Use generics to ensure the correct class types stored inside the costs tags.

Mage.Sets/src/mage/cards/a/AlteredEgo.java

@@ -70,7 +70,7 @@ class AlteredEgoCopyApplier extends CopyApplier {
             // except it enters with an additional X +1/+1 counters on it
             blueprint.getAbilities().add(
                     new EntersBattlefieldAbility(new AddCountersSourceEffect(CounterType.P1P1.createInstance(
-                            (int)CardUtil.getSourceCostsTag(game, source, "X", 0)
+                            CardUtil.getSourceCostsTag(game, source, "X", 0)
                     )))
             );
 

Mage/src/main/java/mage/abilities/Ability.java

@@ -170,13 +170,6 @@ public interface Ability extends Controllable, Serializable {
      * Set tag to the value, initializes this ability's tags map if it is null
      */
     void setCostsTag(String tag, Object value);
-    /**
-     * Returns the value of the tag or defaultValue if the tag is not found in this ability's tag map
-     * does NOT check the source permanent's tags, use CardUtil.getSourceCostsTag for that
-     *
-     * @return The given tag value (or the default if not found)
-     */
-    Object getCostsTagOrDefault(String tag, Object defaultValue);
 
     /**
      * Retrieves the effects that are put into the place by the resolution of

Mage/src/main/java/mage/abilities/AbilityImpl.java

@@ -721,12 +721,6 @@ public abstract class AbilityImpl implements Ability {
         }
         costsTagMap.put(tag, value);
     }
-    public Object getCostsTagOrDefault(String tag, Object defaultValue){
-        if (costsTagMap != null && costsTagMap.containsKey(tag)){
-            return costsTagMap.get(tag);
-        }
-        return defaultValue;
-    }
 
     @Override
     public Effects getEffects() {

Mage/src/main/java/mage/abilities/dynamicvalue/common/GetKickerXValue.java

@@ -19,7 +19,7 @@ public enum GetKickerXValue implements DynamicValue {
     public int calculate(Game game, Ability sourceAbility, Effect effect) {
         // Currently identical logic to the Manacost X value
         // which should be fine since you can only have one X at a time
-        return (int) CardUtil.getSourceCostsTag(game, sourceAbility, "X", 0);
+        return CardUtil.getSourceCostsTag(game, sourceAbility, "X", 0);
     }
 
     @Override

Mage/src/main/java/mage/abilities/dynamicvalue/common/GetXLoyaltyValue.java

@@ -14,7 +14,7 @@ public enum GetXLoyaltyValue implements DynamicValue {
 
     @Override
     public int calculate(Game game, Ability sourceAbility, Effect effect) {
-        return (int) CardUtil.getSourceCostsTag(game, sourceAbility, "X", 0);
+        return CardUtil.getSourceCostsTag(game, sourceAbility, "X", 0);
     }
 
     @Override

Mage/src/main/java/mage/abilities/dynamicvalue/common/GetXValue.java

@@ -14,7 +14,7 @@ public enum GetXValue implements DynamicValue {
 
     @Override
     public int calculate(Game game, Ability sourceAbility, Effect effect) {
-        return (int) CardUtil.getSourceCostsTag(game, sourceAbility, "X", 0);
+        return CardUtil.getSourceCostsTag(game, sourceAbility, "X", 0);
     }
 
     @Override

Mage/src/main/java/mage/abilities/dynamicvalue/common/ManacostVariableValue.java

@@ -16,7 +16,7 @@ public enum ManacostVariableValue implements DynamicValue {
 
     @Override
     public int calculate(Game game, Ability sourceAbility, Effect effect) {
-        return (int) CardUtil.getSourceCostsTag(game, sourceAbility, "X", 0);
+        return CardUtil.getSourceCostsTag(game, sourceAbility, "X", 0);
     }
 
     @Override

Mage/src/main/java/mage/abilities/effects/common/EntersBattlefieldWithXCountersEffect.java

@@ -57,7 +57,7 @@ public class EntersBattlefieldWithXCountersEffect extends OneShotEffect {
             }
         }
         if (permanent != null) {
-            int amount = ((int) CardUtil.getSourceCostsTag(game, source, "X", 0)) * multiplier;
+            int amount = CardUtil.getSourceCostsTag(game, source, "X", 0) * multiplier;
             if (amount > 0) {
                 Counter counterToAdd = counter.copy();
                 counterToAdd.add(amount - counter.getCount());

