How do I install FastMapCodec?

Install the matching mod loader (Forge, Fabric or NeoForge) for your Minecraft version. Download the .jar. Put it in the .minecraft/mods folder and launch that loader profile. Opens on Modrinth — not every file is mirrored on our own servers.

What Minecraft version does FastMapCodec work with?

The listing gives Minecraft 1.21, 1.21.1, 1.21.2, among others. This is the version stated by the author or source site, not something we verified ourselves.

Home›Java›Mods›FastMapCodec

ModsJava

FastMapCodec

Java mod listed for Minecraft 1.21–26.x. Downloads from the MC Java Mods app on Android.

by neko_L · on Modrinth

Which build do you need?

This mod ships a separate file for every mod loader and every version of the game. Pick both, then download — the wrong build installs fine and then does nothing in the game.

Mod loader

Mod loader Fabric1 build
Mod loader NeoForge1 build

Minecraft version

Minecraft 1.21–26.2Newest

Minecraft 1.21–26.2Newest

⬇ Download for 1.21–26.2 · Fabric ⬇ Download for 1.21–26.2 · NeoForge

I don't know my version or loader

Open the Minecraft launcher and look at the profile you play on — it names both, like fabric-loader-1.21.4. The version also shows in the bottom-right corner of the game's main menu.

No loader in the profile name? Then it's plain Minecraft, and these mods won't run — you need Fabric or NeoForge installed first.

Open in MC Java Mods Get it on Google Play →

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:

nekotxs/BaseMapCodec-bench

The fix has also been submitted upstream to Mojang's DataFixerUpper:

Mojang/DataFixerUpper#110

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.

Quick facts

Edition: Minecraft Java
File type: .jar
Minecraft versions listed: 1.21, 1.21.1, 1.21.2
How to install: Install the matching mod loader (Forge, Fabric or NeoForge) for your Minecraft version. → Download the .jar. → Put it in the .minecraft/mods folder and launch that loader profile.
Where to get it: Opens on Modrinth — not every file is mirrored on our own servers.

Install steps are the general flow for this file type — How to install Minecraft Java mods & modpacks walks through it step by step.

File checked by MCModsHub

Ships as fastmapcodec-neoforge-1.0.4.jar — drop this file into the mods folder
Download size: 28 KB
Download link checked 5 Oct 2026 — working

These come from our own check of the pack file, not from the source page.

Explore more

All Mods for Java All Minecraft Java mods Java mods for 1.21 Java mods for 26