Items can use more than name to pass their original id to client for resource packs

common/src/main/java/com/viaversion/viabackwards/ViaBackwardsConfig.java

@@ -47,6 +47,7 @@ public class ViaBackwardsConfig extends Config implements com.viaversion.viaback
     private boolean dialogsViaChests;
     private DialogStyleConfig dialogStyleConfig;
     private boolean codeOfConductAsDialog;
+    private boolean passOriginalItemNameToResourcePacks;
 
     public ViaBackwardsConfig(File configFile, Logger logger) {
         super(configFile, logger);
@@ -75,6 +76,7 @@ private void loadFields() {
         dialogsViaChests = getBoolean("dialogs-via-chests", true);
         dialogStyleConfig = loadDialogStyleConfig(getSection("dialog-style"));
         codeOfConductAsDialog = getBoolean("code-of-conduct-as-dialog", true);
+        passOriginalItemNameToResourcePacks = getBoolean("pass-original-item-name-to-resource-packs", true);
     }
 
     private DialogStyleConfig loadDialogStyleConfig(final ConfigSection section) {
@@ -179,6 +181,11 @@ public boolean codeOfConductAsDialog() {
         return codeOfConductAsDialog;
     }
 
+    @Override
+    public boolean passOriginalItemNameToResourcePacks() {
+        return passOriginalItemNameToResourcePacks;
+    }
+
     @Override
     public URL getDefaultConfigURL() {
         return getClass().getClassLoader().getResource("assets/viabackwards/config.yml");

common/src/main/java/com/viaversion/viabackwards/api/ViaBackwardsConfig.java

@@ -133,4 +133,13 @@ public interface ViaBackwardsConfig extends Config {
      * @return true if enabled
      */
     boolean codeOfConductAsDialog();
+
+    /**
+     * Injects the original vanilla 1.21.4+ item name into custom_model_data strings for resource packs.
+     * Disable if your server creates custom items using modern items as their base.
+     * Tip: For server custom items, base them on items in the game before 1.21.4 (e.g. saddle) to ensure compatibility.
+     *
+     * @return true if enabled
+     */
+    boolean passOriginalItemNameToResourcePacks();
 }

common/src/main/java/com/viaversion/viabackwards/api/rewriters/BackwardsStructuredItemRewriter.java

@@ -24,6 +24,7 @@
 import com.viaversion.nbt.tag.ListTag;
 import com.viaversion.nbt.tag.StringTag;
 import com.viaversion.nbt.tag.Tag;
+import com.viaversion.viabackwards.ViaBackwards;
 import com.viaversion.viabackwards.api.BackwardsProtocol;
 import com.viaversion.viabackwards.api.data.BackwardsMappingData;
 import com.viaversion.viabackwards.api.data.MappedItem;
@@ -40,6 +41,7 @@
 import com.viaversion.viaversion.api.protocol.packet.ServerboundPacketType;
 import com.viaversion.viaversion.api.protocol.version.ProtocolVersion;
 import com.viaversion.viaversion.rewriter.StructuredItemRewriter;
+import com.viaversion.viaversion.util.ArrayUtil;
 import com.viaversion.viaversion.util.Key;
 import java.util.ArrayList;
 import java.util.List;
@@ -51,9 +53,13 @@ public class BackwardsStructuredItemRewriter<C extends ClientboundPacketType, S
     T extends BackwardsProtocol<C, ?, ?, S>> extends StructuredItemRewriter<C, S, T> {
 
     private static final int[] EMPTY_INT_ARRAY = new int[0];
+    private static final String GLOBAL_MODEL_DATA_MARKER = "VB|injected_cmd";
+
+    private final String nbtTagName;
 
     public BackwardsStructuredItemRewriter(T protocol) {
         super(protocol);
+        this.nbtTagName = "VB|" + protocol.getClass().getSimpleName();
     }
 
     @Override
@@ -70,17 +76,11 @@ protected void backupInconvertibleData(final UserConnection connection, final It
         customTag.putInt(nbtTagName("id"), item.identifier()); // Save original id
 
         // Add custom model data
-        if (mappedItem.customModelData() != null) {
+        final boolean addOriginalIdentifier = ViaBackwards.getConfig().passOriginalItemNameToResourcePacks();
+        if (mappedItem.customModelData() != null || addOriginalIdentifier) {
             if (connection.getProtocolInfo().protocolVersion().newerThanOrEqualTo(ProtocolVersion.v1_21_4)) {
-                if (!dataContainer.has(StructuredDataKey.CUSTOM_MODEL_DATA1_21_4)) {
-                    dataContainer.set(StructuredDataKey.CUSTOM_MODEL_DATA1_21_4, new CustomModelData1_21_4(
-                        new float[]{mappedItem.customModelData().floatValue()},
-                        new boolean[0],
-                        new String[0],
-                        EMPTY_INT_ARRAY
-                    ));
-                }
-            } else if (!dataContainer.has(StructuredDataKey.CUSTOM_MODEL_DATA1_20_5)) {
+                addCustomModelData(item, addOriginalIdentifier, mappedItem, customTag);
+            } else if (mappedItem.customModelData() != null && !dataContainer.has(StructuredDataKey.CUSTOM_MODEL_DATA1_20_5)) {
                 dataContainer.set(StructuredDataKey.CUSTOM_MODEL_DATA1_20_5, mappedItem.customModelData());
             }
         }
@@ -92,13 +92,59 @@ protected void backupInconvertibleData(final UserConnection connection, final It
         }
     }
 
