diff --git a/core/src/mindustry/ai/RtsAI.java b/core/src/mindustry/ai/RtsAI.java
index 56643587b3..7242afbfe8 100644
--- a/core/src/mindustry/ai/RtsAI.java
+++ b/core/src/mindustry/ai/RtsAI.java
@@ -33,6 +33,7 @@ public class RtsAI{
     //in order of priority??
     static final BlockFlag[] flags = {BlockFlag.generator, BlockFlag.factory, BlockFlag.core, BlockFlag.battery};
     static final ObjectFloatMap<Building> weights = new ObjectFloatMap<>();
+    //TODO configurable, perhaps
     static final int minSquadSize = 4;
     //TODO max squad size
     static final boolean debug = OS.hasProp("mindustry.debug");
diff --git a/core/src/mindustry/entities/comp/BuildingComp.java b/core/src/mindustry/entities/comp/BuildingComp.java
index b4a628a39d..156c303032 100644
--- a/core/src/mindustry/entities/comp/BuildingComp.java
+++ b/core/src/mindustry/entities/comp/BuildingComp.java
@@ -70,7 +70,7 @@ abstract class BuildingComp implements Posc, Teamc, Healthc, Buildingc, Timerc,
     transient boolean wasDamaged; //used only by the indexer
     transient float visualLiquid;
 
-    /** TODO Each bit corresponds to a team ID. Only 64 are supported. */
+    /** TODO Each bit corresponds to a team ID. Only 64 are supported. Does not work on servers. */
     transient long visibleFlags;
     transient boolean wasVisible; //used only by the block renderer when fog is on (TODO replace with discovered check?)
 
@@ -165,10 +165,12 @@ abstract class BuildingComp implements Posc, Teamc, Healthc, Buildingc, Timerc,
     //region io
 
     public final void writeBase(Writes write){
+        boolean writeVisibility = state.rules.fog && visibleFlags != 0;
+
         write.f(health);
         write.b(rotation | 0b10000000);
         write.b(team.id);
-        write.b(3); //version
+        write.b(writeVisibility ? 4 : 3); //version
         write.b(enabled ? 1 : 0);
         //write presence of items/power/liquids/cons, so removing/adding them does not corrupt future saves.
         write.b(moduleBitmask());
@@ -179,6 +181,11 @@ abstract class BuildingComp implements Posc, Teamc, Healthc, Buildingc, Timerc,
         //efficiency is written as two bytes to save space
         write.b((byte)(Mathf.clamp(efficiency) * 255f));
         write.b((byte)(Mathf.clamp(optionalEfficiency) * 255f));
+
+        //only write visibility when necessary, saving 8 bytes - implies new version
+        if(writeVisibility){
+            write.l(visibleFlags);
+        }
     }
 
     public final void readBase(Reads read){
@@ -220,6 +227,11 @@ abstract class BuildingComp implements Posc, Teamc, Healthc, Buildingc, Timerc,
             efficiency = read.ub() / 255f;
             optionalEfficiency = read.ub() / 255f;
         }
+
+        //version 4 (and only 4 at the moment) has visibility flags
+        if(version == 4){
+            visibleFlags = read.l();
+        }
     }
 
     public int moduleBitmask(){
diff --git a/core/src/mindustry/graphics/BlockRenderer.java b/core/src/mindustry/graphics/BlockRenderer.java
index 489a2117ed..8499849c98 100644
--- a/core/src/mindustry/graphics/BlockRenderer.java
+++ b/core/src/mindustry/graphics/BlockRenderer.java
@@ -83,7 +83,7 @@ public class BlockRenderer{
                     updateFloors.add(new UpdateRenderState(tile, tile.floor()));
                 }
 
-                if(tile.build != null && (tile.team() == player.team() || !state.rules.fog)){
+                if(tile.build != null && (tile.team() == player.team() || !state.rules.fog || (tile.build.visibleFlags & (1L << player.team().id)) != 0)){
                     tile.build.wasVisible = true;
                 }
 
@@ -416,6 +416,7 @@ public class BlockRenderer{
                             updateShadow(build);
                             renderer.minimap.update(tile);
                         }
+                        build.visibleFlags |= (1L << player.team().id);
                         build.wasVisible = true;
                     }