Fix validation for item and fluid tags

src/main/java/com/gregtechceu/gtceu/api/recipe/ingredient/FluidIngredient.java

@@ -1,5 +1,8 @@
 package com.gregtechceu.gtceu.api.recipe.ingredient;
 
+import com.gregtechceu.gtceu.GTCEu;
+import com.gregtechceu.gtceu.integration.kjs.recipe.KJSHelpers;
+
 import net.minecraft.core.Holder;
 import net.minecraft.core.registries.BuiltInRegistries;
 import net.minecraft.core.registries.Registries;
@@ -327,6 +330,10 @@ public record TagValue(TagKey<Fluid> tag) implements Value {
 
         @Override
         public Collection<Fluid> getFluids() {
+            if (GTCEu.Mods.isKubeJSLoaded()) {
+                var resolved = KJSHelpers.getFluidsDuringLoad(this.tag);
+                if (resolved != null) return resolved;
+            }
             ArrayList<Fluid> list = Lists.newArrayList();
             for (Holder<Fluid> holder : BuiltInRegistries.FLUID.getTagOrEmpty(this.tag)) {
                 list.add(holder.value());

src/main/java/com/gregtechceu/gtceu/integration/kjs/recipe/GTRecipeSchema.java

@@ -24,6 +24,8 @@
 import com.gregtechceu.gtceu.common.item.behavior.IntCircuitBehaviour;
 import com.gregtechceu.gtceu.common.recipe.condition.*;
 import com.gregtechceu.gtceu.config.ConfigHolder;
+import com.gregtechceu.gtceu.core.mixins.IngredientAccessor;
+import com.gregtechceu.gtceu.core.mixins.TagValueAccessor;
 import com.gregtechceu.gtceu.data.recipe.builder.GTRecipeBuilder;
 import com.gregtechceu.gtceu.integration.kjs.recipe.components.CapabilityMap;
 import com.gregtechceu.gtceu.integration.kjs.recipe.components.ExtendedOutputItem;
@@ -792,6 +794,15 @@ private void validateItems(@NotNull String type, InputItem... items) {
                 if (stack == null || stack.isEmpty()) {
                     throw new RecipeExceptionJS(String.format("Invalid or empty %s item (recipe ID: %s)", type, id));
                 }
+                if (stack.ingredient.getItems().length == 0) {
+                    String tagInfo = "";
+                    var values = ((IngredientAccessor) stack.ingredient).getValues();
+                    if (values.length == 1 && values[0] instanceof Ingredient.TagValue tagValue) {
+                        tagInfo = " (empty or unknown tag: #" + ((TagValueAccessor) tagValue).getTag().location() + ")";
+                    }
+                    throw new RecipeExceptionJS(
+                            String.format("Invalid or empty %s item (recipe ID: %s)%s", type, id, tagInfo));
+                }
             }
         }
 
@@ -839,6 +850,15 @@ private void validateFluids(@NotNull String type, GTRecipeComponents.FluidIngred
                                 String.format("Invalid or empty %s fluid (recipe ID: %s)", type, id));
                     }
                 }
+                if (fluid.ingredient().getStacks().length == 0) {
+                    String tagInfo = "";
+                    var values = fluid.ingredient().values;
+                    if (values.length == 1 && values[0] instanceof FluidIngredient.TagValue tagValue) {
+                        tagInfo = " (empty or unknown tag: #" + tagValue.tag().location() + ")";
+                    }
+                    throw new RecipeExceptionJS(String.format(
+                            "Invalid or empty %s fluid (recipe ID: %s)%s", type, id, tagInfo));
+                }
             }
         }
 

src/main/java/com/gregtechceu/gtceu/integration/kjs/recipe/KJSHelpers.java

@@ -3,12 +3,20 @@
 import com.gregtechceu.gtceu.api.capability.recipe.IO;
 import com.gregtechceu.gtceu.api.recipe.ingredient.EnergyStack;
 
+import net.minecraft.core.Holder;
+import net.minecraft.tags.TagKey;
+import net.minecraft.world.level.material.Fluid;
+
+import com.google.common.collect.Lists;
+import dev.latvian.mods.kubejs.item.ingredient.TagContext;
+import dev.latvian.mods.kubejs.recipe.RecipesEventJS;
 import dev.latvian.mods.kubejs.util.MapJS;
 import dev.latvian.mods.kubejs.util.UtilsJS;
 import dev.latvian.mods.rhino.Wrapper;
 import it.unimi.dsi.fastutil.longs.LongLongPair;
 import org.jetbrains.annotations.Nullable;
 
+import java.util.ArrayList;
 import java.util.Map;
 import java.util.Set;
 import java.util.regex.Matcher;
@@ -104,4 +112,21 @@ public static EnergyStack parseEnergyStack(Object o) {
         }
         return LongLongPair.of(voltage, amperage);
     }
+
+    /**
+     * Resolves a fluid tag using KubeJS's TagContext during recipe (re)loading, since the server registry
+     * hasn't bound the tags yet. Returns null if a recipe event is not currently active.
+     * Analogous to KubeJS' TagContext::patchIngredientTags for items.
+     */
+    @Nullable
+    public static ArrayList<Fluid> getFluidsDuringLoad(TagKey<Fluid> tag) {
+        if (RecipesEventJS.instance == null) return null;
+
+        var holders = TagContext.INSTANCE.getValue().getTag(tag);
+        ArrayList<Fluid> list = Lists.newArrayList();
+        for (Holder<Fluid> holder : holders) {
+            list.add(holder.value());
+        }
+        return list;
+    }
 }