diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index caa3223..2354f9c 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -111,9 +111,9 @@ jobs:
           if-no-files-found: error
           retention-days: 30
           path: |
-            build/libs/SkysGrassSlabs-1.1.2.2603002.jar
-            build/libs/SkysGrassSlabs-1.1.2.2603002-sources.jar
-            build/libs/SkysGrassSlabs-1.1.2.2603002-javadoc.jar
+            build/libs/SkysGrassSlabs-1.1.3.2603002.jar
+            build/libs/SkysGrassSlabs-1.1.3.2603002-sources.jar
+            build/libs/SkysGrassSlabs-1.1.3.2603002-javadoc.jar
             build/release/SHA256SUMS
             CHANGELOG.md
 
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 156d3cd..d642568 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,8 +1,15 @@
 # Changelog
 
+## 1.1.3.2603002 - Minecraft 26.3 (NeoForge)
+
+- Fixed crashes when starting the mod or loading a world with newer NeoForge
+  26.3 builds. Earlier 26.3 builds still work.
+- You can keep your existing worlds and `skysgrassslabs-common.toml`. Blocks,
+  recipes and terrain smoothing are unchanged.
+
 ## 1.1.2.2603002 - Minecraft 26.3 (NeoForge)
 
-- Ported the accepted NeoForge release to Minecraft 26.3.
+- Added the Minecraft 26.3 NeoForge build.
 - Preserved blocks, recipes, configuration, world data and legacy upgrades.
 
 ## 1.1.2.2602002 - Minecraft 26.2 (NeoForge)
@@ -13,7 +20,7 @@
 ## 1.1.1.2602002 - Minecraft 26.2 (NeoForge)
 
 - Added the NeoForge 26.2 build with the same blocks, recipes, world
-  smoothing and upgrade support as the accepted 26.1.2 build.
+  smoothing and upgrade support as the 26.1.2 build.
 - Updated grass light checks, carpet references and recipe-book displays for
   Minecraft 26.2.
 - Retained translated names for all four items in 18 language variants.
@@ -81,7 +88,7 @@
 
 ## 1.0.1.118021 - Minecraft 1.18.2
 
-- Brought the accepted dirt, grass and dirt path slabs, turf and terrain
+- Brought dirt, grass and dirt path slabs, turf and terrain
   smoothing forward to Minecraft 1.18.2.
 - Kept both slab orientations, waterlogging, snowy caps, path alignment,
   grass spreading, turf recipes and sheep interaction.
diff --git a/README.md b/README.md
index a7bc2b3..e203b9c 100644
--- a/README.md
+++ b/README.md
@@ -1,12 +1,11 @@
 # Sky's Grass Slabs
 
-Sky's Grass Slabs adds dirt, grass and lowered dirt path slabs, along with a
-thin layer of turf. Grass spreads naturally between full blocks and slabs,
-while new Overworld terrain uses grass slabs to soften suitable one block
-steps.
+Sky's Grass Slabs adds dirt, grass and lowered dirt path slabs, plus a thin
+layer of turf. Grass spreads between blocks, slabs and turf, and grass slabs
+soften one block steps in newly generated Overworld terrain.
 
-This branch is for Minecraft 26.3. Version `1.1.2.2603002` requires NeoForge
-26.3.0.1-beta and Java 25.
+This branch is for Minecraft 26.3. Version `1.1.3.2603002` requires NeoForge
+26.3.0.1-beta or a compatible newer 26.3 build, and Java 25.
 
 ## Installation
 
@@ -14,27 +13,35 @@ Place the mod jar in the `mods` folder. It must be installed on both the client
 and server for multiplayer. Back up an existing world before adding, removing
 or updating world changing mods.
 
