IllegalStateException: BlockCanBuildEvent may only be triggered synchronously.

[Zoriot]

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

[PierreSchwang]

I've not reproduced the issue, but based on the NMS code it checks out. If the item is a BlockItem, useOn will attempt to place the block and call the event. Similar things happen with other items, for example axes (for stripping logs).

Not sure if simply executing on the main thread is the correct approach here though (or if there is even another sane solution)