Modded Minecraft Registry Sync / Mismatch Error

Direct answer

Registry sync failed is often a shorter expression of client/server mod or datapack registry disagreement. Use the first detailed log message to find the missing ID or incompatible mod before changing files.

What it means

Registry data describes content the game needs to agree on. A generic failure can originate from one dependency or a changed world datapack rather than all installed mods.

Practical example: Example: if a new mod adds an item registry and one client still has the old pack, the client may be rejected immediately after protocol negotiation. If all clients fail only after the server updated its datapacks, investigate server registry generation and the world’s backup snapshot instead of reinstalling random client mods.

If you are a player

Compare installed modpack version, Minecraft version and loader type with the server.
Save the complete disconnect detail and the client log around the first registry error.
Do not remove mods from an existing multiplayer world without coordinating with the server owner.

If you run the server

Compare the first registry exception on both client and server, including named IDs.
Check mod versions and dependencies as a coherent pack rather than changing one at random.
Inspect custom datapacks and data migrations on a backup/staging world.
Restore the last known-good pack and world combination if a recent update introduced the mismatch.
Test with a fresh client profile containing precisely the supported mods and loader.

Important notes

Registry errors are sensitive to the exact mod loader/version and world history. Do not delete registry/world data based only on a generic guide.

References

Use the documentation for the exact software and release you run. These sources support the settings or diagnostic behavior used in this fix.

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