MixinRuntime

Standard mixins must be declared before the game starts. MixinRuntime removes that limit by registering new mixin configurations dynamically, using Java Instrumentation to retransform classes that are already loaded in memory. Authors can therefore inject into classes that could not be reached before, and the change takes effect without restarting the game.

The mod works in several stages during bootstrap. It patches core platform classes such as MixinProcessor, ModuleClassLoader, ModLauncher and ClassTracker, so runtime injection is allowed, and it applies class transformers that suppress errors raised during injection. A ModuleUtils helper opens the Java Platform Module System deep enough for reflective access. Injection is done through MixinProcessorUtils, which adds new IMixinConfig instances to the active mixin processor's configuration lists.

It works on either side, and neither needs it, so it can run on a client, a server or both. It is published for Forge and NeoForge under the MIT License, and its source code is open source. Files exist for 1.20.1, 1.21, 1.21.1, 1.21.2, 1.21.3 and 1.21.10, with the latest Forge file at 1.0 for 1.20.1 and the latest NeoForge file at 1.0.1 for 1.21.10, both dated 2026-06-07.