From bd42bdcaebab1ed0ef39ddf3a9a82f457f936b17 Mon Sep 17 00:00:00 2001
From: olcxja <olcxja@olcxja.ovh>
Date: Sat, 24 Oct 2026 18:36:56 +0200
Subject: [PATCH] Fix unknown block states in registry data for pre-26.3
 clients

---
 .../rewriters/BackwardsRegistryRewriter.java  | 23 +++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/common/src/main/java/com/viaversion/viabackwards/api/rewriters/BackwardsRegistryRewriter.java b/common/src/main/java/com/viaversion/viabackwards/api/rewriters/BackwardsRegistryRewriter.java
index faf45cd1e..0e1b0a8a9 100644
--- a/common/src/main/java/com/viaversion/viabackwards/api/rewriters/BackwardsRegistryRewriter.java
+++ b/common/src/main/java/com/viaversion/viabackwards/api/rewriters/BackwardsRegistryRewriter.java
@@ -21,6 +21,7 @@
 import com.viaversion.nbt.tag.Tag;
 import com.viaversion.viabackwards.api.BackwardsProtocol;
 import com.viaversion.viaversion.api.connection.UserConnection;
+import com.viaversion.viaversion.api.data.FullMappings;
 import com.viaversion.viaversion.api.minecraft.RegistryEntry;
 import com.viaversion.viaversion.rewriter.RegistryDataRewriter;
 import com.viaversion.viaversion.util.Key;
@@ -69,6 +70,28 @@ public void updateJukeboxSongs(final UserConnection connection, final RegistryEn
         }
     }
 
+    @Override
+    protected boolean updateBlockState(final Tag blockStateTag) {
+        if (!(blockStateTag instanceof CompoundTag blockState) || blockState.getStringTag("Name") == null) {
+            return super.updateBlockState(blockStateTag);
+        }
+
+        // Up to 26.2, block states use "Name" and "Properties" instead of "id" and "properties"
+        final FullMappings blockMappings = protocol.getMappingData().getFullBlockMappings();
+        if (blockMappings == null) {
+            return false;
+        }
+
+        final int blockId = blockMappings.id(blockState.getString("Name"));
+        if (blockId == -1 || protocol.getMappingData().hasBlockChanged(blockId)) {
+            // Replace unknown or changed blocks with a dummy state, else the client will fail to load the registry
+            blockState.putString("Name", "minecraft:dirt");
+            blockState.remove("Properties");
+            return true;
+        }
+        return false;
+    }
+
     private void updateBiomeEffects(final CompoundTag effects) {
         updateSound(effects.getCompoundTag("mood_sound"), "sound");
         updateSound(effects.getCompoundTag("additions_sound"), "sound");