-## Highlights
-
-- Place dirt, grass and dirt path slabs in either half of a block.
-- Combine two matching slabs into the corresponding vanilla block.
-- Let grass spread between full blocks, slabs and turf.
-- Use a shovel on dirt or grass slabs to create lowered dirt path slabs.
-- Cut turf from grass without consuming the shovel.
-- Let sheep eat turf and regrow their wool.
-- Add grass slabs to suitable slopes in newly generated Overworld chunks.
-- Display biome colours and snowy edges that match the surrounding terrain.
-- Use standard Minecraft waterlogging behaviour on slabs.
-- Use translated names in 18 supported language variants.
-- Recover selected grass slabs and grass carpet when replacing a compatible
-  older mod.
+## Using the mod
+
+- Place slabs in either half of a block. Two matching slabs combine into the
+  corresponding vanilla block.
+- Use a shovel on dirt or grass slabs to make a dirt path slab.
+- Craft grass and a shovel into turf. The shovel is returned unchanged.
+- Let sheep eat turf to regrow their wool.
+
+Grass follows the surrounding biome colour, and slabs show snow when it is
+nearby. Slabs can also be waterlogged. Block names are translated into 18
+language variants.
+
+World smoothing affects new chunks only and can be turned off in the config.
+The mod can also recover supported slabs and grass carpet from older mods;
+read the upgrade guide before removing one from an existing world.
+
+## This update
+
+Version `1.1.3.2603002` fixes crashes when starting the mod or loading a world
+with newer NeoForge 26.3 builds. Earlier 26.3 builds still work. You can keep
+your existing worlds and `skysgrassslabs-common.toml`; no settings need to be
+changed.
 
 ## Guides
 
 - [Gameplay and recipes](docs/GAMEPLAY.md)
 - [Configuration](docs/CONFIGURATION.md)
 - [Upgrading an older world](docs/WORLD-UPGRADES.md)
-- [Release notes](docs/RELEASE-1.1.2.2603002.md)
+- [Release notes](docs/RELEASE-1.1.3.2603002.md)
 
 ## License
 
diff --git a/docs/CONFIGURATION.md b/docs/CONFIGURATION.md
index 479f27b..e0f2fae 100644
--- a/docs/CONFIGURATION.md
+++ b/docs/CONFIGURATION.md
@@ -5,6 +5,10 @@ first time. Stop the game or server before editing it, then restart to apply
 changes. On a dedicated server, the server's configuration controls world
 generation and world upgrades.
 
+Newer NeoForge 26.3 builds call this configuration scope `LOCAL` rather than
+`COMMON`. The file remains `skysgrassslabs-common.toml`; existing settings do
+not need to be moved or recreated.
+
 When upgrading from an earlier release, the mod copies valid settings from
 `skysgrassslabs.cfg` if the TOML file does not already exist. The old file is
 kept. An existing TOML file always takes priority.
@@ -46,9 +50,10 @@ the other legacy replacement option.
 
 ## Related game rules
 
-Turf and grass behaviour use Minecraft random ticks. Setting `randomTickSpeed`
-to zero pauses spreading, grass decay and delayed turf support checks.
+Turf and grass behaviour use Minecraft random ticks. Setting
+`minecraft:random_tick_speed` to zero pauses spreading, grass decay and delayed
+turf support checks.
 
-Sheep respect the `mobGriefing` game rule. With mob griefing enabled they eat
+Sheep respect the `minecraft:mob_griefing` game rule. With mob griefing enabled they eat
 and remove turf without a drop. With it disabled they can still perform their
 normal eating behaviour and regrow wool, but the turf remains.
diff --git a/docs/GAMEPLAY.md b/docs/GAMEPLAY.md
index 1b4b240..d2f3cee 100644
--- a/docs/GAMEPLAY.md
+++ b/docs/GAMEPLAY.md
@@ -37,7 +37,7 @@ Turf spreads grass to nearby dirt and dirt slabs. Placing turf directly on a
 dirt slab consumes the turf and changes the slab into a grass slab without
 changing whether it is in the top or bottom half.
 
-Sheep can eat turf and regrow their wool. Turf eaten while `mobGriefing` is
+Sheep can eat turf and regrow their wool. Turf eaten while `minecraft:mob_griefing` is
 enabled is removed without dropping an item.
 
 ## Crafting
diff --git a/docs/RELEASE-1.1.3.2603002.md b/docs/RELEASE-1.1.3.2603002.md
new file mode 100644
index 0000000..c0d9986
--- /dev/null
+++ b/docs/RELEASE-1.1.3.2603002.md
@@ -0,0 +1,11 @@
+# Sky's Grass Slabs 1.1.3 for Minecraft 26.3
+
+This update fixes crashes when starting the mod or loading a world with
+newer NeoForge 26.3 builds. Earlier 26.3 builds still work.
+
+You can keep your existing worlds and `skysgrassslabs-common.toml`. No
+settings need to be changed, and blocks, recipes and terrain smoothing work
+as before.
+
+Requires Minecraft 26.3, NeoForge 26.3.0.1-beta or a compatible newer 26.3
+build, and Java 25.
diff --git a/docs/WORLD-UPGRADES.md b/docs/WORLD-UPGRADES.md
index 374e559..e464ab8 100644
--- a/docs/WORLD-UPGRADES.md
+++ b/docs/WORLD-UPGRADES.md
@@ -6,7 +6,7 @@ world has been checked in game.
 
