diff --git a/adapters/bukkit/nms/v26_3_R1/src/test/java/art/arcane/iris/platform/bukkit/nms/v26_3_R1/CustomBiomeCodecTest.java b/adapters/bukkit/nms/v26_3_R1/src/test/java/art/arcane/iris/platform/bukkit/nms/v26_3_R1/CustomBiomeCodecTest.java
index a30e593d8..637cc6abc 100644
--- a/adapters/bukkit/nms/v26_3_R1/src/test/java/art/arcane/iris/platform/bukkit/nms/v26_3_R1/CustomBiomeCodecTest.java
+++ b/adapters/bukkit/nms/v26_3_R1/src/test/java/art/arcane/iris/platform/bukkit/nms/v26_3_R1/CustomBiomeCodecTest.java
@@ -13,6 +13,7 @@ import net.minecraft.data.registries.VanillaRegistries;
 import net.minecraft.resources.RegistryOps;
 import net.minecraft.server.Bootstrap;
 import net.minecraft.world.attribute.EnvironmentAttributes;
+import net.minecraft.world.attribute.modifier.MobSpawnSettingsModifier;
 import net.minecraft.world.entity.MobCategory;
 import net.minecraft.world.level.biome.Biome;
 import net.minecraft.world.level.biome.MobSpawnSettings;
@@ -53,5 +54,7 @@ public class CustomBiomeCodecTest {
         assertEquals(2, decoded.count().minInclusive());
         assertEquals(4, decoded.count().maxInclusive());
         assertEquals(0x102030, biome.getWaterColor() & 0xFFFFFF);
+        assertEquals(MobSpawnSettingsModifier.overlay(),
+                biome.getAttributes().get(EnvironmentAttributes.NATURAL_MOB_SPAWNS).modifier());
     }
 }
diff --git a/core/src/main/java/art/arcane/iris/pack/datapack/v263/DataFixerV263.java b/core/src/main/java/art/arcane/iris/pack/datapack/v263/DataFixerV263.java
index 8c3228f12..a17f1e67d 100644
--- a/core/src/main/java/art/arcane/iris/pack/datapack/v263/DataFixerV263.java
+++ b/core/src/main/java/art/arcane/iris/pack/datapack/v263/DataFixerV263.java
@@ -34,9 +34,12 @@ public final class DataFixerV263 extends DataFixerV1217 {
                         .put("max_inclusive", maximum));
             }
         }
+        // Vanilla writes spawns as an overlay. A bare value decodes as an override, which NeoForge biome modifiers refuse to edit.
         attributes.put("minecraft:gameplay/natural_mob_spawns", new JSONObject()
-                .put("spawns_by_category", spawners)
-                .put("spawn_costs", fixed.getJSONObject("spawn_costs")));
+                .put("argument", new JSONObject()
+                        .put("spawns_by_category", spawners)
+                        .put("spawn_costs", fixed.getJSONObject("spawn_costs")))
+                .put("modifier", "overlay"));
         Object probability = fixed.remove("creature_spawn_probability");
         if (probability != null) {
             attributes.put("minecraft:gameplay/creature_world_gen_spawn_probability", probability);