diff --git a/annotations/src/main/java/mindustry/annotations/BaseProcessor.java b/annotations/src/main/java/mindustry/annotations/BaseProcessor.java
index c341349971..37cbff235f 100644
--- a/annotations/src/main/java/mindustry/annotations/BaseProcessor.java
+++ b/annotations/src/main/java/mindustry/annotations/BaseProcessor.java
@@ -68,6 +68,25 @@ public abstract class BaseProcessor extends AbstractProcessor{
         }
     }
 
+    //in bytes
+    public static int typeSize(String kind){
+        switch(kind){
+            case "boolean":
+            case "byte":
+                return 1;
+            case "short":
+                return 2;
+            case "float":
+            case "char":
+            case "int":
+                return 4;
+            case "long":
+                return 8;
+            default:
+                throw new IllegalArgumentException("Invalid primitive type: " + kind + "");
+        }
+    }
+
     public static String simpleName(String str){
         return str.contains(".") ? str.substring(str.lastIndexOf('.') + 1) : str;
     }
diff --git a/annotations/src/main/java/mindustry/annotations/entity/EntityIO.java b/annotations/src/main/java/mindustry/annotations/entity/EntityIO.java
index 1d116cbb97..93b07ace1a 100644
--- a/annotations/src/main/java/mindustry/annotations/entity/EntityIO.java
+++ b/annotations/src/main/java/mindustry/annotations/entity/EntityIO.java
@@ -1,59 +1,185 @@
 package mindustry.annotations.entity;
 
+import arc.files.*;
+import arc.struct.*;
+import arc.util.serialization.*;
 import com.squareup.javapoet.*;
-import com.squareup.javapoet.MethodSpec.*;
+import mindustry.annotations.Annotations.*;
 import mindustry.annotations.*;
 import mindustry.annotations.util.TypeIOResolver.*;
 
+import javax.lang.model.element.*;
+
 import static mindustry.annotations.BaseProcessor.instanceOf;
 
 public class EntityIO{
-    final MethodSpec.Builder builder;
-    final boolean write;
-    final ClassSerializer serializer;
+    final static Json json = new Json();
 
-    EntityIO(Builder builder, boolean write, ClassSerializer serializer){
-        this.builder = builder;
+    final ClassSerializer serializer;
+    final String name;
+    final TypeSpec.Builder type;
+    final Fi directory;
+    final Array<Revision> revisions = new Array<>();
+
+    boolean write;
+    MethodSpec.Builder method;
+    ObjectSet<String> presentFields = new ObjectSet<>();
+
+    EntityIO(String name, TypeSpec.Builder type, ClassSerializer serializer, Fi directory){
+        this.directory = directory;
+        this.type = type;
         this.serializer = serializer;
-        this.write = write;
+        this.name = name;
+
+        directory.mkdirs();
+
+        //load old revisions
+        for(Fi fi : directory.list()){
+            revisions.add(json.fromJson(Revision.class, fi));
+        }
+
+        //next revision to be used
+        int nextRevision = revisions.isEmpty() ? 0 : revisions.max(r -> r.version).version + 1;
+
+        //resolve preferred field order based on fields that fit
+        Array<FieldSpec> fields = Array.with(type.fieldSpecs).select(spec ->
+            !spec.hasModifier(Modifier.TRANSIENT) &&
+            !spec.hasModifier(Modifier.STATIC) &&
+            !spec.hasModifier(Modifier.FINAL) &&
+            (spec.type.isPrimitive() || serializer.has(spec.type.toString())));
+
+        //sort to keep order
+        fields.sortComparing(f -> f.name);
+
+        //keep track of fields present in the entity
+        presentFields.addAll(fields.map(f -> f.name));
+
+        //add new revision if it doesn't match or there are no revisions
+        if(revisions.isEmpty() || !revisions.peek().equal(fields)){
+            revisions.add(new Revision(nextRevision, fields.map(f -> new RevisionField(f.name, f.type.toString(), f.type.isPrimitive() ? BaseProcessor.typeSize(f.type.toString()) : -1))));
+            //write revision
+            directory.child(nextRevision + ".json").writeString(json.toJson(revisions.peek()));
+        }
     }
 
-    void io(TypeName type, String field) throws Exception{
+    void write(MethodSpec.Builder method, boolean write) throws Exception{
+        this.method = method;
+        this.write = write;
 