 ## Sky's Grass Slabs content
 
-Install `SkysGrassSlabs-1.1.2.2603002.jar` before opening a world that used an
+Install `SkysGrassSlabs-1.1.3.2603002.jar` before opening a world that used an
 earlier release. The permanent dirt slab, grass slab, path slab and turf IDs
 are unchanged.
 
diff --git a/gradle.properties b/gradle.properties
index 476b8f5..83e162d 100644
--- a/gradle.properties
+++ b/gradle.properties
@@ -43,7 +43,7 @@ mod_name=Sky's Grass Slabs
 # The license of the mod. Review your options at https://choosealicense.com/. All Rights Reserved is the default.
 mod_license=LGPL-2.1-only
 # The mod version. See https://semver.org/
-mod_version=1.1.2.2603002
+mod_version=1.1.3.2603002
 # The group ID for the mod. It is only important when publishing as an artifact to a Maven repository.
 # This should match the base package used for the mod sources.
 # See https://maven.apache.org/guides/mini/guide-naming-conventions.html
diff --git a/src/gameTest/java/zone/moddev/mc/skysgrassslabs/gametest/WorldgenGameTests.java b/src/gameTest/java/zone/moddev/mc/skysgrassslabs/gametest/WorldgenGameTests.java
index 46e97ea..9f6b422 100644
--- a/src/gameTest/java/zone/moddev/mc/skysgrassslabs/gametest/WorldgenGameTests.java
+++ b/src/gameTest/java/zone/moddev/mc/skysgrassslabs/gametest/WorldgenGameTests.java
@@ -12,6 +12,8 @@
 import net.minecraft.world.level.levelgen.Heightmap;
 import net.minecraft.world.level.levelgen.placement.PlacedFeature;
 import net.neoforged.neoforge.registries.NeoForgeRegistries;
+import net.neoforged.neoforge.common.world.BiomeModifier.Phase;
+import net.neoforged.neoforge.common.world.ModifiableBiomeInfo.BiomeInfo;
 import zone.moddev.mc.skysgrassslabs.SkysGrassSlabs;
 import zone.moddev.mc.skysgrassslabs.init.ModBlocks;
 import zone.moddev.mc.skysgrassslabs.world.GrassSlabSmoothingFeature;
@@ -40,6 +42,46 @@ public static void smoothingIsFirstVegetationFeature(GameTestHelper helper) {
         helper.succeed();
     }
 
