[1.21.1 NeoForge] Incompatibility with Vercte's Satchels / YYZ's Backpack causes Accessories inventory to disconnect with IndexOutOfBoundsException

[Sijan079]

Environment

• Minecraft: 1.21.1
• NeoForge: 21.1.233
• Curios API: 9.5.1
• Accessories: 1.1.0-beta.53
• YYZ's Backpack: 21.1.11
• Vercte's Satchels: 1.2.0

Description

I found a compatibility issue between YYZ's Backpack and Vercte's Satchels.

Each mod works correctly on its own, but installing both causes the game to disconnect (singleplayer world exits with a Network Protocol Error) whenever the Accessories/inventory menu is opened.

The exception is an IndexOutOfBoundsException while handling the container_set_content packet.

Example:

java.lang.IndexOutOfBoundsException: Index 111 out of bounds for length 111

The crash occurs during:

AbstractContainerMenu.initializeContents()
ClientboundContainerSetContentPacket

The stack includes both mods' menu mixins:

satchels.mixins.json:menu.AbstractContainerMenuMixin
yyzsbackpack.mixins.json:AbstractContainerMenuMixin
yyzsbackpackcompat.mixins.json:AbstractContainerMenuSyncMixin

This suggests the client and server disagree on the final container size, resulting in one extra slot being synchronized.

Reproduction Steps

1. Install:
   • Accessories
   • Curios
   • YYZ's Backpack
   • Vercte's Satchels
2. Launch the game.
3. Create or load a world.
4. Open the Accessories/inventory screen.
5. The world immediately disconnects with a Network Protocol Error.

Troubleshooting Performed

I tested multiple combinations:

Configuration | Result -- | -- Accessories + Curios + YYZ | ✅ Works Accessories + Curios + Vercte's Satchels | ✅ Works Accessories + Curios + YYZ + Vercte | ❌ Crashes Removed Accessories Compatibility Layer | ❌ Still crashes Removed YYZ | ✅ Works Removed Vercte | ✅ Works

I also verified that:

• Updating YYZ's Backpack to 21.1.11 did not resolve the issue.
• Removing the Accessories Compatibility Layer does not resolve the issue.
• With Vercte removed, I can equip the YYZ backpack into the Accessories backpack slot normally.

Expected Behavior

Both mods should coexist and synchronize inventory/container contents correctly.

Actual Behavior

Opening the Accessories/inventory menu causes an immediate disconnect due to a container synchronization mismatch (IndexOutOfBoundsException).

Attachments

Previous disconnect reports showing the same issue with different slot counts (77 and 111)

disconnect-2026-07-20_15.00.40-client.txt

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I believe this is a compatibility issue rather than a bug in either mod individually, since each functions correctly when the other is absent. If there's any additional logging or a development build you'd like me to test, I'd be happy to help narrow it down further.