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If catalog is defined in the same settings script then no. The build script classpath is already composed to run actual settings script which loads catalogs. Of course you could try load them manually in settings plugin but it would be fragile. Much better just to use catalog in an included build and apply required settings plugins from your settings plugin

It's defined in a toml file

It shouldn't matter since it's loaded via same API in the

dependencyResolutionManagement

https://github.com/gradle/gradle/blob/master/subprojects/core/src/main/java/org/gradle/configuration/BuildTreePreparingProjectsPreparer.java#L72-L88

iirc you cannot use the version catalog in the settings script at all - no matter where, as they are only created during evaluation of the settings script. Without parsing the TOML file manually that is.