+    public static void bothModifierSignaturesPreservePhaseOrderAndDeduplication(GameTestHelper helper) {
+        var biome = helper.getLevel().getBiome(helper.absolutePos(BlockPos.ZERO));
+        var value = biome.value();
+        var original = new BiomeInfo(value.getModifiedClimateSettings(), value.getAttributes(),
+                value.getModifiedSpecialEffects(), value.getGenerationSettings());
+        var key = biome.unwrapKey().orElseThrow();
+        var legacy = BiomeInfo.Builder.copyOf(key, original);
+        var current = BiomeInfo.Builder.copyOf(key, original);
+        var step = GenerationStep.Decoration.VEGETAL_DECORATION;
+        for (var builder : java.util.List.of(legacy, current)) {
+            builder.getGenerationSettings().getFeatures(step).removeIf(feature ->
+                    feature.value().feature().value() instanceof GrassSlabSmoothingFeature);
+        }
+        var before = java.util.List.copyOf(current.getGenerationSettings().getFeatures(step));
+        var modifier = zone.moddev.mc.skysgrassslabs.world.SmoothingBiomeModifier.INSTANCE;
+        for (Phase phase : Phase.values()) {
+            if (phase == Phase.AFTER_EVERYTHING) {
+                continue;
+            }
+            modifier.modify(biome, phase, legacy);
+            modifier.modify(helper.getLevel().registryAccess(), biome, phase, current);
+            require(helper, legacy.getGenerationSettings().getFeatures(step).equals(before)
+                    && current.getGenerationSettings().getFeatures(step).equals(before),
+                    "a nonfinal phase changed vegetation features");
+        }
+        modifier.modify(biome, Phase.AFTER_EVERYTHING, legacy);
+        modifier.modify(helper.getLevel().registryAccess(), biome, Phase.AFTER_EVERYTHING, current);
+        var expected = java.util.List.copyOf(legacy.getGenerationSettings().getFeatures(step));
+        require(helper, current.getGenerationSettings().getFeatures(step).equals(expected),
+                "the two loader entry points changed feature order");
+        require(helper, expected.size() == before.size() + 1
+                && expected.getFirst().value().feature().value() instanceof GrassSlabSmoothingFeature,
+                "smoothing was not inserted exactly once at index zero");
+        modifier.modify(helper.getLevel().registryAccess(), biome, Phase.AFTER_EVERYTHING, current);
+        modifier.modify(biome, Phase.AFTER_EVERYTHING, current);
+        require(helper, current.getGenerationSettings().getFeatures(step).equals(expected),
+                "repeat calls duplicated the smoothing feature");
+        helper.succeed();
+    }
+
     public static void smoothingIsBorderSafeAndIdempotent(GameTestHelper helper) {
         ChunkPos owner = ChunkPos.containing(helper.absolutePos(new BlockPos(1, 2, 1)));
         int y = 120;
diff --git a/src/gameTest/resources/data/skysgrassslabs/test_instance/worldgen/both_modifier_signatures_preserve_phase_order_and_deduplication.json b/src/gameTest/resources/data/skysgrassslabs/test_instance/worldgen/both_modifier_signatures_preserve_phase_order_and_deduplication.json
new file mode 100644
index 0000000..173c3dc
--- /dev/null
+++ b/src/gameTest/resources/data/skysgrassslabs/test_instance/worldgen/both_modifier_signatures_preserve_phase_order_and_deduplication.json
@@ -0,0 +1,7 @@
+{
+  "type": "minecraft:function",
+  "environment": "minecraft:default",
+  "function": "skysgrassslabs:worldgen/both_modifier_signatures_preserve_phase_order_and_deduplication",
+  "max_ticks": 100,
+  "structure": "minecraft:empty"
+}
diff --git a/src/main/java/zone/moddev/mc/skysgrassslabs/SkysGrassSlabs.java b/src/main/java/zone/moddev/mc/skysgrassslabs/SkysGrassSlabs.java
index 7fa1558..0491ef8 100644
--- a/src/main/java/zone/moddev/mc/skysgrassslabs/SkysGrassSlabs.java
+++ b/src/main/java/zone/moddev/mc/skysgrassslabs/SkysGrassSlabs.java
@@ -27,7 +27,7 @@ public final class SkysGrassSlabs {
     /** Stable mod identifier and resource namespace. */
     public static final String MOD_ID = "skysgrassslabs";
     public static final String NAME = "Sky's Grass Slabs";
-    public static final String VERSION = "1.1.2.2603002";
+    public static final String VERSION = "1.1.3.2603002";
     public static final Logger LOGGER = LogManager.getLogger();
 
     /** Registers content, configuration, world generation, and persistent state. */
diff --git a/src/main/java/zone/moddev/mc/skysgrassslabs/config/NeoForgeConfigType.java b/src/main/java/zone/moddev/mc/skysgrassslabs/config/NeoForgeConfigType.java
new file mode 100644
index 0000000..410bd5e
--- /dev/null
+++ b/src/main/java/zone/moddev/mc/skysgrassslabs/config/NeoForgeConfigType.java
@@ -0,0 +1,18 @@
+package zone.moddev.mc.skysgrassslabs.config;
+
+import net.neoforged.fml.config.ModConfig;
+
+/** Keeps the same local configuration scope across the NeoForge 26.3 rename. */
+final class NeoForgeConfigType {
+    private NeoForgeConfigType() {
+    }
+
+    static ModConfig.Type local() {
+        // Resolve names at runtime so one jar supports both loader versions.
+        try {
+            return ModConfig.Type.valueOf("LOCAL");
+        } catch (IllegalArgumentException missingLocalName) {
+            return ModConfig.Type.valueOf("COMMON");
+        }
+    }
+}
diff --git a/src/main/java/zone/moddev/mc/skysgrassslabs/config/SkysGrassSlabsConfig.java b/src/main/java/zone/moddev/mc/skysgrassslabs/config/SkysGrassSlabsConfig.java
index 5525e29..93801c4 100644
--- a/src/main/java/zone/moddev/mc/skysgrassslabs/config/SkysGrassSlabsConfig.java
+++ b/src/main/java/zone/moddev/mc/skysgrassslabs/config/SkysGrassSlabsConfig.java
@@ -7,7 +7,6 @@
 import java.util.List;
 import org.apache.commons.lang3.tuple.Pair;
 import net.neoforged.neoforge.common.ModConfigSpec;
-import net.neoforged.fml.config.ModConfig;
 import net.neoforged.fml.loading.FMLPaths;
 import net.neoforged.fml.ModContainer;
 import zone.moddev.mc.skysgrassslabs.SkysGrassSlabs;
@@ -32,7 +31,7 @@ public final class SkysGrassSlabsConfig {
     }
 
