Fix: prevent pristine tool stack from re-merging with damaged tool on hurtAndBreak/damage

YoannDev90 · YoannDev90 · commit 922c8007197b · 2026-06-20T13:04:29.000+02:00
When a stacked tool takes damage, the mixin now temporarily marks the
original stack as damaged (damage=1) before creating and inserting the
leftover pristine tools. This prevents InventoryMixin.onInsertStack
from merging the pristine leftovers back into the original stack via
canStackItems. The damage is reset to 0 immediately after insertion,
so the original hurtAndBreak/damage method applies it normally.

Fixes the bug where all tools in a stack would take damage instead of
only the single tool being used.
diff --git a/src/mc-1.20-1.20.3/kotlin/stackabletools/mixin/ItemStackMixin.kt b/src/mc-1.20-1.20.3/kotlin/stackabletools/mixin/ItemStackMixin.kt
@@ -26,6 +26,8 @@ abstract class ItemStackMixin {
 
         processingDamage = true
         try {
+            stack.damage = 1
+
             val leftover = stack.copy()
             leftover.count = stack.count - 1
             leftover.damage = 0
@@ -36,6 +38,8 @@ abstract class ItemStackMixin {
             if (!entity.inventory.insertStack(leftover)) {
                 entity.dropItem(leftover, false)
             }
+
+            stack.damage = 0
         } finally {
             processingDamage = false
         }
diff --git a/src/mc-1.20.4/kotlin/stackabletools/mixin/ItemStackMixin.kt b/src/mc-1.20.4/kotlin/stackabletools/mixin/ItemStackMixin.kt
@@ -26,6 +26,8 @@ abstract class ItemStackMixin {
 
         processingDamage = true
         try {
+            stack.damage = 1
+
             val leftover = stack.copy()
             leftover.count = stack.count - 1
             leftover.damage = 0
@@ -36,6 +38,8 @@ abstract class ItemStackMixin {
             if (!entity.inventory.insertStack(leftover)) {
                 entity.dropItem(leftover, false)
             }
+
+            stack.damage = 0
         } finally {
             processingDamage = false
         }
diff --git a/src/mc-1.20.5-1.20.6/kotlin/stackabletools/mixin/ItemStackMixin.kt b/src/mc-1.20.5-1.20.6/kotlin/stackabletools/mixin/ItemStackMixin.kt
@@ -27,6 +27,8 @@ abstract class ItemStackMixin {
 
         processingDamage = true
         try {
+            stack.damage = 1
+
             val leftover = stack.copy()
             leftover.count = stack.count - 1
             leftover.damage = 0
@@ -37,6 +39,8 @@ abstract class ItemStackMixin {
             if (!entity.inventory.insertStack(leftover)) {
                 entity.dropItem(leftover, false)
             }
+
+            stack.damage = 0
         } finally {
             processingDamage = false
         }
diff --git a/src/mc-1.21.0-1.21.3/kotlin/stackabletools/mixin/ItemStackMixin.kt b/src/mc-1.21.0-1.21.3/kotlin/stackabletools/mixin/ItemStackMixin.kt
@@ -33,6 +33,7 @@ abstract class ItemStackMixin {
                     val countBefore = stack.count
 
                     stack.count = 1
+                    stack.damage = 1
 
                     val leftover = stack.copy()
                     leftover.count = countBefore - 1
@@ -43,6 +44,8 @@ abstract class ItemStackMixin {
                     if (!player.inventory.insertStack(leftover)) {
                         player.dropItem(leftover, false)
                     }
+
+                    stack.damage = 0
                 } finally {
                     isProcessingDamageInternal.set(false)
                 }
diff --git a/src/mc-1.21.11/kotlin/stackabletools/mixin/ItemStackMixin.kt b/src/mc-1.21.11/kotlin/stackabletools/mixin/ItemStackMixin.kt
@@ -34,6 +34,7 @@ abstract class ItemStackMixin {
                     val countBefore = stack.count
 
                     stack.count = 1
+                    stack.damage = 1
 
                     val leftover = stack.copy()
                     leftover.count = countBefore - 1
@@ -44,6 +45,8 @@ abstract class ItemStackMixin {
                     if (!player.inventory.insertStack(leftover)) {
                         player.dropItem(leftover, false)
                     }
+
+                    stack.damage = 0
                 } finally {
                     isProcessingDamageInternal.set(false)
                 }
diff --git a/src/mc-1.21.4/kotlin/stackabletools/mixin/ItemStackMixin.kt b/src/mc-1.21.4/kotlin/stackabletools/mixin/ItemStackMixin.kt
@@ -33,6 +33,7 @@ abstract class ItemStackMixin {
                     val countBefore = stack.count
 