Mage/src/main/java/mage/abilities/keyword/KickerAbility.java

@@ -137,7 +137,13 @@ public class KickerAbility extends StaticAbility implements OptionalAdditionalSo
         String finalActivationKey = getActivationKey(needKickerCost);
         Stream<Map.Entry<String, Object>> tagStream = costsTags.entrySet().stream()
                 .filter(x -> x.getKey().startsWith(finalActivationKey));
-        return tagStream.mapToInt(x -> (int)x.getValue()).sum();
+        return tagStream.mapToInt(x -> {
+                Object value = x.getValue();
+                if (!(value instanceof Integer)){
+                    throw new IllegalStateException("Wrong code usage: Kicker tag "+x.getKey()+" needs Integer but has "+(value==null?"null":value.getClass().getName()));
+                }
+                return (int) value;
+            }).sum();
     }
 
     /**
@@ -182,7 +188,7 @@ public class KickerAbility extends StaticAbility implements OptionalAdditionalSo
         game.fireEvent(GameEvent.getEvent(GameEvent.EventType.KICKED, source.getSourceId(), source, source.getControllerId()));
 
         String activationKey = getActivationKey(kickerCost.getText(true));
-        Integer next = (int)source.getCostsTagOrDefault(activationKey,0)+1;
+        Integer next = CardUtil.getSourceCostsTag(game, source, activationKey,0)+1;
         source.setCostsTag(activationKey,next);
     }
 

Mage/src/main/java/mage/abilities/keyword/SquadAbility.java

@@ -121,7 +121,7 @@ class SquadTriggerAbility extends EntersBattlefieldTriggeredAbility {
 
     @Override
     public boolean checkInterveningIfClause(Game game) {
-        int squadCount = (int)CardUtil.getSourceCostsTag(game, this, SquadAbility.SQUAD_ACTIVATION_VALUE_KEY,0);
+        int squadCount = CardUtil.getSourceCostsTag(game, this, SquadAbility.SQUAD_ACTIVATION_VALUE_KEY,0);
         return (squadCount > 0);
     }
     @Override
@@ -148,7 +148,7 @@ class SquadEffectETB extends OneShotEffect {
 
     @Override
     public boolean apply(Game game, Ability source) {
-        int squadCount = (int)CardUtil.getSourceCostsTag(game, source, SquadAbility.SQUAD_ACTIVATION_VALUE_KEY,0);
+        int squadCount = CardUtil.getSourceCostsTag(game, source, SquadAbility.SQUAD_ACTIVATION_VALUE_KEY,0);
         CreateTokenCopySourceEffect effect = new CreateTokenCopySourceEffect(squadCount);
         return effect.apply(game, source);
     }

Mage/src/main/java/mage/game/stack/StackAbility.java

@@ -408,10 +408,6 @@ public class StackAbility extends StackObjectImpl implements Ability {
         ability.setCostsTag(tag, value);
     }
     @Override
-    public Object getCostsTagOrDefault(String tag, Object defaultValue){
-        return ability.getCostsTagOrDefault(tag, defaultValue);
-    }
-    @Override
     public AbilityType getAbilityType() {
         return ability.getAbilityType();
     }

Mage/src/main/java/mage/util/CardUtil.java

@@ -1711,6 +1711,7 @@ public final class CardUtil {
     /**
      * Find a specific tag in the cost tags of either the source ability, or the permanent source of the ability.
      * Works in any moment (even before source ability activated)
+     * Do not use with null values, use checkSourceCostsTagExists instead
      *
      * @param game
      * @param source
@@ -1718,10 +1719,17 @@ public final class CardUtil {
      * @param defaultValue A default value to return if the tag is not found
      * @return The object stored by the tag if found, the default if not
      */
-    public static Object getSourceCostsTag(Game game, Ability source, String tag, Object defaultValue){
+    public static <T> T getSourceCostsTag(Game game, Ability source, String tag, T defaultValue){
         Map<String, Object> costTags = getSourceCostsTagsMap(game, source);
         if (costTags != null) {
-            return costTags.getOrDefault(tag, defaultValue);
+            Object value = costTags.getOrDefault(tag, defaultValue);
+            if (value == null) {
+                throw new IllegalStateException("Wrong code usage: Costs tag " + tag + " has value stored of type null but is trying to be read. Use checkSourceCostsTagExists");
+            }
+            if (value.getClass() != defaultValue.getClass()) {
+                throw new IllegalStateException("Wrong code usage: Costs tag " + tag + " has value stored of type " + value.getClass().getName() + " different from default of type " + defaultValue.getClass().getName());
+            }
+            return (T) value;
         }
         return defaultValue;
     }