Exception when loading some old maps

[Brokkonaut]

Expected behavior

The map should load

Observed/Actual behavior

Exception: https://cpaste.de/raw/daraxareya

Steps/models to reproduce

1. Add this map to your server (the exception is happening with many maps on our server but not all): https://webinterface.cubeside.de/temp/map_5423.dat
2. Use the code mentioned here to load the map: https://cpaste.de/raw/daraxareya
3. Join the server
4. Exception: https://cpaste.de/raw/daraxareya

Plugin and Datapack List

just a test plugin

Paper version

This server is running Paper version 1.21.5-87-main@fa360aa (2025-05-18T22:35:50Z) (Implementing API version 1.21.5-R0.1-SNAPSHOT)
You are running the latest version

Other

No response

[Owen1212055]

After investigating, there are a large amount of issues with Vanilla datafixers with old maps:

• They incorrectly wrap the entire data in data (MapIdFix was incorrectly updated at one point)
• They don't datafix dimension until 25w18a, so that is incorrect (25w05a caused this breaking change)
• They don't define the map schema correctly in many places, assuming the data is present, or not.
• We should be using optional codecs in our createUUIDBackedDimensionKeyCodec

[Tsoccerguy3]

The data converter is not finished, this is why 1.21.5 is experimental and not stable. If you want to use your old maps, run the version updater in the minecraft 1.21.5 client first. then add the map to your server