[1.21.11 → 26.2] EnderChest interaction causes DecoderException (unrecognized NBT tag ID -35)

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General Information

Issue: EnderChest packet decode error when connecting 1.21.11 client to 26.2 server

Summary
When connecting a 1.21.11 client to a 26.2 server via ViaFabricPlus, any Ender Chest interaction (or logging in with non-empty Ender Chest) causes immediate disconnection:

Internal Exception: io.netty.handler.codec.DecoderException: Failed to decode packet 'clientbound/minecraft:custom_payload'

Root cause (from stack trace)
The client's NBT decoder fails with:
Caused by: java.io.IOException: Invalid tag id: -35

text
This indicates the server is sending NBT data with tag ID -35, which the 1.21.11 client does not recognize. This is a core version incompatibility between 26.2 and 1.21.11 NBT formats.

Important: This occurs even with no mods installed – simply placing an item in an Ender Chest on a vanilla 26.2 server triggers the same error. The issue is NOT mod-related.

Environment

• Client: Minecraft 1.21.11 Fabric (ViaFabricPlus 4.4.14, latest dev also tested)
• Server: Minecraft 26.2 Fabric (vanilla behavior also affected)
• Client mods (for reference): fabric-api 0.141.4, sodium, quickshulker 3.0.1, inventoryprofilesnext, litematica, tweakeroo, Jade, carpet-ams-addition, carpet-tis-addition, syncmatica, etc.
• Server mods (for reference): fabric-api 0.152.2, fabric-carpet 26.2, quickshulker 3.0.2, servux, syncmatica, carpet extensions, etc.

Reproduce

1. Join a 26.2 server from a 1.21.11 client.
2. Open Ender Chest and place any item.
3. Server sends Ender Chest sync packet.
4. Client fails to decode tag ID -35 and disconnects.

Full error stack
[16:50:50] [Netty NIO IO #2/ERROR] [ViaFabricPlus]: An exception occurred while handling a packet
io.netty.handler.codec.DecoderException: Failed to decode packet 'clientbound/minecraft:custom_payload'
...
Caused by: java.io.IOException: Invalid tag id: -35
at net.minecraft.class_4614$1.method_39878(class_4614.java:54)
...
at net.minecraft.class_2507.method_52892(class_2507.java:192)

text

Request
Please review the NBT tag mapping for 26.2 → 1.21.11, specifically tag ID -35 which appears to be invalid in the older version. The 26.2 Data Component system may have introduced new NBT tags that the current translation layer does not remap.

Bug Description

Steps to Reproduce

1. Launch Minecraft 1.21.11 client (Fabric or Vanilla) with ViaFabricPlus latest version (4.4.14 or a newer dev build).
2. Connect to a 26.2 server (Vanilla or Fabric – no extra mods required).
3. Once in-game, open the Ender Chest.
4. Place any item (e.g., a stone) into the Ender Chest.
5. Wait 1–2 seconds – the client disconnects immediately with the error.

(If the Ender Chest already contains items before login, the disconnection may happen instantly upon entering the world.)

Expected Behavior

The client should successfully decode the Ender Chest data sent by the server and remain connected without any error.

[Barvalg]

[Kichura]

Cannot reproduce on vanilla server, This seems to be caused by server mods / corrupted inventory sending back garbage data.