Access widener or class tweaker not working

Checked against Fabric’s documentation on 2 November 2026.

An access widener makes a private or final part of Minecraft’s code usable from your mod. Fabric now calls the file a class tweaker, because it can do more than widen access (Fabric: Class Tweakers). If your code still can’t see the member, or the game won’t start, check these.

1. The file is named in two places

Both are needed (Fabric: Class Tweakers):

build.gradle

loom {
    accessWidenerPath = file("src/main/resources/example-mod.classtweaker")
}

fabric.mod.json

"accessWidener": "example-mod.classtweaker"

The keys keep the old “access widener” name, for backwards compatibility.

2. The first line is right for your version

Your setup	First line
Minecraft 26.1 and newer	classTweaker v1 official
Class tweaker before 26.1 (Linen’s Fabric templates)	classTweaker v1 named
Older access widener (Linen’s Quilt and Legacy Fabric templates)	accessWidener v2 named

When you port to 26.1, change named to official in the first line (Fabric: Porting to 26.1). Class tweakers need Fabric Loader 0.18.0 and Loom 1.12 or newer (Fabric: Class Tweakers).

3. The targets are written the right way

From Fabric: Access Widening:

• Classes use internal names: slashes, not dots (net/minecraft/...), and $ before an inner class.
• Fields and methods need the class name, the member’s name and its bytecode descriptor.
• accessible works on classes, methods and fields; extendable on classes and methods; mutable on fields.
• It only works on Minecraft’s own classes, not on other mods. For another mod’s private members, use an accessor mixin.
• The names must be the ones for your version. After moving from Yarn to Mojang names, migrate this file too with migrateClassTweakerMappings (Loom’s migrateMappings task).

4. Validate the file

An entry that points at something that doesn’t exist is ignored without an error. To check every entry:

./gradlew validateAccessWidener

The errors point to the bad entry, though not always to the bad part of it (Fabric: Class Tweakers).

5. Refresh after a change

Refresh the Gradle project and regenerate sources, then reopen the class you changed (Fabric: Access Widening, Generating Sources).

In Linen

• Every Fabric project from New mod has a class tweaker already named in both places, with the right first line for its version.
• Minecraft’s code browses the decompiled game for your project’s version, on your machine, so you can find the exact class, member and descriptor.
• Hovers show names in Mojang, Yarn, intermediary and Parchment, where the version has them.