-        if(type.isPrimitive()){
-            s(type == TypeName.BOOLEAN ? "bool" : type.toString().charAt(0) + "", field);
-        //}else if(type.toString().equals("java.lang.String")){
-        //    s("str", field);
-        }else if(instanceOf(type.toString(), "mindustry.ctype.Content")){
+        //subclasses *have* to call this method
+        method.addAnnotation(CallSuper.class);
+
+        if(write){
+            //write short revision
+            st("write.s($L)", revisions.peek().version);
+            //write uses most recent revision
+            for(RevisionField field : revisions.peek().fields){
+                io(field.type, "this." + field.name);
+            }
+        }else{
+            //read revision
+            st("short REV = read.s()");
+
+            for(int i = 0; i < revisions.size; i++){
+                //check for the right revision
+                Revision rev = revisions.get(i);
+                if(i == 0){
+                    cont("if(REV == $L)", rev.version);
+                }else{
+                    ncont("else if(REV == $L)", rev.version);
+                }
+
+                //add code for reading revision
+                for(RevisionField field : rev.fields){
+                    //if the field doesn't exist, the result will be an empty string, it won't get assigned
+                    io(field.type, presentFields.contains(field.name) ? "this." + field.name + " = " : "");
+                }
+            }
+
+            //throw exception on illegal revisions
+            ncont("else");
+            st("throw new IllegalArgumentException(\"Unknown revision '\" + REV + \"' for entity type '" + name + "'\")");
+            econt();
+        }
+    }
+
+    private void io(String type, String field) throws Exception{
+        if(BaseProcessor.isPrimitive(type)){
+            s(type.equals("boolean") ? "bool" : type.charAt(0) + "", field);
+        }else if(instanceOf(type, "mindustry.ctype.Content")){
             if(write){
                 s("s", field + ".id");
             }else{
-                st(field + " = mindustry.Vars.content.getByID(mindustry.ctype.ContentType.$L, read.s())", BaseProcessor.simpleName(type.toString()).toLowerCase().replace("type", ""));
+                st(field + "mindustry.Vars.content.getByID(mindustry.ctype.ContentType.$L, read.s())", BaseProcessor.simpleName(type).toLowerCase().replace("type", ""));
             }
-        }else if(serializer.writers.containsKey(type.toString()) && write){
-            st("$L(write, $L)", serializer.writers.get(type.toString()), field);
-        }else if(serializer.readers.containsKey(type.toString()) && !write){
-            st("$L = $L(read)", field, serializer.readers.get(type.toString()));
+        }else if(serializer.writers.containsKey(type) && write){
+            st("$L(write, $L)", serializer.writers.get(type), field);
+        }else if(serializer.readers.containsKey(type) && !write){
+            st("$L$L(read)", field, serializer.readers.get(type));
         }
     }
 
     private void cont(String text, Object... fmt){
-        builder.beginControlFlow(text, fmt);
+        method.beginControlFlow(text, fmt);
     }
 
-    private void cont(){
-        builder.endControlFlow();
+    private void econt(){
+        method.endControlFlow();
+    }
+
+    private void ncont(String text, Object... fmt){
+        method.nextControlFlow(text, fmt);
     }
 
     private void st(String text, Object... args){
-        builder.addStatement(text, args);
+        method.addStatement(text, args);
     }
 
     private void s(String type, String field){
         if(write){
-            builder.addStatement("write.$L($L)", type, field);
+            method.addStatement("write.$L($L)", type, field);
         }else{
-            builder.addStatement("$L = read.$L()", field, type);
+            method.addStatement("$Lread.$L()", field, type);
         }
     }
+
+    public static class Revision{
+        int version;
+        Array<RevisionField> fields;
+
+        Revision(int version, Array<RevisionField> fields){
+            this.version = version;
+            this.fields = fields;
+        }
+
+        Revision(){}
+
+        /** @return whether these two revisions are compatible */
+        boolean equal(Array<FieldSpec> specs){
+            if(fields.size != specs.size) return false;
+
+            for(int i = 0; i < fields.size; i++){
+                RevisionField field = fields.get(i);
+                FieldSpec spec = specs.get(i);
+                //TODO when making fields, their primitive size may be overwritten by an annotation; check for that
+                if(!(field.type.equals(spec.type.toString()) && (!spec.type.isPrimitive() || BaseProcessor.typeSize(spec.type.toString()) == field.size))){
+                    return false;
+                }
+            }
+            return true;
+        }
+    }
+
+    public static class RevisionField{
+        String name, type;
+        int size; //in bytes
+
+        RevisionField(String name, String type, int size){
+            this.name = name;
+            this.size = size;
+            this.type = type;
+        }
+
+        RevisionField(){}
+    }
 }
diff --git a/annotations/src/main/java/mindustry/annotations/entity/EntityProcess.java b/annotations/src/main/java/mindustry/annotations/entity/EntityProcess.java
index 8a39f97804..4affd92b7e 100644
--- a/annotations/src/main/java/mindustry/annotations/entity/EntityProcess.java
+++ b/annotations/src/main/java/mindustry/annotations/entity/EntityProcess.java
@@ -275,6 +275,8 @@ public class EntityProcess extends BaseProcessor{
                     .addModifiers(Modifier.PUBLIC)
                     .addStatement("return $S + $L", name + "#", "id").build());
 
