IllegalStateException: BlockCanBuildEvent may only be triggered synchronously.

Server Implementation

Paper

Server Version

1.21.10

Describe the bug

2026-08-05 02:53:09.894    
Zoriot issued server command: //mask mossy_stone_bricks
Zoriot issued server command: //brush apply sphere item red_wool
Zoriot issued server command: //gmask #air
Error occurred executing player action
ava.lang.IllegalStateException: BlockCanBuildEvent may only be triggered synchronously.
    at io.papermc.paper.plugin.manager.PaperEventManager.callEvent(PaperEventManager.java:42) ~[paper-1.21.10.jar:1.21.10-130-8043efd]
    at io.papermc.paper.plugin.manager.PaperPluginManagerImpl.callEvent(PaperPluginManagerImpl.java:131) ~[paper-1.21.10.jar:1.21.10-130-8043efd]
    at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:628) ~[paper-api-1.21.10-R0.1-SNAPSHOT.jar:?]
    at net.minecraft.world.item.BlockItem.canPlace(BlockItem.java:180) ~[paper-1.21.10.jar:1.21.10-130-8043efd]
    at net.minecraft.world.item.BlockItem.getPlacementState(BlockItem.java:152) ~[paper-1.21.10.jar:1.21.10-130-8043efd]
    at net.minecraft.world.item.BlockItem.place(BlockItem.java:58) ~[paper-1.21.10.jar:1.21.10-130-8043efd]
    at net.minecraft.world.item.BlockItem.useOn(BlockItem.java:42) ~[paper-1.21.10.jar:1.21.10-130-8043efd]
    at net.minecraft.world.item.ItemStack.useOn(ItemStack.java:391) ~[paper-1.21.10.jar:1.21.10-130-8043efd]
    at FastAsyncWorldEdit-Paper-2.15.3.jar/com.sk89q.worldedit.bukkit.adapter.ext.fawe.v1_21_9.PaperweightAdapter.simulateItemUse(PaperweightAdapter.java:679) ~[FastAsyncWorldEdit-Paper-2.15.3.jar:?]
    at FastAsyncWorldEdit-Paper-2.15.3.jar/com.fastasyncworldedit.bukkit.adapter.IDelegateBukkitImplAdapter.simulateItemUse(IDelegateBukkitImplAdapter.java:96) ~[FastAsyncWorldEdit-Paper-2.15.3.jar:?]
    at FastAsyncWorldEdit-Paper-2.15.3.jar/com.sk89q.worldedit.bukkit.BukkitWorld.useItem(BukkitWorld.java:670) ~[FastAsyncWorldEdit-Paper-2.15.3.jar:?]
    at FastAsyncWorldEdit-Paper-2.15.3.jar/com.fastasyncworldedit.core.wrappers.WorldWrapper.useItem(WorldWrapper.java:97) ~[FastAsyncWorldEdit-Paper-2.15.3.jar:?]

To Reproduce

Not sure yet

Expected behaviour

No exception

Screenshots / Videos

No response

Error log (if applicable)

No response

Fawe Debugpaste

https://athion.net/ISPaster/paste/view/941d03c7ecee4db38d7d942474f31c30 (not this run)

Are you interested in contributing a fix?

No

Start by investigating the reported command sequence and stack trace, then read BukkitWorld.java at the useItem entry point and follow the calls through IDelegateBukkitImplAdapter.java and PaperweightAdapter.java. Confirm a reproducible case and define done as completing the reported actions without the synchronous BlockCanBuildEvent exception.