                     stack.count = 1
+                    stack.damage = 1
 
                     val leftover = stack.copy()
                     leftover.count = countBefore - 1
@@ -43,6 +44,8 @@ abstract class ItemStackMixin {
                     if (!player.inventory.insertStack(leftover)) {
                         player.dropItem(leftover, false)
                     }
+
+                    stack.damage = 0
                 } finally {
                     isProcessingDamageInternal.set(false)
                 }
diff --git a/src/mc-1.21.5-1.21.8/kotlin/stackabletools/mixin/ItemStackMixin.kt b/src/mc-1.21.5-1.21.8/kotlin/stackabletools/mixin/ItemStackMixin.kt
@@ -33,6 +33,7 @@ abstract class ItemStackMixin {
                     val countBefore = stack.count
 
                     stack.count = 1
+                    stack.damage = 1
 
                     val leftover = stack.copy()
                     leftover.count = countBefore - 1
@@ -43,6 +44,8 @@ abstract class ItemStackMixin {
                     if (!player.inventory.insertStack(leftover)) {
                         player.dropItem(leftover, false)
                     }
+
+                    stack.damage = 0
                 } finally {
                     isProcessingDamageInternal.set(false)
                 }
diff --git a/src/mc-1.21.9-1.21.10/kotlin/stackabletools/mixin/ItemStackMixin.kt b/src/mc-1.21.9-1.21.10/kotlin/stackabletools/mixin/ItemStackMixin.kt
@@ -34,6 +34,7 @@ abstract class ItemStackMixin {
                     val countBefore = stack.count
 
                     stack.count = 1
+                    stack.damage = 1
 
                     val leftover = stack.copy()
                     leftover.count = countBefore - 1
@@ -44,6 +45,8 @@ abstract class ItemStackMixin {
                     if (!player.inventory.insertStack(leftover)) {
                         player.dropItem(leftover, false)
                     }
+
+                    stack.damage = 0
                 } finally {
                     isProcessingDamageInternal.set(false)
                 }
diff --git a/src/mc-26.1/kotlin/stackabletools/mixin/ItemStackMixin.kt b/src/mc-26.1/kotlin/stackabletools/mixin/ItemStackMixin.kt
@@ -33,6 +33,7 @@ abstract class ItemStackMixin {
                     val countBefore = stack.count
 
                     stack.count = 1
+                    stack.damageValue = 1
 
                     val leftover = stack.copy()
                     leftover.count = countBefore - 1
@@ -43,6 +44,8 @@ abstract class ItemStackMixin {
                     if (!player.inventory.add(leftover)) {
                         player.drop(leftover, false)
                     }
+
+                    stack.damageValue = 0
                 } finally {
                     isProcessingDamageInternal.set(false)
                 }

Original file line number	Diff line number	Diff line change
`@@ -33,6 +33,7 @@ abstract class ItemStackMixin {`
`33`	`33`	`val countBefore = stack.count`
`34`	`34`
`35`	`35`	`stack.count = 1`
	`36`	`+ stack.damage = 1`
`36`	`37`
`37`	`38`	`val leftover = stack.copy()`
`38`	`39`	`leftover.count = countBefore - 1`
`@@ -43,6 +44,8 @@ abstract class ItemStackMixin {`
`43`	`44`	`if (!player.inventory.insertStack(leftover)) {`
`44`	`45`	`player.dropItem(leftover, false)`
`45`	`46`	`}`
	`47`	`+`
	`48`	`+ stack.damage = 0`
`46`	`49`	`} finally {`
`47`	`50`	`isProcessingDamageInternal.set(false)`
`48`	`51`	`}`
Original file line number	Diff line number	Diff line change
`@@ -34,6 +34,7 @@ abstract class ItemStackMixin {`
`34`	`34`	`val countBefore = stack.count`
`35`	`35`
`36`	`36`	`stack.count = 1`
	`37`	`+ stack.damage = 1`
`37`	`38`
`38`	`39`	`val leftover = stack.copy()`
`39`	`40`	`leftover.count = countBefore - 1`
`@@ -44,6 +45,8 @@ abstract class ItemStackMixin {`
`44`	`45`	`if (!player.inventory.insertStack(leftover)) {`
`45`	`46`	`player.dropItem(leftover, false)`
`46`	`47`	`}`
	`48`	`+`
	`49`	`+ stack.damage = 0`
`47`	`50`	`} finally {`
`48`	`51`	`isProcessingDamageInternal.set(false)`
`49`	`52`	`}`