Fixing OptiFabric Crashes and Mod Conflicts on Fabric

Identify the conflicting mod

OptiFabric works by patching OptiFine at runtime, so anything else that rewrites the same rendering or entity code will fight it. The usual suspects are performance mods that replace the vanilla renderer, and any mod that already bundles OptiFine-style features. Read the crash report's top stack trace: if it names a rendering class from another mod, that mod is the problem.

Resolve it step by step

1. Move every mod except OptiFabric and OptiFine out of the mods folder, then launch. If the game starts, the conflict is confirmed.

2. Add mods back in small batches, launching between each batch, until the crash returns. The last batch contains the culprit.

3. Remove or replace the offending mod, or check whether it offers a compatibility mode.

Also check that your Fabric loader and Fabric API are current, since OptiFabric depends on loader internals. If the crash happens only when entering a world rather than at the main menu, the cause is more likely an entity or block-entity rendering mod than a core renderer replacement. Keep a copy of the working mod list so you can roll back quickly after future updates.