     public static void register(ModContainer container) {
-        container.registerConfig(ModConfig.Type.COMMON, SPEC, FILE_NAME);
+        container.registerConfig(NeoForgeConfigType.local(), SPEC, FILE_NAME);
     }
 
     public static void migrateLegacyConfig() {
diff --git a/src/main/java/zone/moddev/mc/skysgrassslabs/world/SmoothingBiomeModifier.java b/src/main/java/zone/moddev/mc/skysgrassslabs/world/SmoothingBiomeModifier.java
index 1b99a67..df1c54e 100644
--- a/src/main/java/zone/moddev/mc/skysgrassslabs/world/SmoothingBiomeModifier.java
+++ b/src/main/java/zone/moddev/mc/skysgrassslabs/world/SmoothingBiomeModifier.java
@@ -3,6 +3,7 @@
 import com.mojang.serialization.MapCodec;
 import java.util.List;
 import net.minecraft.core.Holder;
+import net.minecraft.core.RegistryAccess;
 import net.minecraft.tags.BiomeTags;
 import net.minecraft.world.level.biome.Biome;
 import net.minecraft.world.level.levelgen.GenerationStep;
@@ -34,6 +35,12 @@ public void modify(Holder<Biome> biome, Phase phase, BiomeInfo.Builder builder)
         }
     }
 
+    /** Newer NeoForge builds provide registry access alongside the biome. */
+    public void modify(RegistryAccess registries, Holder<Biome> biome,
+            Phase phase, BiomeInfo.Builder builder) {
+        modify(biome, phase, builder);
+    }
+
     @Override
     public MapCodec<? extends BiomeModifier> codec() {
         return CODEC;
diff --git a/src/test/java/zone/moddev/mc/skysgrassslabs/ProjectContractTest.java b/src/test/java/zone/moddev/mc/skysgrassslabs/ProjectContractTest.java
index 14c56fe..6ecb066 100644
--- a/src/test/java/zone/moddev/mc/skysgrassslabs/ProjectContractTest.java
+++ b/src/test/java/zone/moddev/mc/skysgrassslabs/ProjectContractTest.java
@@ -35,14 +35,14 @@ public void playerAndMaintainerDocumentsExist() {
         assertTrue(Files.isRegularFile(Path.of("docs/GAMEPLAY.md")));
         assertTrue(Files.isRegularFile(Path.of("docs/WORLD-UPGRADES.md")));
         assertFalse(Files.exists(Path.of("docs/REPOSITORY.md")));
-        assertTrue(Files.isRegularFile(Path.of("docs/RELEASE-1.1.2.2603002.md")));
+        assertTrue(Files.isRegularFile(Path.of("docs/RELEASE-1.1.3.2603002.md")));
         assertTrue(Files.isRegularFile(Path.of("docs/BETA-0.2.0.118021.md")));
     }
 
