Merged forced datapack tags

- Staged packs that write the same data/*/tags file now merge into the union of their values
- Tag entries dedupe by value or id and replace stays false unless a contributor set it
- Every non-tag file keeps strict conflict detection

adapters/modded-common/src/main/java/art/arcane/iris/modded/ModdedDatapackTagMerge.java

@@ -0,0 +1,83 @@
+/*
+ * Iris is a World Generator for Minecraft Servers
+ * Copyright (c) 2026 Arcane Arts (Volmit Software)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package art.arcane.iris.modded;
+
+import art.arcane.volmlib.util.json.JSONArray;
+import art.arcane.volmlib.util.json.JSONObject;
+
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.Map;
+import java.util.TreeMap;
+
+final class ModdedDatapackTagMerge {
+    private static final String REPLACE = "replace";
+    private static final String VALUES = "values";
+
+    private ModdedDatapackTagMerge() {
+    }
+
+    static boolean isTag(Path relative) {
+        return relative.getNameCount() >= 4
+                && "data".equals(relative.getName(0).toString())
+                && "tags".equals(relative.getName(2).toString())
+                && relative.getFileName().toString().endsWith(".json");
+    }
+
+    static void merge(Path source, Path destination) throws IOException {
+        JSONObject incoming = read(source);
+        JSONObject existing = read(destination);
+        Map<String, Object> values = new TreeMap<>();
+        collect(existing, values, destination);
+        collect(incoming, values, source);
+        JSONArray merged = new JSONArray();
+        for (Object value : values.values()) {
+            merged.put(value);
+        }
+        JSONObject tag = new JSONObject();
+        tag.put(REPLACE, existing.optBoolean(REPLACE, false) || incoming.optBoolean(REPLACE, false));
+        tag.put(VALUES, merged);
+        Files.writeString(destination, tag.toString(4), StandardCharsets.UTF_8);
+    }
+
+    private static JSONObject read(Path tag) throws IOException {
+        try {
+            return new JSONObject(Files.readString(tag, StandardCharsets.UTF_8));
+        } catch (RuntimeException invalid) {
+            throw new IOException("Iris cannot merge the invalid datapack tag " + tag + ".", invalid);
+        }
+    }
+
+    private static void collect(JSONObject tag, Map<String, Object> values, Path origin) throws IOException {
+        JSONArray entries = tag.optJSONArray(VALUES);
+        if (entries == null) {
+            return;
+        }
+        for (int index = 0; index < entries.length(); index++) {
+            Object entry = entries.get(index);
+            String key = entry instanceof JSONObject object ? object.optString("id", "") : entry.toString();
+            if (key.isBlank()) {
+                throw new IOException("Iris cannot merge datapack tag " + origin + ": entry " + index + " has no id.");
+            }
+            values.putIfAbsent(key, entry);
+        }
+    }
+}

adapters/modded-common/src/main/java/art/arcane/iris/modded/ModdedForcedDatapack.java

@@ -464,6 +464,8 @@ public final class ModdedForcedDatapack {
                 Files.createDirectories(destination);
             } else if (!Files.exists(destination)) {
                 Files.copy(source, destination);
+            } else if (Files.isRegularFile(destination) && ModdedDatapackTagMerge.isTag(relative)) {
+                ModdedDatapackTagMerge.merge(source, destination);
             } else if (!Files.isRegularFile(destination) || Files.mismatch(source, destination) != -1L) {
                 throw new IOException("Conflicting Iris datapack output at " + destination + ".");
             }

adapters/modded-common/src/test/java/art/arcane/iris/modded/ModdedForcedDatapackTest.java

@@ -260,6 +260,61 @@ public class ModdedForcedDatapackTest {
         }
     }
 
+    @Test
+    public void mergesTagFilesFromDifferentPacksIntoTheUnionOfTheirValues() throws IOException {
+        Path root = Files.createTempDirectory("iris-forced-pack-tag-merge");
+        try {
+            Path tag = Path.of("data/minecraft/tags/worldgen/biome/water_on_map_outlines.json");
+            Path source = root.resolve("source");
+            Path destination = root.resolve("destination");
+            Files.createDirectories(source.resolve(tag).getParent());
+            Files.createDirectories(destination.resolve(tag).getParent());
+            Files.writeString(source.resolve(tag),
+                    "{\"replace\":false,\"values\":[\"iris:biomes/b\",\"iris:biomes/c\",{\"id\":\"mod:river\",\"required\":false}]}",
+                    StandardCharsets.UTF_8);
+            Files.writeString(destination.resolve(tag),
+                    "{\"replace\":false,\"values\":[\"iris:biomes/a\",\"iris:biomes/b\"]}",
+                    StandardCharsets.UTF_8);
+
+            ModdedForcedDatapack.mergeDirectory(source, destination);
+
+            JSONObject merged = new JSONObject(Files.readString(destination.resolve(tag), StandardCharsets.UTF_8));
+            assertFalse(merged.getBoolean("replace"));
+            List<String> values = new ArrayList<>();
+            for (int index = 0; index < merged.getJSONArray("values").length(); index++) {
+                Object value = merged.getJSONArray("values").get(index);
+                values.add(value instanceof JSONObject entry ? entry.getString("id") : value.toString());
+            }
+            assertEquals(List.of("iris:biomes/a", "iris:biomes/b", "iris:biomes/c", "mod:river"), values);
+        } finally {
+            deleteTree(root);
+        }
+    }
+
+    @Test
+    public void tagMergeKeepsStrictConflictsForNonTagFilesInTheSameMerge() throws IOException {
+        Path root = Files.createTempDirectory("iris-forced-pack-tag-strict");
+        try {
+            Path tag = Path.of("data/minecraft/tags/worldgen/biome/is_overworld.json");
+            Path biome = Path.of("data/iris/worldgen/biome/shared.json");
+            for (String side : List.of("source", "destination")) {
+                Files.createDirectories(root.resolve(side).resolve(tag).getParent());
+                Files.createDirectories(root.resolve(side).resolve(biome).getParent());
+                Files.writeString(root.resolve(side).resolve(tag),
+                        "{\"replace\":false,\"values\":[\"iris:biomes/" + side + "\"]}", StandardCharsets.UTF_8);
+            }
+            Files.writeString(root.resolve("source").resolve(biome), "{\"temperature\":1}", StandardCharsets.UTF_8);
+            Files.writeString(root.resolve("destination").resolve(biome), "{\"temperature\":2}", StandardCharsets.UTF_8);
+
+            assertThrows(
+                    IOException.class,
+                    () -> ModdedForcedDatapack.mergeDirectory(root.resolve("source"), root.resolve("destination"))
+            );
+        } finally {
+            deleteTree(root);
+        }
+    }
+
     private List<String> directoryEntries(Path root) throws IOException {
         try (Stream<Path> entries = Files.list(root)) {
             return entries.map((Path path) -> path.getFileName().toString())