (c) NeoForged and contributors
+ * SPDX-License-Identifier: LGPL-2.1-only
+ */
+
+package net.neoforged.neoforge.flag;
+
+import it.unimi.dsi.fastutil.objects.ReferenceSet;
+import java.util.Collection;
+import java.util.Set;
+import java.util.stream.Stream;
+import net.neoforged.neoforge.common.extensions.ILevelReaderExtension;
+import org.jetbrains.annotations.ApiStatus;
+
+/**
+ * The main manager for all {@link Flag} related tasks.
+ * <p>
+ * An instance of this can be looked up via {@link ILevelReaderExtension#getModdedFlagManager()}.
+ */
+public sealed interface FlagManager permits ImmutableFlagManager, EmptyFlagManager, ServerFlagManager {
+    /**
+     * Empty instance to be used when no other can be found.
+     */
+    FlagManager EMPTY = new EmptyFlagManager();
+
+    /**
+     * Returns {@code true} if the given {@link Flag} is enabled, {@code false} otherwise.
+     *
+     * @param flag The {@link Flag} to be tested.
+     * @return {@code true} if the given {@link Flag} is enabled, {@code false} otherwise.
+     */
+    default boolean isEnabled(Flag flag) {
+        return getEnabledFlags().contains(flag);
+    }
+
+    /**
+     * Returns {@code true} if all given {@link Flag Flags} are enabled, {@code false} otherwise.
+     *
+     * @param flag  A {@link Flag} to be tested.
+     * @param flags Additional {@link Flag Flags} to also be tested.
+     * @return {@code true} if all given {@link Flag Flags} are enabled, {@code false} otherwise.
+     */
+    default boolean isEnabled(Flag flag, Flag... flags) {
+        if (!isEnabled(flag))
+            return false;
+
+        for (var other : flags) {
+            if (!isEnabled(other))
+                return false;
+        }
+
+        return true;
+    }
+
+    /**
+     * Returns {@code true} if all given {@link Flag Flags} are enabled, {@code false} otherwise.
+     *
+     * @param flags {@link Flag Flags} to be tested.
+     * @return {@code true} if all given {@link Flag Flags} are enabled, {@code false} otherwise.
+     */
+    default boolean isEnabled(Collection<Flag> flags) {
+        for (var flag : flags) {
+            if (!isEnabled(flag))
+                return false;
+        }
+
+        return true;
+    }
+
+    /**
+     * Sets the state of the given {@link Flag}.
+     *
+     * @param flag  The {@link Flag} which to have its state changed.
+     * @param state The new state for the given {@link Flag}.
+     * @return {@code true} if the state of the {@link Flag} changed, {@code false} otherwise.
+     */
+    boolean set(Flag flag, boolean state);
+
+    /**
+     * Toggles the state of the given {@link Flag}.
+     *
+     * @param flag {@link Flag} which to have its state toggled.
+     */
+    default void toggle(Flag flag) {
+        set(flag, !isEnabled(flag));
+    }
+
+    /**
+     * Returns an immutable collection containing all currently enabled {@link Flag Flags}.
+     *
+     * @return Immutable collection containing all currently enabled {@link Flag Flags}.
+     */
+    ReferenceSet<Flag> getEnabledFlags();
+
+    /**
+     * Returns {@link Stream} representing all currently enabled {@link Flag Flags}.
+     *
+     * @return {@link Stream} representing all currently enabled {@link Flag Flags}.
+     */
+    default Stream<Flag> enabledFlags() {
+        return getEnabledFlags().stream();
+    }
+
+    /**
+     * Creates a dummy {@link FlagManager} for the provided {@link Set}.
+     *
+     * @param enabledFlags {@link Set} of enabled {@link Flag flags} to be wrapped.
+     * @return Dummy {@link FlagManager} wrapping the provided {@link Set}.