     @Test
     public void releaseIdentityAndLicenseAreStable() throws Exception {
         String properties = Files.readString(Path.of("gradle.properties"), StandardCharsets.UTF_8);
-        assertTrue(properties.contains("mod_version=1.1.2.2603002"));
+        assertTrue(properties.contains("mod_version=1.1.3.2603002"));
         assertTrue(properties.contains("mod_license=LGPL-2.1-only"));
         assertEquals("LGPL-2.1-only", Files.readString(Path.of("LICENSE.spdx"), StandardCharsets.UTF_8).trim());
         assertTrue(Files.readString(Path.of("NOTICE"), StandardCharsets.UTF_8)
@@ -117,7 +117,7 @@ public void commonConfigAndWorldStateUsePermanentKeys() throws Exception {
         assertTrue(config.contains("push(\"compat\")"));
         assertTrue(config.contains("define(FORCE_REPLACE_BUILDINGBRICKS_SLABS, false)"));
         assertTrue(config.contains("define(FORCE_REPLACE_GRASS_SLABS_MOD_CONTENT, false)"));
-        assertTrue(main.contains("VERSION = \"1.1.2.2603002\""));
+        assertTrue(main.contains("VERSION = \"1.1.3.2603002\""));
         assertTrue(state.contains("skysgrassslabs_world_state"));
         assertTrue(state.contains("SCHEMA_VERSION = 1"));
         assertTrue(state.contains("schema_version"));
@@ -302,9 +302,9 @@ public void continuousIntegrationTargetsTheStableBranchAndArtifacts() throws Exc
         }
         String ci = Files.readString(Path.of(".github/workflows/ci.yml"),
                 StandardCharsets.UTF_8);
-        assertTrue(ci.contains("SkysGrassSlabs-1.1.2.2603002.jar"));
-        assertTrue(ci.contains("SkysGrassSlabs-1.1.2.2603002-sources.jar"));
-        assertTrue(ci.contains("SkysGrassSlabs-1.1.2.2603002-javadoc.jar"));
+        assertTrue(ci.contains("SkysGrassSlabs-1.1.3.2603002.jar"));
+        assertTrue(ci.contains("SkysGrassSlabs-1.1.3.2603002-sources.jar"));
+        assertTrue(ci.contains("SkysGrassSlabs-1.1.3.2603002-javadoc.jar"));
         assertTrue(ci.contains("if-no-files-found: error"));
         assertTrue(ci.contains("java-version: '25.0.3+9.0.LTS'"));
         assertTrue(ci.contains("--offline --no-daemon"));
diff --git a/src/test/java/zone/moddev/mc/skysgrassslabs/config/NeoForgeConfigTypeTest.java b/src/test/java/zone/moddev/mc/skysgrassslabs/config/NeoForgeConfigTypeTest.java
new file mode 100644
index 0000000..24e1894
--- /dev/null
+++ b/src/test/java/zone/moddev/mc/skysgrassslabs/config/NeoForgeConfigTypeTest.java
@@ -0,0 +1,95 @@
+package zone.moddev.mc.skysgrassslabs.config;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.fail;
+
+import java.io.InputStream;
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import javax.tools.ToolProvider;
+import net.neoforged.fml.config.ModConfig;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.TemporaryFolder;
+
+public class NeoForgeConfigTypeTest {
+    private static final String HELPER =
+            "zone.moddev.mc.skysgrassslabs.config.NeoForgeConfigType";
+
+    @Rule
+    public final TemporaryFolder temporary = new TemporaryFolder();
+
+    @Test
+    public void actualLoaderUsesItsUnsyncedConfigType() throws Exception {
+        String expected = java.util.Arrays.stream(ModConfig.Type.values())
+                .anyMatch(type -> type.name().equals("LOCAL")) ? "LOCAL" : "COMMON";
+        Method local = Class.forName(HELPER).getDeclaredMethod("local");
+        local.setAccessible(true);
+        assertEquals(ModConfig.Type.valueOf(expected), local.invoke(null));
+    }
+
+    @Test
+    public void olderLoaderKeepsCommonScope() throws Exception {
+        assertEquals("COMMON", resolveWith("COMMON, CLIENT, SERVER, STARTUP"));
+    }
+
+    @Test
+    public void newerLoaderUsesLocalScope() throws Exception {
+        assertEquals("LOCAL", resolveWith("LOCAL, CLIENT, SYNCED, STARTUP"));
+    }
+
+    @Test
+    public void localWinsWhenBothNamesExist() throws Exception {
+        assertEquals("LOCAL", resolveWith("COMMON, LOCAL, CLIENT, SYNCED, STARTUP"));
+    }
+
+    @Test
+    public void missingUnsyncedScopeFailsInsteadOfUsingClientOrSynced() throws Exception {
+        try {
