Index: binaries/data/mods/public/simulation/components/Foundation.js
===================================================================
--- binaries/data/mods/public/simulation/components/Foundation.js
+++ binaries/data/mods/public/simulation/components/Foundation.js
@@ -75,6 +75,16 @@
 	return Math.floor(this.GetBuildProgress() * 100);
 };
 
+/**
+ * Returns the current builders.
+ *
+ * @return {number[]} An array containing the entity IDs of assigned builders.
+ */
+Foundation.prototype.GetBuilders = function()
+{
+	return Array.from(this.builders.keys());
+};
+
 Foundation.prototype.GetNumBuilders = function()
 {
 	return this.builders.size;
Index: binaries/data/mods/public/simulation/components/Repairable.js
===================================================================
--- binaries/data/mods/public/simulation/components/Repairable.js
+++ binaries/data/mods/public/simulation/components/Repairable.js
@@ -33,6 +33,16 @@
 	return hitpoints / maxHitpoints;
 };
 
+/**
+ * Returns the current builders.
+ *
+ * @return {number[]} An array containing the entity IDs of assigned builders.
+ */
+Repairable.prototype.GetBuilders = function()
+{
+	return Array.from(this.builders.keys());
+};
+
 Repairable.prototype.GetNumBuilders = function()
 {
 	return this.builders.size;
Index: binaries/data/mods/public/simulation/components/UnitAI.js
===================================================================
--- binaries/data/mods/public/simulation/components/UnitAI.js
+++ binaries/data/mods/public/simulation/components/UnitAI.js
@@ -2687,37 +2687,41 @@
 
 					this.order.data.force = false;
 
-					this.repairTarget = this.order.data.target;	// temporary, deleted in "leave".
-					// Check we can still reach and repair the target
-					if (!this.CanRepair(this.repairTarget))
+					let target = this.order.data.target;
+
+					// Needed to remove the entity from the builder list when leaving this state.
+					this.repairTarget = target;
+
+					// Check we can still reach and repair the target.
+					if (!this.CanRepair(target))
 					{
-						// Can't reach it, no longer owned by ally, or it doesn't exist any more
+						// Can't reach it, no longer owned by ally, or it doesn't exist any more.
 						this.FinishOrder();
 						return true;
 					}
 
-					if (!this.CheckTargetRange(this.repairTarget, IID_Builder))
+					if (!this.CheckTargetRange(target, IID_Builder))
 					{
 						this.SetNextState("APPROACHING");
 						return true;
 					}
-					// Check if the target is still repairable
-					var cmpHealth = Engine.QueryInterface(this.repairTarget, IID_Health);
+					// Check if the target is still repairable.
+					let cmpHealth = Engine.QueryInterface(target, IID_Health);
 					if (cmpHealth && cmpHealth.GetHitpoints() >= cmpHealth.GetMaxHitpoints())
 					{
 						// The building was already finished/fully repaired before we arrived;
 						// let the ConstructionFinished handler handle this.
-						this.OnGlobalConstructionFinished({"entity": this.repairTarget, "newentity": this.repairTarget});
+						this.OnGlobalConstructionFinished({"entity": target, "newentity": target});
 						return true;
 					}
 
 					this.StopMoving();
 
-					let cmpBuilderList = QueryBuilderListInterface(this.repairTarget);
+					let cmpBuilderList = QueryBuilderListInterface(target);
 					if (cmpBuilderList)
 						cmpBuilderList.AddBuilder(this.entity);
 
-					this.FaceTowardsTarget(this.order.data.target);
+					this.FaceTowardsTarget(target);
 
 					this.SelectAnimation("build");
 					this.StartTimer(1000, 1000);
@@ -2734,24 +2738,29 @@
 				},
 
 				"Timer": function(msg) {
-					// Check we can still reach and repair the target
-					if (!this.CanRepair(this.repairTarget))
+					let target = this.order.data.target;
+
+					// Needed to remove the entity from the builder list when leaving this state.
+					this.repairTarget = target;
+
+					// Check we can still reach and repair the target.
+					if (!this.CanRepair(target))
 					{
-						// No longer owned by ally, or it doesn't exist any more
+						// No longer owned by ally, or it doesn't exist any more.
 						this.FinishOrder();
 						return;
 					}
 
-					this.FaceTowardsTarget(this.order.data.target);
+					this.FaceTowardsTarget(target);
 
 					let cmpBuilder = Engine.QueryInterface(this.entity, IID_Builder);
-					cmpBuilder.PerformBuilding(this.repairTarget);
-					// if the building is completed, the leave() function will be called
-					// by the ConstructionFinished message
-					// in that case, the repairTarget is deleted, and we can just return
+					cmpBuilder.PerformBuilding(target);
+					// If the building is completed, the leave() function will be called
+					// by the ConstructionFinished message.
+					// In that case, the repairTarget is deleted, and we can just return.
 					if (!this.repairTarget)
 						return;
-					if (!this.CheckTargetRange(this.repairTarget, IID_Builder))
+					if (!this.CheckTargetRange(target, IID_Builder))
 						this.SetNextState("APPROACHING");
 				},
 			},
Index: binaries/data/mods/public/simulation/components/tests/test_Pack.js
===================================================================
--- binaries/data/mods/public/simulation/components/tests/test_Pack.js
+++ binaries/data/mods/public/simulation/components/tests/test_Pack.js
@@ -4,12 +4,14 @@
 Engine.LoadHelperScript("ValueModification.js");
 Engine.LoadComponentScript("interfaces/ModifiersManager.js");
 Engine.LoadComponentScript("interfaces/Capturable.js");
+Engine.LoadComponentScript("interfaces/Foundation.js");
 Engine.LoadComponentScript("interfaces/GarrisonHolder.js");
 Engine.LoadComponentScript("interfaces/Guard.js");
 Engine.LoadComponentScript("interfaces/Health.js");
 Engine.LoadComponentScript("interfaces/Pack.js");
 Engine.LoadComponentScript("interfaces/Player.js");
 Engine.LoadComponentScript("interfaces/Promotion.js");
+Engine.LoadComponentScript("interfaces/Repairable.js");
 Engine.LoadComponentScript("interfaces/ResourceGatherer.js");
 Engine.LoadComponentScript("interfaces/Timer.js");
 Engine.LoadComponentScript("interfaces/UnitAI.js");
Index: binaries/data/mods/public/simulation/helpers/Transform.js
===================================================================
--- binaries/data/mods/public/simulation/helpers/Transform.js
+++ binaries/data/mods/public/simulation/helpers/Transform.js
@@ -53,6 +53,12 @@
 		cmpNewHealth.SetHitpoints(cmpNewHealth.GetMaxHitpoints() * healthLevel);
 	}
 
+	let cmpBuilderList = QueryBuilderListInterface(oldEnt);
+	let cmpNewBuilderList = QueryBuilderListInterface(newEnt);
+	if (cmpBuilderList && cmpNewBuilderList)
+		for (let builder of cmpBuilderList.GetBuilders())
+			cmpNewBuilderList.AddBuilder(builder);
+
 	var cmpUnitAI = Engine.QueryInterface(oldEnt, IID_UnitAI);
 	var cmpNewUnitAI = Engine.QueryInterface(newEnt, IID_UnitAI);
 	if (cmpUnitAI && cmpNewUnitAI)