+     */
+    @ApiStatus.Internal
+    static FlagManager createImmutable(ReferenceSet<Flag> enabledFlags) {
+        return new ImmutableFlagManager(enabledFlags);
+    }
+}
diff --git a/src/main/java/net/neoforged/neoforge/flag/FlagsCommand.java b/src/main/java/net/neoforged/neoforge/flag/FlagsCommand.java
new file mode 100644
index 0000000000..2b3eb89ca3
--- /dev/null
+++ b/src/main/java/net/neoforged/neoforge/flag/FlagsCommand.java
@@ -0,0 +1,101 @@
+/*
+ * Copyright (c) NeoForged and contributors
+ * SPDX-License-Identifier: LGPL-2.1-only
+ */
+
+package net.neoforged.neoforge.flag;
+
+import static com.mojang.brigadier.Command.SINGLE_SUCCESS;
+import static net.minecraft.commands.Commands.argument;
+import static net.minecraft.commands.Commands.literal;
+
+import com.google.common.collect.Sets;
+import com.mojang.brigadier.builder.ArgumentBuilder;
+import com.mojang.brigadier.context.CommandContext;
+import com.mojang.brigadier.exceptions.CommandSyntaxException;
+import java.util.Set;
+import java.util.stream.Collectors;
+import net.minecraft.commands.CommandSourceStack;
+import net.minecraft.commands.Commands;
+import net.minecraft.commands.SharedSuggestionProvider;
+import net.minecraft.commands.arguments.ResourceLocationArgument;
+import net.minecraft.network.chat.ClickEvent;
+import net.minecraft.network.chat.Component;
+import net.minecraft.network.chat.HoverEvent;
+import org.jetbrains.annotations.ApiStatus;
+
+/**
+ * Command used for easy {@link Flag} manipulation.
+ * <p>
+ * This command requires permission level: {@link Commands#LEVEL_ADMINS}.
+ *
+ * <ul>
+ * <li>{@code /neoforge flags list} - Lists out all Enabled and Disabled {@link Flag Flags}</li>
+ * <li>{@code /neoforge flags enabled} - Lists out all Enabled {@link Flag Flags}</li>
+ * <li>{@code /neoforge flags disabled} - Lists out all Disabled {@link Flag Flags}</li>
+ * <li>{@code /neoforge flags set <flag> enabled} - Attempts to Enable the given {@link Flag}</li>
+ * <li>{@code /neoforge flags set <flag> disabled} - Attempts to Disable the given {@link Flag}</li>
+ * </ul>
+ * <em>Note:</em> Replace {@code <flag>} with any valid {@link Flag} identifier
+ */
+@ApiStatus.Internal
+public interface FlagsCommand {
+    static ArgumentBuilder<CommandSourceStack, ?> register() {
+        return literal("flags")
+                .requires(src -> src.hasPermission(Commands.LEVEL_ADMINS))
+                .then(literal("list").executes(context -> {
+                    listFlagsFor(context, true);
+                    listFlagsFor(context, false);
+                    return SINGLE_SUCCESS;
+                }))
+                .then(literal("enabled").executes(context -> listFlagsFor(context, true)))
+                .then(literal("disabled").executes(context -> listFlagsFor(context, false)))
+                .then(literal("set")
+                        .then(argument("flag", ResourceLocationArgument.id())
+                                .suggests((context, builder) -> {
+                                    var flags = Flag.flags().map(Flag::toStringShort);
+                                    return SharedSuggestionProvider.suggest(flags, builder);
+                                })
+                                .then(literal("enabled").executes(context -> setFlagState(context, true)))
+                                .then(literal("disabled").executes(context -> setFlagState(context, false)))));
+    }
+
+    private static int setFlagState(CommandContext<CommandSourceStack> context, boolean state) throws CommandSyntaxException {
+        var flag = Flag.of(ResourceLocationArgument.getId(context, "flag"));
+        var src = context.getSource();
+        var flagManager = src.getServer().getModdedFlagManager();
+        var stateKey = state ? "enabled" : "disabled";