+            resolveWith("CLIENT, SYNCED, STARTUP");
+            fail("A loader without either local scope must fail clearly");
+        } catch (InvocationTargetException exception) {
+            assertTrue(exception.getCause() instanceof IllegalArgumentException);
+            assertTrue(exception.getCause().getMessage().contains("COMMON"));
+        }
+    }
+
+    @Test
+    public void registrationRetainsTheExistingConfigFile() throws Exception {
+        String source = Files.readString(Path.of(
+                "src/main/java/zone/moddev/mc/skysgrassslabs/config/SkysGrassSlabsConfig.java"));
+        assertEquals("skysgrassslabs-common.toml", SkysGrassSlabsConfig.FILE_NAME);
+        assertTrue(source.contains("registerConfig(NeoForgeConfigType.local(), SPEC, FILE_NAME)"));
+    }
+
+    private String resolveWith(String constants) throws Exception {
+        Path directory = temporary.newFolder().toPath();
+        Path source = directory.resolve("net/neoforged/fml/config/ModConfig.java");
+        Files.createDirectories(source.getParent());
+        Files.writeString(source, "package net.neoforged.fml.config; public final class ModConfig {"
+                + " public enum Type { " + constants + " } }");
+        var compiler = ToolProvider.getSystemJavaCompiler();
+        assertNotNull("The enum compatibility test requires a JDK", compiler);
+        assertEquals(0, compiler.run(null, null, null, "-proc:none", "-d",
+                directory.toString(), source.toString()));
+        Path helper = directory.resolve(HELPER.replace('.', '/') + ".class");
+        Files.createDirectories(helper.getParent());
+        try (InputStream bytes = getClass().getClassLoader().getResourceAsStream(
+                HELPER.replace('.', '/') + ".class")) {
+            assertNotNull("The shipped config helper must be present", bytes);
+            Files.copy(bytes, helper);
+        }
+        try (URLClassLoader isolated = new URLClassLoader(
+                new URL[] {directory.toUri().toURL()}, ClassLoader.getPlatformClassLoader())) {
+            Method local = isolated.loadClass(HELPER).getDeclaredMethod("local");
+            local.setAccessible(true);
+            return ((Enum<?>) local.invoke(null)).name();
+        }
+    }
+}
diff --git a/src/test/java/zone/moddev/mc/skysgrassslabs/world/BiomeModifierCompatibilityTest.java b/src/test/java/zone/moddev/mc/skysgrassslabs/world/BiomeModifierCompatibilityTest.java
new file mode 100644
index 0000000..ef906f4
--- /dev/null
+++ b/src/test/java/zone/moddev/mc/skysgrassslabs/world/BiomeModifierCompatibilityTest.java
@@ -0,0 +1,32 @@
+package zone.moddev.mc.skysgrassslabs.world;
+
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
+
+import net.minecraft.core.Holder;
+import net.minecraft.core.RegistryAccess;
+import net.neoforged.neoforge.common.world.BiomeModifier.Phase;
+import net.neoforged.neoforge.common.world.ModifiableBiomeInfo.BiomeInfo;
+import org.junit.Test;
+
+public class BiomeModifierCompatibilityTest {
+    @Test
+    public void bothLoaderEntryPointsRemainAvailable() throws Exception {
+        Class<?> modifier = Class.forName(SmoothingBiomeModifier.class.getName(), false,
+                getClass().getClassLoader());
+        assertNotNull(modifier.getMethod("modify", Holder.class, Phase.class,
+                BiomeInfo.Builder.class));
+        assertNotNull(modifier.getMethod("modify", RegistryAccess.class, Holder.class,
+                Phase.class, BiomeInfo.Builder.class));
+    }
+
+    @Test
+    public void newerEntryPointDelegatesWithoutChangingTheSmoothingRules() throws Exception {
+        String source = java.nio.file.Files.readString(java.nio.file.Path.of(
+                "src/main/java/zone/moddev/mc/skysgrassslabs/world/SmoothingBiomeModifier.java"));
+        assertTrue(source.contains("modify(biome, phase, builder);"));
+        assertTrue(source.contains("phase != Phase.AFTER_EVERYTHING"));
+        assertTrue(source.contains("features.add(0, smoothing)"));
+        assertTrue(source.contains("features.stream().noneMatch"));
+    }
+}