Update Forge and change the max UTF value.

common/src/main/java/dev/tonimatas/packetfixer/mixins/FriendlyByteBufMixin.java

@@ -12,4 +12,19 @@ public abstract class FriendlyByteBufMixin {
     public long newSize(long constant) {
         return Config.getNbtMaxSize();
     }
+
+    @ModifyConstant(method = "readUtf()Ljava/lang/String;", constant = @Constant(intValue = 32767))
+    private int readUtf$newSize(int constant) {
+        return Config.getUtf8Size();
+    }
+
+    @ModifyConstant(method = "writeUtf(Ljava/lang/String;)Lnet/minecraft/network/FriendlyByteBuf;", constant = @Constant(intValue = 32767))
+    private int writeUtf$newSize(int constant) {
+        return Config.getUtf8Size(); // It should work. The error is correct, with the Packet Fixer values. You can change the max in the config.
+    }
+
+    @ModifyConstant(method = "readResourceLocation", constant = @Constant(intValue = 32767))
+    private int readResourceLocation$newSize(int constant) {
+        return Config.getUtf8Size();
+    }
 }

common/src/main/java/dev/tonimatas/packetfixer/util/Config.java

@@ -27,6 +27,7 @@ public static void runProperties() {
                 properties.setProperty("packetSize", Integer.toString(1048576 * 100));
                 properties.setProperty("decoderSize", Integer.toString(8388608 * 100));
                 properties.setProperty("varInt21", Integer.toString(8));
+                properties.setProperty("utfSize", Integer.toString(32767));
 
                 save(propertiesFile);
             }
@@ -37,6 +38,7 @@ public static void runProperties() {
             checkVariable("packetSize", Integer.toString(1048576 * 100));
             checkVariable("decoderSize", Integer.toString(8388608 * 100));
             checkVariable("varInt21", Integer.toString(8));
+            checkVariable("utfSize", Integer.toString(32767 * 100));
             save(propertiesFile);
         } catch (IOException e) {
             throw new RuntimeException(e);
@@ -63,6 +65,11 @@ public static int getVarInt21Size() {
         return Integer.parseInt(properties.getProperty("varInt21"));
     }
 
+    public static int getUtf8Size() {
+        if (properties == null) runProperties();
+        return Integer.parseInt(properties.getProperty("utfSize"));
+    }
+
     private static void checkVariable(String variable, String defaultValue) {
         if (properties.getProperty(variable) == null) {
             properties.setProperty(variable, defaultValue);

gradle.properties

@@ -9,7 +9,7 @@ minecraftVersion=1.20.1
 minecraftVersionRange=1.19,1.20.1
 
 # Forge Properties
-forgeVersion=47.2.20
+forgeVersion=47.3.12
 
 # Fabric Properties
 fabricLoaderVersion=0.15.6