+        var changed = flagManager.set(flag, state);
+
+        src.sendSuccess(() -> {
+            if (changed) {
+                return Component.translatable("commands.neoforge.flags." + stateKey, flag.toStringShort()).withStyle(style -> {
+                    var notice = Component.translatable("commands.neoforge.flags.notice");
+                    var hoverEvent = new HoverEvent(HoverEvent.Action.SHOW_TEXT, notice);
+                    var clickEvent = new ClickEvent(ClickEvent.Action.SUGGEST_COMMAND, "/reload");
+                    return style.withHoverEvent(hoverEvent).withClickEvent(clickEvent);
+                });
+            }
+
+            return Component.translatable("commands.neoforge.flags.already_" + stateKey, flag.toStringShort());
+        }, false);
+
+        return SINGLE_SUCCESS;
+    }
+
+    private static int listFlagsFor(CommandContext<CommandSourceStack> context, boolean state) {
+        var src = context.getSource();
+        var stateStr = state ? "enabled" : "disabled";
+        var flagManager = src.getServer().getModdedFlagManager();
+        Set<Flag> flags = flagManager.getEnabledFlags();
+
+        if (!state) {
+            var knownFlags = Flag.getFlags();
+            flags = Sets.difference(knownFlags, flags);
+        }
+
+        var flagsStr = flags.stream().map(Flag::toStringShort).collect(Collectors.joining(", ", "[", "]"));
+        src.sendSuccess(() -> Component.translatable("commands.neoforge.flags.list_" + stateStr, flagsStr), false);
+        return SINGLE_SUCCESS;
+    }
+}
diff --git a/src/main/java/net/neoforged/neoforge/flag/ImmutableFlagManager.java b/src/main/java/net/neoforged/neoforge/flag/ImmutableFlagManager.java
new file mode 100644
index 0000000000..3cf1569459
--- /dev/null
+++ b/src/main/java/net/neoforged/neoforge/flag/ImmutableFlagManager.java
@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) NeoForged and contributors
+ * SPDX-License-Identifier: LGPL-2.1-only
+ */
+
+package net.neoforged.neoforge.flag;
+
+import it.unimi.dsi.fastutil.objects.ReferenceSet;
+import it.unimi.dsi.fastutil.objects.ReferenceSets;
+
+final class ImmutableFlagManager implements FlagManager {
+    private final ReferenceSet<Flag> enabledFlags;
+
+    public ImmutableFlagManager(ReferenceSet<Flag> enabledFlags) {
+        this.enabledFlags = ReferenceSets.unmodifiable(enabledFlags);
+    }
+
+    @Override
+    public boolean set(Flag flag, boolean state) {
+        return false;
+    }
+
+    @Override
+    public ReferenceSet<Flag> getEnabledFlags() {
+        return enabledFlags;
+    }
+}
diff --git a/src/main/java/net/neoforged/neoforge/flag/RequiredFlagsCondition.java b/src/main/java/net/neoforged/neoforge/flag/RequiredFlagsCondition.java
new file mode 100644
index 0000000000..6b7ec5d721
--- /dev/null
+++ b/src/main/java/net/neoforged/neoforge/flag/RequiredFlagsCondition.java
@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) NeoForged and contributors
+ * SPDX-License-Identifier: LGPL-2.1-only
+ */
+
+package net.neoforged.neoforge.flag;
+
+import com.mojang.serialization.MapCodec;
+import com.mojang.serialization.codecs.RecordCodecBuilder;
+import it.unimi.dsi.fastutil.objects.ReferenceOpenHashSet;
+import java.util.Collection;
+import java.util.Set;
+import java.util.stream.Collectors;
+import net.neoforged.neoforge.common.conditions.ICondition;
+import net.neoforged.neoforge.common.util.NeoForgeExtraCodecs;
+
+/**
+ * {@link ICondition Condition} used to conditionally load data files based on state of given set of {@link Flag Flags}.
+ * <p>
+ * All provided {@link #requiredFlags flags} must be enabled in order for data to pass this {@link ICondition condition}.
+ *
+ * @apiNote A {@code /reload} is be required to correctly reload data after toggling the state of a given {@link Flag}.
