diff --git a/CHANGELOG.txt b/CHANGELOG.txt
index 2cfc395a4..86ebc4753 100644
--- a/CHANGELOG.txt
+++ b/CHANGELOG.txt
@@ -1,3 +1,10 @@
+Unreleased 1.13.2 migration repair
+
+- Leave an already-correct Forge leaves data fixer unchanged instead of logging
+  a transformer failure.
+- Still reject unrecognised fixer bytecode, and check both repair and repeat-load
+  paths.
+
 Base Metals 3.0.1.113021
 
 - Add the configuration screen to Forge's Mods list and OreSpawn's cog, with
diff --git a/src/main/resources/coremods/basemetals_113_compatibility.js b/src/main/resources/coremods/basemetals_113_compatibility.js
index df22fa547..529277807 100644
--- a/src/main/resources/coremods/basemetals_113_compatibility.js
+++ b/src/main/resources/coremods/basemetals_113_compatibility.js
@@ -14,8 +14,17 @@ function initializeCoreMod() {
                 var patched = false;
                 for (var i = 0; i < classNode.methods.size(); ++i) {
                     var method = classNode.methods.get(i);
+                    if (method.desc !== '(Lcom/mojang/datafixers/Typed;)Lcom/mojang/datafixers/Typed;') continue;
                     for (var instruction = method.instructions.getFirst(); instruction !== null;
                             instruction = instruction.getNext()) {
+                        // Some Forge installations already contain the corrected erased overload.
+                        if (instruction.getOpcode() === Opcodes.INVOKEVIRTUAL
+                                && instruction.owner === 'com/mojang/datafixers/Typed' && instruction.name === 'set'
+                                && instruction.desc === '(Lcom/mojang/datafixers/OpticFinder;' +
+                                        'Ljava/lang/Object;)Lcom/mojang/datafixers/Typed;') {
+                            patched = true;
+                            continue;
+                        }
                         if (instruction.getOpcode() !== Opcodes.CHECKCAST
                                 || instruction.desc !== 'com/mojang/datafixers/Typed') continue;
                         var call = instruction.getNext();
diff --git a/src/test/java/zone/moddev/mc/basemetals/migration/LeavesTransformerTest.java b/src/test/java/zone/moddev/mc/basemetals/migration/LeavesTransformerTest.java
new file mode 100644
index 000000000..dc094c9d6
--- /dev/null
+++ b/src/test/java/zone/moddev/mc/basemetals/migration/LeavesTransformerTest.java
@@ -0,0 +1,75 @@
+package zone.moddev.mc.basemetals.migration;
+
+import java.io.Reader;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Paths;
+import javax.script.ScriptEngine;
+import javax.script.ScriptEngineManager;
+import jdk.nashorn.api.scripting.ScriptObjectMirror;
+import org.junit.jupiter.api.Test;
+import org.objectweb.asm.Opcodes;
+import org.objectweb.asm.tree.ClassNode;
+import org.objectweb.asm.tree.MethodNode;
+import org.objectweb.asm.tree.MethodInsnNode;
+import org.objectweb.asm.tree.TypeInsnNode;
+import static org.junit.jupiter.api.Assertions.*;
+
+class LeavesTransformerTest {
+    private static final String OWNER = "com/mojang/datafixers/Typed";
+    private static final String METHOD = "(L" + OWNER + ";)L" + OWNER + ";";
+    private static final String PREFIX = "(Lcom/mojang/datafixers/OpticFinder;";
+
+    private ClassNode transform(ClassNode node) throws Exception {
+        ScriptEngine engine = new ScriptEngineManager().getEngineByName("nashorn");
+        assertNotNull(engine, "Run the native Java 8 coremod engine");
+        try (Reader reader = Files.newBufferedReader(Paths.get(
+                "src/main/resources/coremods/basemetals_113_compatibility.js"), StandardCharsets.UTF_8)) {
+            engine.eval(reader);
+        }
+        ScriptObjectMirror modules = (ScriptObjectMirror)engine.eval("initializeCoreMod()");
+        ScriptObjectMirror entry = (ScriptObjectMirror)modules.get("basemetals_forge25_leaves_fixer");
+        return (ClassNode)entry.callMember("transformer", node);
+    }
+
+    private ClassNode fixture(boolean broken) {
+        ClassNode node = new ClassNode();
+        node.name = "net/minecraft/util/datafix/fixes/LeavesFix$Section";
+        MethodNode method = new MethodNode(Opcodes.ACC_PUBLIC, "func_208465_a", METHOD, null, null);
+        node.methods.add(method);
+        if (broken) method.instructions.add(new TypeInsnNode(Opcodes.CHECKCAST, OWNER));
+        method.instructions.add(new MethodInsnNode(Opcodes.INVOKEVIRTUAL, OWNER, "set",
+                PREFIX + (broken ? "L" + OWNER + ";" : "Ljava/lang/Object;") + ")L" + OWNER + ";", false));
+        return node;
+    }
+
+    @Test void repairsBrokenCallAndRemainsIdempotent() throws Exception {
+        ClassNode node = fixture(true);
+        assertSame(node, transform(node));
+        assertEquals(1, node.methods.get(0).instructions.size());
+        String descriptor = ((MethodInsnNode)node.methods.get(0).instructions.getFirst()).desc;
+        assertTrue(descriptor.contains("Ljava/lang/Object;"));
+        assertSame(node, transform(node));
+        assertEquals(descriptor, ((MethodInsnNode)node.methods.get(0).instructions.getFirst()).desc);
+    }
+
+    @Test void preservesAlreadyCorrectNativeCall() throws Exception {
+        ClassNode node = fixture(false);
+        Object instruction = node.methods.get(0).instructions.getFirst();
+        assertSame(node, transform(node));
+        assertSame(instruction, node.methods.get(0).instructions.getFirst());
+        assertEquals(1, node.methods.get(0).instructions.size());
+    }
+
+    @Test void rejectsUnknownStructure() {
+        ClassNode node = fixture(false);
+        ((MethodInsnNode)node.methods.get(0).instructions.getFirst()).name = "notSet";
+        assertThrows(Exception.class, () -> transform(node));
+    }
+
+    @Test void unrelatedObjectSetDoesNotMaskMissingFixer() {
+        ClassNode node = fixture(false);
+        node.methods.get(0).desc = "()V";
+        assertThrows(Exception.class, () -> transform(node));
+    }
+}