Keys for feature seeds in world configs do not migrate from 1.21.11 to 26.1.2 properly

[youFailerEXE]

Expected behavior

The server should automatically convert names for keys regarding feature seeds in paper-world.yml configurations in each world folder from their 1.21.11 format to their 26.1.2 format, respectively.

Observed/Actual behavior

When upgrading the server from 1.21.11 to 26.1.2, some feature seeds can not be deserialized from the paper-world configuration file because the name of the key of the feature seed is different across the versions. This causes an error like this to be shown for each feature seed that can't be deserialized:

[14:59:04] [Server thread/ERROR]: [MapSerializer] Could not deserialize key minecraft:pale_forest_flowers into net.minecraft.core.Holder<net.minecraft.world.level.levelgen.feature.ConfiguredFeature<?, ?>> at [feature-seeds, features]: Missing holder in ResourceKey[minecraft:root / minecraft:worldgen/configured_feature] with key ResourceKey[minecraft:worldgen/configured_feature / minecraft:pale_forest_flowers]

Steps/models to reproduce

1. Create a paper server on version 1.21.11
2. Set this in the paper-world-defaults.yml configuration:
   feature-seeds: generate-random-seeds-for-all: true
3. Start the server again and let the server generate random feature seeds
4. Swap the server jar for 26.1.2 and start the server
5. When the world starts loading, an error will be thrown for each feature seed that can't be deserialized, and the server will generate new feature seeds for each non-deserializable feature seed.

Plugin and Datapack List

plugins
[22:34:13 INFO]: ℹ Server Plugins (0):

datapack list
[22:34:35 INFO]: There are 3 data pack(s) enabled: [vanilla (built-in)], [file/bukkit (world)], [paper (built-in)]
[22:34:35 INFO]: There are no more data packs available

Paper version

version
[22:34:54 INFO]: Checking version, please wait...
[22:34:54 INFO]: This server is running Paper version 26.1.2-64-main@2186e1e (2026-05-16T22:33:57Z) (Implementing API version 26.1.2.build.64-stable)
You are running the latest version
Previous version: 1.21.11-132-c5eb079 (MC: 1.21.11)

Other

No response

[Warriorrrr]

I don't think there was never any migration for removed configured features? I always thought the expectation was for you to just remove the invalid ones upon updating, this was previously also reported in #8277 which was closed as WAI

[Doc94]

Maybe can be good add a message for notice comes from the config?
because the error just says cannot serialize and before of that the dimension (include the path of config but not mention nothing about config directly.. not?)....

[youFailerEXE]

The key that the server says cannot be deserialized is found in the paper-world.yml config before upgrading, and disappears after upgrading, so it is correct that they are getting removed. However, upon checking the paper-world.yml after upgrading, I found many keys with similar names to the ones that got removed, many of which just have the word "patch_" removed from the beginning of the key (ex: minecraft:patch_pumpkin -> minecraft:pumpkin). I cannot say for certain that the old and the new keys with similar names are used for the same thing when generating the world, but if they are, Im not sure changing feature seeds is healthy for the world...

Also, I counted 25 keys that could not be deserialized and got removed, and the server only regenerated 22 keys after the 25 keys got removed, so three keys were completely removed from the config

[electronicboy]

The past issue was marked as WAI as the error is kinda expected when stuff doesn't exist, there isn't any real means of automating the upgrades of this stuff too well, so it kinda falls by the wayside; That stuff is all backed off of Mojang's registries, and given that this is all basically generation stuff which doesn't get serialised, there is no migration logic inside of vanilla at all, and nothing we have to hook.