+ */
+public final class RequiredFlagsCondition implements ICondition {
+    public static final MapCodec<RequiredFlagsCondition> CODEC = RecordCodecBuilder.mapCodec(builder -> builder.group(
+            NeoForgeExtraCodecs.setOf(Flag.CODEC).fieldOf("flags").forGetter(condition -> condition.requiredFlags)).apply(builder, RequiredFlagsCondition::new));
+
+    private final Set<Flag> requiredFlags;
+
+    public RequiredFlagsCondition(Flag... requiredFlags) {
+        this.requiredFlags = new ReferenceOpenHashSet<>(requiredFlags);
+    }
+
+    private RequiredFlagsCondition(Collection<Flag> requiredFlags) {
+        this.requiredFlags = new ReferenceOpenHashSet<>(requiredFlags);
+    }
+
+    @Override
+    public boolean test(IContext context) {
+        return context.getModdedFlagManager().isEnabled(requiredFlags);
+    }
+
+    @Override
+    public MapCodec<RequiredFlagsCondition> codec() {
+        return CODEC;
+    }
+
+    @Override
+    public String toString() {
+        return "required_flags(" + requiredFlags.stream().map(Flag::toStringShort).collect(Collectors.joining(", ", "\"", "\"")) + ')';
+    }
+}
diff --git a/src/main/java/net/neoforged/neoforge/flag/ServerFlagManager.java b/src/main/java/net/neoforged/neoforge/flag/ServerFlagManager.java
new file mode 100644
index 0000000000..2ceb179578
--- /dev/null
+++ b/src/main/java/net/neoforged/neoforge/flag/ServerFlagManager.java
@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) NeoForged and contributors
+ * SPDX-License-Identifier: LGPL-2.1-only
+ */
+
+package net.neoforged.neoforge.flag;
+
+import it.unimi.dsi.fastutil.objects.ReferenceOpenHashSet;
+import it.unimi.dsi.fastutil.objects.ReferenceSet;
+import it.unimi.dsi.fastutil.objects.ReferenceSets;
+import java.util.Set;
+import net.minecraft.server.MinecraftServer;
+import net.minecraft.world.level.WorldDataConfiguration;
+import net.neoforged.neoforge.network.PacketDistributor;
+
+public final class ServerFlagManager implements FlagManager {
+    private final ReferenceSet<Flag> enabledFlags = new ReferenceOpenHashSet<>();
+
+    private final MinecraftServer server;
+
+    public ServerFlagManager(MinecraftServer server, Set<Flag> initialEnabledFlags) {
+        this.server = server;
+        enabledFlags.addAll(initialEnabledFlags);
+    }
+
+    private void flagChanged(Flag flag, boolean state) {
+        // notify clients of state change
+        PacketDistributor.sendToAllPlayers(new ClientboundSyncFlags(enabledFlags));
+
+        var data = server.getWorldData();
+        var config = data.getDataConfiguration();
+
+        // update backing config file
+        // will be saved to disk later during autosave/server shutdown
+        data.setDataConfiguration(new WorldDataConfiguration(config.dataPacks(), config.enabledFeatures(), enabledFlags));
+    }
+
+    @Override
+    public boolean set(Flag flag, boolean state) {
+        var changed = state ? enabledFlags.add(flag) : enabledFlags.remove(flag);
+
+        if (changed)
+            flagChanged(flag, state);
+
+        return changed;
+    }
+
+    @Override
+    public ReferenceSet<Flag> getEnabledFlags() {
+        return ReferenceSets.unmodifiable(enabledFlags);
+    }
+}
diff --git a/src/main/java/net/neoforged/neoforge/flag/package-info.java b/src/main/java/net/neoforged/neoforge/flag/package-info.java
new file mode 100644
index 0000000000..91c6c0d04b
--- /dev/null
+++ b/src/main/java/net/neoforged/neoforge/flag/package-info.java
@@ -0,0 +1,13 @@
+/*
+ * Copyright (c) NeoForged and contributors
+ * SPDX-License-Identifier: LGPL-2.1-only
+ */
+
+@FieldsAreNonnullByDefault
+@MethodsReturnNonnullByDefault
+@ParametersAreNonnullByDefault
+package net.neoforged.neoforge.flag;
+
+import javax.annotation.ParametersAreNonnullByDefault;
+import net.minecraft.FieldsAreNonnullByDefault;
+import net.minecraft.MethodsReturnNonnullByDefault;
