Inventory Weight

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Sophisticated Backpacks Inventory Weight is an add-on that connects Sophisticated Backpacks with MT Inventory Weight.

It allows accurate weight calculation for Sophisticated Backpacks by reading the actual contents stored in the world's BackpackStorage (via UUID) rather than the limited NBT data on the item stack.

Generic container NBT readers often under-count or mis-count the weight of Sophisticated Backpacks. This add-on fixes that by using the Sophisticated Backpacks storage API.

It registers a high-priority Inventory Weight item weight provider so Sophisticated Backpacks items are handled accurately while all other items continue to use default behavior.

Required Mods ​

Install these mods:

How It Works ​

MT Inventory Weight supports custom item weight providers via the Inventory Weight entrypoint.

Sophisticated Backpacks Inventory Weight registers its provider at priority 8500 (higher than the built-in generic backpacks provider at 8000).

When MT Inventory Weight asks for the weight of an item:

  1. The provider checks if the item belongs to Sophisticated Backpacks.
  2. For real backpacks, it reads the actual contents using the Fabric Transfer API (which resolves the UUID-backed BackpackStorage).
  3. It recurses into the contents using context.nested() so nested containers, datapack weights, and other add-ons are respected.
  4. For other Sophisticated Backpacks items (upgrades, etc.), it returns a small fixed weight.

If Sophisticated Backpacks is not present, the provider stays inactive and MT Inventory Weight falls back to its normal behavior.

No configuration file is needed. All behavior is automatic once the required mods are installed.

Weight Model ​

The add-on matches the built-in backpack calculation used by MT Inventory Weight:

effectiveWeight = emptyBackpackWeight + (contentsWeight / 2)
  • contentsWeight is the summed weight of everything inside the backpack (calculated recursively).
  • The returned WeightResult carries both the effective (halved) weight and the full contentsWeight for display purposes.
  • The empty backpack base weight is currently a constant (1.0 by default).

Client Tooltip Behavior ​

A client mixin ensures that only actual backpacks from Sophisticated Backpacks are treated as weight containers in tooltips.

Non-backpack Sophisticated Backpacks items (such as upgrades) will not incorrectly show container weight information.

Configuration ​

The empty backpack weight is currently hardcoded to 1.0.

Future versions may expose this value through a config system (such as fzzy_config).

Examples ​

Default Behavior ​

A Sophisticated Backpack with 10 kg of contents (after all inner weights are calculated):

effectiveWeight = 1.0 + (10 / 2) = 6.0

The backpack contributes 6.0 to carried weight, while the full 10.0 contents weight is recorded for display and overload calculations.

Nested Backpacks ​

Nested backpacks are handled safely because the provider uses context.nested() when recursing, preventing infinite loops.

Troubleshooting ​

Sophisticated Backpacks items show incorrect weight ​

Make sure both Sophisticated Backpacks (Unofficial Fabric port) and this add-on are installed on the same side (server or client) as MT Inventory Weight.

The add-on does nothing ​

Check the server/client log for messages from sb-Inventory Weight.

If Sophisticated Backpacks is missing, the add-on will log that it is inactive and will not register its provider.

Tooltips still treat non-backpack SB items as containers ​

Ensure the client-side add-on is installed. The mixin that filters Sophisticated Backpacks items only runs on the client.

Performance concerns with large backpacks ​

The add-on only reads contents when MT Inventory Weight requests a weight. It uses efficient Transfer API iteration and respects the nested() depth guard provided by MT Inventory Weight.