🌉 LegacyBridge

Run selected Fabric 1.20.1 mods natively on Minecraft 1.21.1 — without modifying a single original JAR.

LegacyBridge is an advanced compatibility layer and runtime transformation engine designed to bridge the gap between Minecraft 1.20.1 and 1.21.1. It analyzes, adapts, and injects legacy mods on-the-fly during game launch, allowing you to bring your favorite 1.20.1 mods into modern Minecraft 1.21.1 modpacks.

✨ Features

🔄 Dynamic Bytecode Transformation (ASM)

• Real-time Remapping: Adapts bytecode instructions, class signatures, method descriptors, and field accesses from 1.20.1 Intermediary/Yarn mappings to 1.21.1.

• Registry Relocation: Remaps relocated vanilla registries and classes, including changes from Registry to Registries, renamed packages, and package moves.

• Zero Disk Mutation: Original mod JARs remain untouched. All bytecode rewriting is performed in-memory during startup and cached safely in a temporary folder.

💉 Direct KnotClassLoader Injection

• Global Visibility: Injects transformed classes directly into Fabric's KnotClassLoader, avoiding isolated classloaders that prevent Minecraft from accessing legacy mod contents.

• Seamless Game Recognition: Allows blocks, items, entities, screens, and network packets registered by 1.20.1 mods to integrate with Minecraft 1.21.1.

🎨 Automatic Resource & Asset Pipeline

• Built-in Resource Pack Virtualization: Discovers assets/ and data/ directories from detected 1.20.1 JARs and hooks into Minecraft's ResourcePackManager.

• Automatic pack.mcmeta Modernization: Dynamically updates pack format headers to Pack Format 34 for Minecraft 1.21.1, helping textures, JSON models, blockstates, sounds, and language files load without incompatibility warnings.

🧩 Essential API Polyfills & Shims

Minecraft 1.21 introduced major internal API changes. LegacyBridge provides compatibility layers for critical components:

• Item Components Bridge: Translates legacy NBT operations, such as ItemStack.getOrCreateNbt(), getNbt(), getTag(), and setCustomName(), to Minecraft 1.21's new Data Component system.

• FabricBlockSettings & FabricItemSettings Polyfill: Restores compatibility with legacy block and item builder APIs removed from modern Fabric API.

• EntityAttributeModifier Shim: Converts 1.20.1 UUID-based attribute modifier constructors into 1.21.1 Identifier-based constructors.

⚡ Mixin Runtime Pipeline

• Config Patcher: Automatically parses legacy *.mixins.json files, maps target classes to 1.21.1 equivalents, and generates patched configurations.

• @At & Target Remapping: Remaps method invocation targets and injection points in @Inject, @Redirect, and @ModifyArg annotations.

• Dynamic Bootstrap: Injects patched mixins into the SpongePowered Mixin processor during launch.

📊 Diagnostic & Telemetry System

• Real-time logging of adapted calls, remapped methods, and potential missing references.

• Non-fatal warnings and graceful degradation to reduce hard crashes whenever possible.

📋 Requirements

• Minecraft 1.21.1

• Fabric Loader 0.15.0 or newer

• Fabric API — latest version for 1.21.1

• Java 21

❓ FAQ

Does LegacyBridge modify the original mod JARs?
No! All bytecode transformations are performed in-memory, leaving the original files untouched.

Can I use any Fabric 1.20.1 mod?
Not necessarily. Compatibility depends on the mod's code, API usage, mixins, and interactions with Minecraft's internal systems.

How are textures, models, and other assets handled?
LegacyBridge discovers legacy resources and makes them available through its resource pack compatibility pipeline.

Does it support complex mods?
Basic mods and compatible mixins may work, but complex renderers, shaders, and deep engine modifications may require additional adaptations.

Developed by Agnist — released under the All Rights Reserved License.