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A mod will not start

Do not delete the whole game. Most failures are found by checking versions, dependencies, and the first meaningful log error in order.

Check in this order

The numbers must match the mod file. “1.21” does not always mean every 1.21.x release.

Fabric, Forge, and NeoForge use separate builds. Check the filename and launcher profile.

Fabric API, GeckoLib, and similar libraries are installed separately when the author lists them as required.

Modern versions require a current Java runtime. A local LLM adds substantial RAM and VRAM use.

Open .minecraft/logs/latest.log and find the first Caused by block or the problematic mod name. The final lines are often only consequences.

Before experimenting: copy the world and launch the mod in a clean profile. This separates a mod error from a pack conflict.