+                EntityIO io = new EntityIO(type.name(), builder, serializer, rootDirectory.child("annotations/src/main/resources/revisions").child(name));
+
                 //add all methods from components
                 for(ObjectMap.Entry<String, Array<Smethod>> entry : methods){
                     if(entry.value.contains(m -> m.has(Replace.class))){
@@ -332,15 +334,7 @@ public class EntityProcess extends BaseProcessor{
                     //SPECIAL CASE: I/O code
                     //note that serialization is generated even for non-serializing entities for manual usage
                     if((first.name().equals("read") || first.name().equals("write")) && ann.genio()){
-                        Array<FieldSpec> fields = Array.with(builder.fieldSpecs).select(spec -> !spec.hasModifier(Modifier.TRANSIENT) && !spec.hasModifier(Modifier.STATIC) && !spec.hasModifier(Modifier.FINAL));
-                        fields.sortComparing(f -> f.name); //sort to keep order
-                        EntityIO writer = new EntityIO(mbuilder, first.name().equals("write"), serializer);
-                        //subclasses *have* to call this method
-                        mbuilder.addAnnotation(CallSuper.class);
-                        //write or read each non-transient field
-                        for(FieldSpec spec : fields){
-                            writer.io(spec.type, "this." + spec.name);
-                        }
+                        io.write(mbuilder, first.name().equals("write"));
                     }
 
                     for(Smethod elem : entry.value){
diff --git a/annotations/src/main/java/mindustry/annotations/util/TypeIOResolver.java b/annotations/src/main/java/mindustry/annotations/util/TypeIOResolver.java
index 450409f689..d0359ba51c 100644
--- a/annotations/src/main/java/mindustry/annotations/util/TypeIOResolver.java
+++ b/annotations/src/main/java/mindustry/annotations/util/TypeIOResolver.java
@@ -45,5 +45,9 @@ public class TypeIOResolver{
             this.writers = writers;
             this.readers = readers;
         }
+
+        public boolean has(String type){
+            return writers.containsKey(type) && readers.containsKey(type);
+        }
     }
 }
diff --git a/annotations/src/main/resources/classids.properties b/annotations/src/main/resources/classids.properties
deleted file mode 100644
index 3f4db7471a..0000000000
--- a/annotations/src/main/resources/classids.properties
+++ /dev/null
@@ -1,22 +0,0 @@
-#Maps entity names to IDs. Autogenerated.
-
-mindustry.entities.def.PuddleComp=12
-mindustry.entities.def.BulletComp=13
-dagger=7
-mindustry.entities.AllEntities.GenericBuilderDef=6
-mindustry.entities.AllEntities.BulletDef=0
-mindustry.entities.AllEntities.PlayerDef=4
-mindustry.entities.AllEntities.GroundEffectDef=9
-mindustry.entities.AllEntities.FireDef=11
-mindustry.entities.AllEntities.EffectDef=2
-mindustry.entities.AllEntities.GenericUnitDef=5
-mindustry.entities.AllEntities.TileDef=3
-vanguard=10
-mindustry.entities.def.PlayerComp=17
-dagger2=8
-mindustry.entities.def.DecalComp=14
-mindustry.entities.AllEntities.DecalDef=1
-mindustry.entities.def.StandardEffectComp=18
-mindustry.entities.def.TileComp=19
-mindustry.entities.def.FireComp=15
-mindustry.entities.def.GroundEffectComp=16
\ No newline at end of file
diff --git a/core/src/mindustry/entities/def/HealthComp.java b/core/src/mindustry/entities/def/HealthComp.java
index 18001d55df..c7bd269666 100644
--- a/core/src/mindustry/entities/def/HealthComp.java
+++ b/core/src/mindustry/entities/def/HealthComp.java
@@ -9,9 +9,10 @@ import mindustry.gen.*;
 abstract class HealthComp implements Entityc{
     static final float hitDuration = 9f;
 
+    float health;
     transient float hitTime;
-    float health, maxHealth = 1f;
-    boolean dead;
+    transient float maxHealth = 1f;
+    transient boolean dead;
 
     boolean isValid(){
         return !dead && isAdded();
diff --git a/server/src/mindustry/server/ServerControl.java b/server/src/mindustry/server/ServerControl.java
index 30196e9257..8dac088d17 100644
--- a/server/src/mindustry/server/ServerControl.java
+++ b/server/src/mindustry/server/ServerControl.java
@@ -732,7 +732,7 @@ public class ServerControl implements ApplicationListener{
             Core.app.post(() -> {
                 try{
                     SaveIO.load(file);
-                    state.rules.zone = null;
+                    state.rules.sector = null;
                     info("Save loaded.");
                     state.set(State.playing);
                     netServer.openServer();