FastMapCodec
Provides more efficient BaseMapCodec.decode implementation
- Loaders
- FabricNeoForge
- Downloads on Modrinth
- 820
- Followers on Modrinth
- 9
- versions
- 10
- Updated
- August 18, 2026Aug 18
- Published
- in 2026
At a glance
Works both in the game and on the server — install where you need it
Mainly fixes TPS freeze on player join caused by large advancement files (and the same class of freeze anywhere else in the game, or in other mods, that arises from decoding a large map through DFU).
It is okay to use this mod on server only or client only.
Fixes O(N²) behavior in Mojang's BaseMapCodec (part of com.mojang:datafixerupper) when decoding large map-based Codecs. Since the fix lives in BaseMapCodec itself, it applies to every codec built on top of it - UnboundedMapCodec, SimpleMapCodec, and anything else sharing the same decode/encode logic - not just player advancements.
This bug was introduced in 1.21 and is present up to modern versions.
Benchmarks comparing this mod's fix to vanilla behaviour (with some context) are available here:
The fix has also been submitted upstream to Mojang's DataFixerUpper:
If Mojang accepts the PR, this mod won't be needed on newer versions.
NeoForge disclaimer
On NeoForge, this mod uses some cursed techniques to modify DFU behaviour, which may cause incompatibility with other mods. If you notice any issues, please open an issue on GitHub.
It is known to be incompatible with Sodium Extra.
This only applies to NeoForge — the Fabric version is not affected.
FAQ
How to install FastMapCodec?
Install Fabric for your Minecraft version, download FastMapCodec for the same version and loader, put the file into the mods folder and launch the game with the loader profile.
Which Minecraft versions does FastMapCodec support?
FastMapCodec has builds for Minecraft 1.21–26.3.
Is FastMapCodec needed on the server?
Works on either side — install it in the game, on the server, or both