diff --git a/src/main/java/net/neoforged/neoforge/network/NetworkInitialization.java b/src/main/java/net/neoforged/neoforge/network/NetworkInitialization.java
index a3e9c32c35..2e38b27739 100644
--- a/src/main/java/net/neoforged/neoforge/network/NetworkInitialization.java
+++ b/src/main/java/net/neoforged/neoforge/network/NetworkInitialization.java
@@ -7,6 +7,7 @@
 
 import net.neoforged.bus.api.SubscribeEvent;
 import net.neoforged.fml.common.EventBusSubscriber;
+import net.neoforged.neoforge.flag.ClientboundSyncFlags;
 import net.neoforged.neoforge.internal.versions.neoforge.NeoForgeVersion;
 import net.neoforged.neoforge.network.configuration.CheckExtensibleEnums;
 import net.neoforged.neoforge.network.event.RegisterPayloadHandlersEvent;
@@ -94,6 +95,10 @@ private static void register(final RegisterPayloadHandlersEvent event) {
                 .playToClient(
                         ClientboundCustomSetTimePayload.TYPE,
                         ClientboundCustomSetTimePayload.STREAM_CODEC,
-                        ClientPayloadHandler::handle);
+                        ClientPayloadHandler::handle)
+                .playToClient(
+                        ClientboundSyncFlags.TYPE,
+                        ClientboundSyncFlags.STREAM_CODEC,
+                        ClientboundSyncFlags::handle);
     }
 }
diff --git a/src/main/java/net/neoforged/neoforge/server/command/NeoForgeCommand.java b/src/main/java/net/neoforged/neoforge/server/command/NeoForgeCommand.java
index 5967982f0c..6e4b112657 100644
--- a/src/main/java/net/neoforged/neoforge/server/command/NeoForgeCommand.java
+++ b/src/main/java/net/neoforged/neoforge/server/command/NeoForgeCommand.java
@@ -8,6 +8,7 @@
 import com.mojang.brigadier.CommandDispatcher;
 import com.mojang.brigadier.builder.LiteralArgumentBuilder;
 import net.minecraft.commands.CommandSourceStack;
+import net.neoforged.neoforge.flag.FlagsCommand;
 import org.jetbrains.annotations.ApiStatus;
 
 @ApiStatus.Internal
@@ -24,6 +25,7 @@ public static void register(CommandDispatcher<CommandSourceStack> dispatcher) {
                         .then(TagsCommand.register())
                         .then(DumpCommand.register())
                         .then(TimeSpeedCommand.register())
-                        .then(DataComponentCommand.register()));
+                        .then(DataComponentCommand.register())
+                        .then(FlagsCommand.register()));
     }
 }
diff --git a/src/main/resources/assets/neoforge/lang/en_us.json b/src/main/resources/assets/neoforge/lang/en_us.json
index 0d78311974..f2b536146d 100644
--- a/src/main/resources/assets/neoforge/lang/en_us.json
+++ b/src/main/resources/assets/neoforge/lang/en_us.json
@@ -127,6 +127,13 @@
   "commands.neoforge.data_components.list.tooltip.deleted": "Component %s with value %s was deleted",
   "commands.neoforge.data_components.list.tooltip.modified": "Component %s was modified from %s to %s",
   "commands.neoforge.data_components.list.tooltip.added": "Component %s was added with value %s",
+  "commands.neoforge.flags.enabled": "Enabled Flag: %s",
+  "commands.neoforge.flags.notice": "Note: A '/reload' may also be required for flagged datapack entries\nClick to run command",
+  "commands.neoforge.flags.already_enabled": "Flag %s is already Enabled",
+  "commands.neoforge.flags.list_enabled": "Enabled Flags: %s",
+  "commands.neoforge.flags.disabled": "Disabled Flag: %s",
+  "commands.neoforge.flags.already_disabled": "Flag %s is already Disabled",
+  "commands.neoforge.flags.list_disabled": "Disabled Flags: %s",
 
   "commands.config.getwithtype": "Config for %s of type %s found at %s",
   "commands.config.noconfig": "Config for %s of type %s not found",
diff --git a/testframework/src/main/java/net/neoforged/testframework/junit/EphemeralTestServerProvider.java b/testframework/src/main/java/net/neoforged/testframework/junit/EphemeralTestServerProvider.java
