Crash due to attachment sync. Received attachment for unknown target

[MehVahdJukaar]

When using the new synced attachment API I often get this.

22:50:04] [Render thread/ERROR]: Error accepting attachment changes
net.fabricmc.fabric.impl.attachment.sync.AttachmentSyncException: Received attachment change for unknown target!

Does it need to hard fail like that? If the target clearly exists on the server given the attachment is accessed there

[modmuss50]

I think it should fail hard, as it likely means there is a more concerning issue elsewhere. Do you know what target its trying to sync to? Or have a way to reproduce this with a minimal amount of code.

[MehVahdJukaar]

I'm only executing some logic on entity tick, getJumpBoostPower entity method and when an entity is hit by another entity.
I dont see how this could be part of a bigger issue also because a very similar implementation works on neo, which I'm only bringing up since the 2 impls are suspiciously similar which makes me think they were made by the same person.

Anyways one crucual difference is that you dont have control for when stuff is synced here, merely setting the attachment sends the synced packet so i dont even know what one could do to prevent this.

Moreover wouldnt this issue happen all the time when a server entity data is accessed? since client doesnt have most of the server entities as they could be in far away chunks or newly added one and hence the client would fail to find it

[modmuss50]

you dont have control for when stuff is synced here,

You do, use a custom AttachmentSyncPredicate

since client doesnt have most of the server entities as they could be in far away chunks

We only sync the attachment changes to targets that are being tracked by the client. We shouldnt be sending changes to the client that will be ingnored, the data shouldnt be sent in the first place.

I really need some more info or a way to reproduce the issue you are having.

[MehVahdJukaar]

I am working on that. I haven't been able to replicate it myself. People reporting it crashing with just my mods. Still idk what I could be doing wrong, all I'm doing is get and set attachment operations

[MehVahdJukaar]

I can't replicate on my own. People here claimed they can trigger this after having just update my mods which replaced a mixin based impl with a simple data attachment https://github.com/user-attachments/files/23111749/latest.log

I believe that whenever underlying issue might be here should be logged better since it's extremely hard to track down given it happens in connection. Also doesn't happen on neo which had a very similar API at least

[modmuss50]

Attachment identifier: supplementaries:slimed_data
World: minecraft:overworld
Target type: Entity
Entity network ID: 169669

Unfortually this is basically all the info we have, it would be helpful to know what entity has 169669 as its id on the server. Do you set your attachement on all entities or just specific ones?

fabric/fabric-data-attachment-api-v1/src/main/java/net/fabricmc/fabric/mixin/attachment/EntityMixin.java

Lines 75 to 85 in c920b08

	if ((Object) this instanceof ServerPlayerEntity self && predicate.test(this, self)) {
	// Players do not track themselves
	AttachmentSync.trySync(payload, self);
	}
	PlayerLookup.tracking((Entity) (Object) this)
	.forEach(player -> {
	if (predicate.test(this, player)) {
	AttachmentSync.trySync(payload, player);
	}
	});

You can see in the above code that we should only be sending these packets to clients that are tracking this entity. As far as I know there shouldnt be a case where the server thinks the entity is being tracked, but the client doesn't. Maybe there is a race when the entity is being untracked?

[MehVahdJukaar]

I set it to all living entities, on first tick they will basically be all initialized

[MehVahdJukaar]

Could it be a mod adding a server only entity?

[modmuss50]

Could it be a mod adding a server only entity?

Quite possibly yes, the vast amount of mods this user has doesnt make it easy to figure out whats going on. There are other logs from somewhere (A quick github search suggests the accessories mod) that suggest similar issue:

Unable to Sync Container Data for a given Entity as it is null on the Client! [EntityId: 169671]

I notice they are in single player so it wouldnt be impossible to get more info from the intergrated server about this to improve the logging. 🤔

[MehVahdJukaar]

I'll get to the bottom of this. Printing all the entities IDs on server and hoping they match the one on the client

[MehVahdJukaar]

I'm working on getting that. This couldnt be caused by entities that are not adde to the level? that would be something totaly valid that mods might do

[MehVahdJukaar]

here's another log. so many entities ID are mentioned which makes me thing all entities are affected.

latest - 2025-10-25T030444.363.log
Really the only thing i'm doing is accessing those data, i dont have anything weird going on so i really dont see how it could be my issue. Idk how another mod could affect this either

[MehVahdJukaar]

@modmuss50 I found a setup to replicate: Synced attachment, get using a mixin in LivingEntity "tick" method + cobblemod mod (for some reason, I dont think its exclusive to this one as Ive heard it happen without) + create a new world, fly around and relog.

[MehVahdJukaar]

Any update on this?