diff --git a/build.gradle b/build.gradle
index c80d6f659..604ecd9f9 100644
--- a/build.gradle
+++ b/build.gradle
@@ -6,12 +6,17 @@ buildscript {
             name = "forge"
             url = "https://files.minecraftforge.net/maven"
         }
+        maven {
+            name = "mixin"
+            url = "https://dist.creeper.host/Sponge/maven"
+        }
     }
     dependencies {
         classpath 'com.google.code.gson:gson:2.8.1'
         classpath 'net.minecraftforge.gradle:ForgeGradle:3.0.190'
         classpath 'net.sf.proguard:proguard-gradle:6.1.0beta2'
         classpath 'org.ajoberstar.grgit:grgit-gradle:3.0.0'
+        classpath 'org.spongepowered:mixingradle:0.7-SNAPSHOT'
     }
 }
 
@@ -25,6 +30,7 @@ plugins {
 }
 
 apply plugin: 'net.minecraftforge.gradle'
+apply plugin: 'org.spongepowered.mixin'
 apply plugin: 'org.ajoberstar.grgit'
 apply plugin: 'maven-publish'
 apply plugin: 'maven'
@@ -79,6 +85,10 @@ minecraft {
         testServer {
             workingDirectory project.file('test-files/server')
             parent runs.server
+            properties 'mixin.env.disableRefMap': 'true'
+
+            arg "-mixin.config=cctest.mixin.json"
+            arg "--nogui"
 
             mods {
                 cctest {
@@ -93,6 +103,10 @@ minecraft {
     accessTransformer file('src/main/resources/META-INF/accesstransformer.cfg')
 }
 
+mixin {
+    add sourceSets.test, "cctest.refmap.json"
+}
+
 sourceSets {
     main.resources {
         srcDir 'src/generated/resources'
diff --git a/src/test/java/dan200/computercraft/ingame/mixin/MixinTestFunctionInfo.java b/src/test/java/dan200/computercraft/ingame/mixin/MixinTestFunctionInfo.java
new file mode 100644
index 000000000..b4fb6ccd2
--- /dev/null
+++ b/src/test/java/dan200/computercraft/ingame/mixin/MixinTestFunctionInfo.java
@@ -0,0 +1,76 @@
+/*
+ * This file is part of ComputerCraft - http://www.computercraft.info
+ * Copyright Daniel Ratcliffe, 2011-2021. Do not distribute without permission.
+ * Send enquiries to dratcliffe@gmail.com
+ */
+package dan200.computercraft.ingame.mixin;
+
+import net.minecraft.test.TestFunctionInfo;
+import net.minecraft.test.TestTrackerHolder;
+import net.minecraft.util.Rotation;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Mutable;
+import org.spongepowered.asm.mixin.Overwrite;
+import org.spongepowered.asm.mixin.Shadow;
+
+import java.util.function.Consumer;
+
+/**
+ * Mixin to replace final fields and some getters with non-final versions.
+ *
+ * Due to (I assume) the magic of proguard, some getters are replaced with constant
+ * implementations. Thus we need to replace them with a sensible version.
+ */
+@Mixin( TestFunctionInfo.class )
+public class MixinTestFunctionInfo
+{
+    @Shadow
+    @Mutable
+    private String batchName;
+
+    @Shadow
+    @Mutable
+    private String testName;
+
+    @Shadow
+    @Mutable
+    private String structureName;
+
+    @Shadow
+    @Mutable
+    private boolean required;
+
+    @Shadow
+    @Mutable
+    private Consumer<TestTrackerHolder> function;
+
+    @Shadow
+    @Mutable
+    private int maxTicks;
+
+    @Shadow
+    @Mutable
+    private long setupTicks;
+
+    @Shadow
+    @Mutable
+    private Rotation rotation;
+
+    @Overwrite
+    public int getMaxTicks()
+    {
+        return this.maxTicks;
+    }
+
+    @Overwrite
+    public long getSetupTicks()
+    {
+        return setupTicks;
+    }
+
+    @Overwrite
+    public boolean isRequired()
+    {
+        return required;
+    }
+}
diff --git a/src/test/java/dan200/computercraft/ingame/mod/TestLoader.java b/src/test/java/dan200/computercraft/ingame/mod/TestLoader.java
index f2eef6f8a..9c4d7fa80 100644
--- a/src/test/java/dan200/computercraft/ingame/mod/TestLoader.java
+++ b/src/test/java/dan200/computercraft/ingame/mod/TestLoader.java
@@ -19,7 +19,6 @@ import org.objectweb.asm.Type;
 
 import java.lang.reflect.Field;
 import java.lang.reflect.Method;
-import java.lang.reflect.Modifier;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Set;
@@ -110,13 +109,6 @@ class TestLoader
     private static void setFinalField( TestFunctionInfo func, String name, Object value ) throws ReflectiveOperationException
     {
         Field field = TestFunctionInfo.class.getDeclaredField( name );
-        if( (field.getModifiers() & Modifier.FINAL) != 0 )
-        {
-            Field modifiers = Field.class.getDeclaredField( "modifiers" );
-            modifiers.setAccessible( true );
-            modifiers.set( field, field.getModifiers() & ~Modifier.FINAL );
-        }
-
         field.setAccessible( true );
         field.set( func, value );
     }
diff --git a/src/test/resources/cctest.mixin.json b/src/test/resources/cctest.mixin.json
new file mode 100644
index 000000000..f836d2638
--- /dev/null
+++ b/src/test/resources/cctest.mixin.json
@@ -0,0 +1,10 @@
+{
+    "required": true,
+    "package": "dan200.computercraft.ingame.mixin",
+    "compatibilityLevel": "JAVA_8",
+    "refmap": "cctest.refmap.json",
+    "mixins": [
+        "MixinTestFunctionInfo"
+    ],
+    "minVersion": "0.8"
+}