index ca738226a1..cfd046b7f1 100644
--- a/testframework/src/main/java/net/neoforged/testframework/junit/EphemeralTestServerProvider.java
+++ b/testframework/src/main/java/net/neoforged/testframework/junit/EphemeralTestServerProvider.java
@@ -148,7 +148,7 @@ public static JUnitServer create(
                 Thread thread, Path tempDir, LevelStorageSource.LevelStorageAccess access, PackRepository resources) {
             resources.reload();
             WorldDataConfiguration config = new WorldDataConfiguration(
-                    new DataPackConfig(new ArrayList<>(resources.getAvailableIds()), List.of()), FeatureFlags.REGISTRY.allFlags());
+                    new DataPackConfig(new ArrayList<>(resources.getAvailableIds()), List.of()), FeatureFlags.REGISTRY.allFlags(), net.neoforged.neoforge.flag.Flag.getFlags());
             LevelSettings levelsettings = new LevelSettings(
                     "Test Level", GameType.CREATIVE, false, Difficulty.NORMAL, true, TEST_GAME_RULES, config);
             WorldLoader.PackConfig worldloader$packconfig = new WorldLoader.PackConfig(resources, config, false, true);
diff --git a/tests/src/main/java/net/neoforged/neoforge/debug/FlagTests.java b/tests/src/main/java/net/neoforged/neoforge/debug/FlagTests.java
new file mode 100644
index 0000000000..67853b33d9
--- /dev/null
+++ b/tests/src/main/java/net/neoforged/neoforge/debug/FlagTests.java
@@ -0,0 +1,75 @@
+/*
+ * Copyright (c) NeoForged and contributors
+ * SPDX-License-Identifier: LGPL-2.1-only
+ */
+
+package net.neoforged.neoforge.debug;
+
+import net.minecraft.client.renderer.entity.NoopRenderer;
+import net.minecraft.world.entity.EntityType;
+import net.minecraft.world.entity.Mob;
+import net.minecraft.world.entity.MobCategory;
+import net.minecraft.world.item.Item;
+import net.minecraft.world.level.Level;
+import net.minecraft.world.level.block.Blocks;
+import net.minecraft.world.level.block.state.BlockBehaviour;
+import net.neoforged.neoforge.common.DeferredSpawnEggItem;
+import net.neoforged.neoforge.flag.Flag;
+import net.neoforged.testframework.DynamicTest;
+import net.neoforged.testframework.annotation.ForEachTest;
+import net.neoforged.testframework.annotation.TestHolder;
+
+@ForEachTest(groups = "modded_feature_flags")
+public interface FlagTests {
+    @TestHolder(description = "Tests modded feature flags")
+    static void test(DynamicTest test) {
+        var namespace = test.createModId();
+        var registration = test.registrationHelper();
+        var items = registration.items();
+
+        // register various elements which require our flag
+        var testFlag = Flag.of(namespace, "test_flag");
+
+        items.registerSimpleItem("flagged_item", new Item.Properties().requiredFlags(testFlag));
+
+        // block disabled via matching block item
+        var flaggedBlock = registration.blocks().registerSimpleBlock("flagged_block", BlockBehaviour.Properties.ofFullCopy(Blocks.STONE).requiredFlags(testFlag));
+        items.registerSimpleBlockItem(flaggedBlock);
+
+        // spawn egg disabled via matching entity type
+        var flaggedEntity = registration.entityTypes().registerType("flagged_entity", () -> EntityType.Builder
+                .of(DummyEntity::new, MobCategory.MISC)
+                .requiredFlags(testFlag)).withRenderer(() -> NoopRenderer::new).withAttributes(Mob::createMobAttributes);
+
+        items.registerItem("flagged_entity_egg", properties -> new DeferredSpawnEggItem(flaggedEntity, 0, 0, properties));
+
+        // generate recipe which requires our flag
+        // TODO: Figure out why this causes duplicate provider exceptions
+        // Seems to clash with recipe provider in DataGeneratorTest -> gen.addProvider(event.includeServer(), new Recipes(packOutput, lookupProvider));
+        // IngredientTests does the same thing using addProvider which works fine, but here causes dupe providers, why?!?
