--- a/net/minecraft/world/item/component/CustomData.java
+++ b/net/minecraft/world/item/component/CustomData.java
@@ -33,7 +_,17 @@
     private static final Logger LOGGER = LogUtils.getLogger();
     public static final CustomData EMPTY = new CustomData(new CompoundTag());
     private static final String TYPE_TAG = "id";
-    public static final Codec<CustomData> CODEC = Codec.withAlternative(CompoundTag.CODEC, TagParser.FLATTENED_CODEC)
+    // Paper start - Item serialization as json
+    public static ThreadLocal<Boolean> SERIALIZE_CUSTOM_AS_SNBT = ThreadLocal.withInitial(() -> false);
+    public static final Codec<CustomData> CODEC = Codec.either(CompoundTag.CODEC, TagParser.FLATTENED_CODEC)
+        .xmap(com.mojang.datafixers.util.Either::unwrap, data -> { // Both will be used for deserialization, but we decide which one to use for serialization
+            if (!SERIALIZE_CUSTOM_AS_SNBT.get()) {
+                return com.mojang.datafixers.util.Either.left(data); // First codec
+            } else {
+                return com.mojang.datafixers.util.Either.right(data); // Second codec
+            }
+        })
+    // Paper end - Item serialization as json
         .xmap(CustomData::new, customData -> customData.tag);
     public static final Codec<CustomData> CODEC_WITH_ID = CODEC.validate(
         data -> data.getUnsafe().getString("id").isPresent() ? DataResult.success(data) : DataResult.error(() -> "Missing id for entity in: " + data)