+    private void addCustomModelData(final Item item, final boolean addOriginalIdentifier, final MappedItem mappedItem, final CompoundTag customTag) {
+        final StructuredDataContainer dataContainer = item.dataContainer();
+        CustomModelData1_21_4 customModelData = dataContainer.get(StructuredDataKey.CUSTOM_MODEL_DATA1_21_4);
+        if (customModelData == null) {
+            final String[] strings = addOriginalIdentifier
+                ? new String[]{protocol.getMappingData().getFullItemMappings().identifier(item.identifier())}
+                : new String[0];
+            customModelData = new CustomModelData1_21_4(
+                mappedItem.customModelData() != null ? new float[]{mappedItem.customModelData().floatValue()} : new float[0],
+                new boolean[0],
+                strings,
+                EMPTY_INT_ARRAY
+            );
+            dataContainer.set(StructuredDataKey.CUSTOM_MODEL_DATA1_21_4, customModelData);
+            // Add one global marker and one for this specific version, so it is removed at the correct protocol
+            customTag.putBoolean(GLOBAL_MODEL_DATA_MARKER, true);
+            customTag.putBoolean(nbtTagName("added_custom_model_data"), true);
+        } else if (addOriginalIdentifier && !customTag.contains(GLOBAL_MODEL_DATA_MARKER)) {
+            final String identifier = protocol.getMappingData().getFullItemMappings().identifier(item.identifier());
+            dataContainer.set(StructuredDataKey.CUSTOM_MODEL_DATA1_21_4, new CustomModelData1_21_4(
+                customModelData.floats(), customModelData.booleans(), ArrayUtil.add(customModelData.strings(), identifier), customModelData.colors()
+            ));
+            customTag.putBoolean(GLOBAL_MODEL_DATA_MARKER, true);
+            customTag.putString(nbtTagName("injected_custom_model_data"), identifier);
+        }
+    }
+
     @Override
     protected void restoreBackupData(final Item item, final StructuredDataContainer container, final CompoundTag customData) {
         super.restoreBackupData(item, container, customData);
         if (removeBackupTag(customData, "id") instanceof final IntTag originalTag) {
             item.setIdentifier(originalTag.asInt());
             removeCustomTag(container, customData);
         }
+
+        if (removeBackupTag(customData, "injected_custom_model_data") instanceof StringTag injectedCustomModelData) {
+            customData.remove(GLOBAL_MODEL_DATA_MARKER);
+            container.replace(StructuredDataKey.CUSTOM_MODEL_DATA1_21_4, customModelData -> {
+                final String target = injectedCustomModelData.getValue();
+                final String[] strings = customModelData.strings();
+                for (int i = 0; i < strings.length; i++) {
+                    if (strings[i].equals(target)) {
+                        // Remove the injected string
+                        final String[] filteredStrings = ArrayUtil.remove(strings, i);
+                        return new CustomModelData1_21_4(customModelData.floats(), customModelData.booleans(), filteredStrings, customModelData.colors());
+                    }
+                }
+                return customModelData;
+            });
+        } else if (removeBackupTag(customData, "added_custom_model_data") != null) {
+            customData.remove(GLOBAL_MODEL_DATA_MARKER);
+            container.remove(StructuredDataKey.CUSTOM_MODEL_DATA1_21_4);
+        }
     }
 
     protected void saveListTag(CompoundTag tag, ListTag<?> original, String name) {
@@ -313,6 +359,6 @@ protected Holder<SoundEvent> restoreSoundEventHolder(final CompoundTag tag, fina
 
     @Override
     public String nbtTagName() {
-        return "VB|" + protocol.getClass().getSimpleName();
+        return this.nbtTagName;
     }
 }

common/src/main/resources/assets/viabackwards/config.yml

@@ -68,3 +68,9 @@ dialog-style:
 # Note that this is not supported for clients below 1.21.6 right now due to missing support for
 # dialogs during the configuration phase.
 code-of-conduct-as-dialog: true
+#
+# Passes the original vanilla 1.21.4+ item name into custom_model_data strings.
+# This allows client resource packs to restore the appearance of newer items entirely missing from this older version.
+# Disable if your server creates custom items using modern items as their base.
+# Tip: For server custom items, base them on items in the game before 1.21.4 (e.g. saddle) to ensure compatibility.
+pass-original-item-name-to-resource-packs: true