+        // Things I have tried
+        // - Renaming method to change mod id
+        // - Adding @GameTest and @EmptyTemplate to match IngredientTests
+        // - Making groups in @ForEachTests dotted
+        // - Passing RegistrationHelper as parameter
+        // - Creating new RegistrationHelper staticly (and registering with @OnInit) and using that rather than one from DynamicTest
+        // - Passing new mod id to test.registrationHelper()
+        // - Changing value in @TestHolder for new mod id
+        // - Setting idPrefix in @ForEachTest
+        /*registration.addProvider(event -> event.getGenerator().addProvider(event.includeServer(), new RecipeProvider(event.getGenerator().getPackOutput(), event.getLookupProvider()) {
+            @Override
+            protected void buildRecipes(RecipeOutput output, HolderLookup.Provider provider) {
+                ShapelessRecipeBuilder.shapeless(RecipeCategory.MISC, Items.DIAMOND, 64)
+                        .requires(ItemTags.DIRT)
+                        .unlockedBy("has_dirt", has(ItemTags.DIRT))
+                        .save(output.withConditions(new RequiredFlagsCondition(testFlag)), ResourceLocation.fromNamespaceAndPath(namespace, "flagged/diamonds_from_dirt"));
+            }
+        }));*/
+    }
+
+    final class DummyEntity extends Mob {
+        DummyEntity(EntityType<? extends Mob> entityType, Level level) {
+            super(entityType, level);
+        }
+    }
+}
diff --git a/tests/src/main/resources/data/neotests_flags/advancement/recipes/misc/flagged/diamonds_from_dirt.json b/tests/src/main/resources/data/neotests_flags/advancement/recipes/misc/flagged/diamonds_from_dirt.json
new file mode 100644
index 0000000000..88c7c37f2c
--- /dev/null
+++ b/tests/src/main/resources/data/neotests_flags/advancement/recipes/misc/flagged/diamonds_from_dirt.json
@@ -0,0 +1,40 @@
+{
+  "neoforge:conditions": [
+    {
+      "type": "neoforge:required_flags",
+      "flags": [
+        "neotests_test:test_flag"
+      ]
+    }
+  ],
+  "parent": "minecraft:recipes/root",
+  "criteria": {
+    "has_dirt": {
+      "conditions": {
+        "items": [
+          {
+            "items": "#minecraft:dirt"
+          }
+        ]
+      },
+      "trigger": "minecraft:inventory_changed"
+    },
+    "has_the_recipe": {
+      "conditions": {
+        "recipe": "neotests_flags:flagged/diamonds_from_dirt"
+      },
+      "trigger": "minecraft:recipe_unlocked"
+    }
+  },
+  "requirements": [
+    [
+      "has_the_recipe",
+      "has_dirt"
+    ]
+  ],
+  "rewards": {
+    "recipes": [
+      "neotests_flags:flagged/diamonds_from_dirt"
+    ]
+  }
+}
\ No newline at end of file
diff --git a/tests/src/main/resources/data/neotests_flags/recipe/flagged/diamonds_from_dirt.json b/tests/src/main/resources/data/neotests_flags/recipe/flagged/diamonds_from_dirt.json
new file mode 100644
index 0000000000..fe7dfca228
--- /dev/null
+++ b/tests/src/main/resources/data/neotests_flags/recipe/flagged/diamonds_from_dirt.json
@@ -0,0 +1,21 @@
+{
+  "neoforge:conditions": [
+    {
+      "type": "neoforge:required_flags",
+      "flags": [
+        "neotests_test:test_flag"
+      ]
+    }
+  ],
+  "type": "minecraft:crafting_shapeless",
+  "category": "misc",
+  "ingredients": [
+    {
+      "tag": "minecraft:dirt"
+    }
+  ],
+  "result": {
+    "count": 64,
+    "id": "minecraft:diamond"
+  }
+}
\ No newline at end of file