How to Fix “Failed to Synchronize Registry Data” When Joining a Server

Direct answer

Failed to synchronize registry data points to disagreement between the registries expected by client and server, commonly mod or datapack content. Compare exact versions and logs before changing worlds.

What it means

Registry sync can include modded blocks, items, entity types and datapack content. The final disconnect message often hides the first missing or conflicting registry entry.

Practical example: Example: a client missing a biome or item registry introduced by a server datapack can fail even with the same Minecraft version. Matching the filename of a mod is also insufficient when the actual mod versions differ. Collect the first registry entry named in logs; do not erase entire world folders to bypass this error.

If you are a player

Use the modpack-provided client files with the exact loader and Minecraft version.
Copy the complete error text, including any named mod, registry or missing entry.
Avoid replacing only one content mod unless the server owner coordinates a pack update.

If you run the server

Check the first client and server registry error around connection time.
Compare mod loader, installed mod IDs, versions and shared dependencies on both sides.
If the problem followed a pack update, test the previous known-good pack against a saved pre-upgrade world copy.
Review datapacks and world registries on the staging copy; do not delete registry/data files in production.
Test a freshly matched client and server pack after correcting the identified mismatch.

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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